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Linux Boot Partitions

💽 Linux Boot Partitions and How to Set Them Up 🚀

Let’s have a look how traditional Linux distributions set up /boot/ and the ESP, and how this could be improved.

How Linux distributions traditionally have been setting up their “boot” file systems has been varying to some degree, but the most common choice has been to have a separate partition mounted to /boot/. Usually the partition is formatted as a Linux file system such as ext2/ext3/ext4. The partition contains the kernel images, the initrd and various boot loader resources. Some distributions, like Debian and Ubuntu, also store ancillary files associated with the kernel here, such as kconfig or System.map. Such a traditional boot partition is only defined within the context of the distribution, and typically not immediately recognizable as such when looking just at the partition table (i.e. it uses the generic Linux partition type UUID).

With the arrival of UEFI a new partition relevant for boot appeared, the EFI System Partition (ESP). This partition is defined by the firmware environment, but typically accessed by Linux to install or update boot loaders. The choice of file system is not up to Linux, but effectively mandated by the UEFI specifications: vFAT. In theory it could be formatted as other file systems too. However, this would require the firmware to support file systems other than vFAT. This is rare and firmware specific though, as vFAT is the only file system mandated by the UEFI specification. In other words, vFAT is the only file system which is guaranteed to be universally supported.

There’s a major overlap of the type of the data typically stored in the ESP and in the traditional boot partition mentioned earlier: a variety of boot loader resources as well as kernels/initrds.

Unlike the traditional boot partition, the ESP is easily recognizable in the partition table via its GPT partition type UUID. The ESP is also a shared resource: all OSes installed on the same disk will share it and put their boot resources into them (as opposed to the traditional boot partition, of which there is one per installed Linux OS, and only that one will put resources there).

To summarize, the most common setup on typical Linux distributions is something like this:

Type Linux Mount Point File System Choice
Linux “Boot” Partition /boot/ Any Linux File System, typically ext2/ext3/ext4
ESP /boot/efi/ vFAT

As mentioned, not all distributions or local installations agree on this. For example, it’s probably worth mentioning that some distributions decided to put kernels onto the root file system of the OS itself. For this setup to work the boot loader itself [sic!] must implement a non-trivial part of the storage stack. This may have to include RAID, storage drivers, networked storage, volume management, disk encryption, and Linux file systems. Leaving aside the conceptual argument that complex storage stacks don’t belong in boot loaders there are very practical problems with this approach. Reimplementing the Linux storage stack in all its combinations is a massive amount of work. It took decades to implement what we have on Linux now, and it will take a similar amount of work to catch up in the boot loader’s reimplementation. Moreover, there’s a political complication: some Linux file system communities made clear they have no interest in supporting a second file system implementation that is not maintained as part of the Linux kernel.

What’s interesting is that the /boot/efi/ mount point is nested below the /boot/ mount point. This effectively means that to access the ESP the Boot partition must exist and be mounted first. A system with just an ESP and without a Boot partition hence doesn’t fit well into the current model. The Boot partition will also have to carry an empty “efi” directory that can be used as the inner mount point, and serves no other purpose.

Given that the traditional boot partition and the ESP may carry similar data (i.e. boot loader resources, kernels, initrds) one may wonder why they are separate concepts. Historically, this was the easiest way to make the pre-UEFI way how Linux systems were booted compatible with UEFI: conceptually, the ESP can be seen as just a minor addition to the status quo ante that way. Today, primarily two reasons remained:

  • Some distributions see a benefit in support for complex Linux file system concepts such as hardlinks, symlinks, SELinux labels/extended attributes and so on when storing boot loader resources. – I personally believe that making use of features in the boot file systems that the firmware environment cannot really make sense of is very clearly not advisable. The UEFI file system APIs know no symlinks, and what is SELinux to UEFI anyway? Moreover, putting more than the absolute minimum of simple data files into such file systems immediately raises questions about how to authenticate them comprehensively (including all fancy metadata) cryptographically on use (see below).

  • On real-life systems that ship with non-Linux OSes the ESP often comes pre-installed with a size too small to carry multiple Linux kernels and initrds. As growing the size of an existing ESP is problematic (for example, because there’s no space available immediately after the ESP, or because some low-quality firmware reacts badly to the ESP changing size) placing the kernel in a separate, secondary partition (i.e. the boot partition) circumvents these space issues.

File System Choices

We already mentioned that the ESP effectively has to be vFAT, as that is what UEFI (more or less) guarantees. The file system choice for the boot partition is not quite as restricted, but using arbitrary Linux file systems is not really an option either. The file system must be accessible by both the boot loader and the Linux OS. Hence only file systems that are available in both can be used. Note that such secondary implementations of Linux file systems in the boot environment – limited as they may be – are not typically welcomed or supported by the maintainers of the canonical file system implementation in the upstream Linux kernel. Modern file systems are notoriously complicated and delicate and simply don’t belong in boot loaders.

In a trusted boot world, the two file systems for the ESP and the /boot/ partition should be considered untrusted: any code or essential data read from them must be authenticated cryptographically before use. And even more, the file system structures themselves are also untrusted. The file system driver reading them must be careful not to be exploitable by a rogue file system image. Effectively this means a simple file system (for which a driver can be more easily validated and reviewed) is generally a better choice than a complex file system (Linux file system communities made it pretty clear that robustness against rogue file system images is outside of their scope and not what is being tested for.).

Some approaches tried to address the fact that boot partitions are untrusted territory by encrypting them via a mechanism compatible to LUKS, and adding decryption capabilities to the boot loader so it can access it. This misses the point though, as encryption does not imply authentication, and only authentication is typically desired. The boot loader and kernel code are typically Open Source anyway, and hence there’s little value in attempting to keep secret what is already public knowledge. Moreover, encryption implies the existence of an encryption key. Physically typing in the decryption key on a keyboard might still be acceptable on desktop systems with a single human user in front, but outside of that scenario unlock via TPM, PKCS#11 or network services are typically required. And even on the desktop FIDO2 unlocking is probably the future. Implementing all the technologies these unlocking mechanisms require in the boot loader is not realistic, unless the boot loader shall become a full OS on its own as it would require subsystems for FIDO2, PKCS#11, USB, Bluetooth network, smart card access, and so on.

File System Access Patterns

Note that traditionally both mentioned partitions were read-only during most parts of the boot. Only later, once the OS is up, write access was required to implement OS or boot loader updates. In today’s world things have become a bit more complicated. A modern OS might want to require some limited write access already in the boot loader, to implement boot counting/boot assessment/automatic fallback (e.g., if the same kernel fails to boot 3 times, automatically revert to older kernel), or to maintain an early storage-based random seed. This means that even though the file system is mostly read-only, we need limited write access after all.

vFAT cannot compete with modern Linux file systems such as btrfs when it comes to data safety guarantees. It’s not a journaled file system, does not use CoW or any form of checksumming. This means when used for the system boot process we need to be particularly careful when accessing it, and in particular when making changes to it (i.e., trying to keep changes local to single sectors). It is essential to use write patterns that minimize the chance of file system corruption. Checking the file system (“fsck”) before modification (and probably also reading) is important, as is ensuring the file system is put into a “clean” state as quickly as possible after each modification.

Code quality of the firmware in typical systems is known to not always be great. When relying on the file system driver included in the firmware it’s hence a good idea to limit use to operations that have a better chance to be correctly implemented. For example, when writing from the UEFI environment it might be wise to avoid any operation that requires allocation algorithms, but instead focus on access patterns that only override already written data, and do not require allocation of new space for the data.

Besides write access from the boot loader code (as described above) these file systems will require write access from the OS, to facilitate boot loader and kernel/initrd updates. These types of accesses are generally not fully random accesses (i.e., never partial file updates) but usually mean adding new files as whole, and removing old files as a whole. Existing files are typically not modified once created, though they might be replaced wholly by newer versions.

Boot Loader Updates

Note that the update cycle frequencies for boot loaders and for kernels/initrds are probably similar these days. While kernels are still vastly more complex than boot loaders, security issues are regularly found in both. In particular, as boot loaders (through “shim” and similar components) carry certificate/keyring and denylist information, which typically require frequent updates. Update cycles hence have to be expected regularly.

Boot Partition Discovery

The traditional boot partition was not recognizable by looking just at the partition table. On MBR systems it was directly referenced from the boot sector of the disk, and on EFI systems from information stored in the ESP. This is less than ideal since by losing this entrypoint information the system becomes unbootable. It’s typically a better, more robust idea to make boot partitions recognizable as such in the partition table directly. This is done for the ESP via the GPT partition type UUID. For traditional boot partitions this was not done though.

Current Situation Summary

Let’s try to summarize the above:

  • Currently, typical deployments use two distinct boot partitions, often using two distinct file system implementations

  • Firmware effectively dictates existence of the ESP, and the use of vFAT

  • In userspace view: the ESP mount is nested below the general Boot partition mount

  • Resources stored in both partitions are primarily kernel/initrd, and boot loader resources

  • The mandatory use of vFAT brings certain data safety challenges, as does quality of firmware file system driver code

  • During boot limited write access is needed, during OS runtime more comprehensive write access is needed (though still not fully random).

  • Less restricted but still limited write patterns from OS environment (only full file additions/updates/removals, during OS/boot loader updates)

  • Boot loaders should not implement complex storage stacks.

  • ESP can be auto-discovered from the partition table, traditional boot partition cannot.

  • ESP and the traditional boot partition are not protected cryptographically neither in structure nor contents. It is expected that loaded files are individually authenticated after being read.

  • The ESP is a shared resource — the traditional boot partition a resource specific to each installed Linux OS on the same disk.

How to Do it Better

Now that we have discussed many of the issues with the status quo ante, let’s see how we can do things better:

  • Two partitions for essentially the same data is a bad idea. Given they carry data very similar or identical in nature, the common case should be to have only one (but see below).

  • Two file system implementations are worse than one. Given that vFAT is more or less mandated by UEFI and the only format universally understood by all players, and thus has to be used anyway, it might as well be the only file system that is used.

  • Data safety is unnecessarily bad so far: both ESP and boot partition are continuously mounted from the OS, even though access is pretty restricted: outside of update cycles access is typically not required.

  • All partitions should be auto-discoverable/self-descriptive

  • The two partitions should not be exposed as nested mounts to userspace

To be more specific, here’s how I think a better way to set this all up would look like:

  • Whenever possible, only have one boot partition, not two. On EFI systems, make it the ESP. On non-EFI systems use an XBOOTLDR partition instead (see below). Only have both in the case where a Linux OS is installed on a system that already contains an OS with an ESP that is too small to carry sufficient kernels/initrds. When a system contains a XBOOTLDR partition put kernels/initrd on that, otherwise the ESP.

  • Instead of the vaguely defined, traditional Linux “boot” partition use the XBOOTLDR partition type as defined by the Discoverable Partitions Specification. This ensures the partition is discoverable, and can be automatically mounted by things like systemd-gpt-auto-generator. Use XBOOTLDR only if you have to, i.e., when dealing with systems that lack UEFI (and where the ESP hence has no value) or to address the mentioned size issues with the ESP. Note that unlike the traditional boot partition the XBOOTLDR partition is a shared resource, i.e., shared between multiple parallel Linux OS installations on the same disk. Because of this it is typically wise to place a per-OS directory at the top of the XBOOTLDR file system to avoid conflicts.

  • Use vFAT for both partitions, it’s the only thing universally understood among relevant firmwares and Linux. It’s simple enough to be useful for untrusted storage. Or to say this differently: writing a file system driver that is not easily vulnerable to rogue disk images is much easier for vFAT than for let’s say btrfs. – But the choice of vFAT implies some care needs to be taken to address the data safety issues it brings, see below.

  • Mount the two partitions via the “automount” logic. For example, via systemd’s automount units, with a very short idle time-out (one second or so). This improves data safety immensely, as the file systems will remain mounted (and thus possibly in a “dirty” state) only for very short periods of time, when they are actually accessed – and all that while the fact that they are not mounted continuously is mostly not noticeable for applications as the file system paths remain continuously around. Given that the backing file system (vFAT) has poor data safety properties, it is essential to shorten the access for unclean file system state as much as possible. In fact, this is what the aforementioned systemd-gpt-auto-generator logic actually does by default.

  • Whenever mounting one of the two partitions, do a file system check (fsck; in fact this is also what systemd-gpt-auto-generatordoes by default, hooked into the automount logic, to run on first access). This ensures that even if the file system is in an unclean state it is restored to be clean when needed, i.e., on first access.

  • Do not mount the two partitions nested, i.e., no more /boot/efi/. First of all, as mentioned above, it should be possible (and is desirable) to only have one of the two. Hence it is simply a bad idea to require the other as well, just to be able to mount it. More importantly though, by nesting them, automounting is complicated, as it is necessary to trigger the first automount to establish the second automount, which defeats the point of automounting them in the first place. Use the two distinct mount points /efi/ (for the ESP) and /boot/ (for XBOOTLDR) instead. You might have guessed, but that too is what systemd-gpt-auto-generator does by default.

  • When making additions or updates to ESP/XBOOTLDR from the OS make sure to create a file and write it in full, then syncfs() the whole file system, then rename to give it its final name, and syncfs() again. Similar when removing files.

  • When writing from the boot loader environment/UEFI to ESP/XBOOTLDR, do not append to files or create new files. Instead overwrite already allocated file contents (for example to maintain a random seed file) or rename already allocated files to include information in the file name (and ideally do not increase the file name in length; for example to maintain boot counters).

  • Consider adopting UKIs, which minimize the number of files that need to be updated on the ESP/XBOOTLDR during OS/kernel updates (ideally down to 1)

  • Consider adopting systemd-boot, which minimizes the number of files that need to be updated on boot loader updates (ideally down to 1)

  • Consider removing any mention of ESP/XBOOTLDR from /etc/fstab, and just let systemd-gpt-auto-generator do its thing.

  • Stop implementing file systems, complex storage, disk encryption, … in your boot loader.

Implementing things like that you gain:

  • Simplicity: only one file system implementation, typically only one partition and mount point

  • Robust auto-discovery of all partitions, no need to even configure /etc/fstab

  • Data safety guarantees as good as possible, given the circumstances

To summarize this in a table:

Type Linux Mount Point File System Choice Automount
ESP /efi/ vFAT yes
XBOOTLDR /boot/ vFAT yes

A note regarding modern boot loaders that implement the Boot Loader Specification: both partitions are explicitly listed in the specification as sources for both Type #1 and Type #2 boot menu entries. Hence, if you use such a modern boot loader (e.g. systemd-boot) these two partitions are the preferred location for boot loader resources, kernels and initrds anyway.

Addendum: You got RAID?

You might wonder, what about RAID setups and the ESP? This comes up regularly in discussions: how to set up the ESP so that (software) RAID1 (mirroring) can be done on the ESP. Long story short: I’d strongly advise against using RAID on the ESP. Firmware typically doesn’t have native RAID support, and given that firmware and boot loader can write to the file systems involved, any attempt to use software RAID on them will mean that a boot cycle might corrupt the RAID sync, and immediately requires a re-synchronization after boot. If RAID1 backing for the ESP is really necessary, the only way to implement that safely would be to implement this as a driver for UEFI – but that creates certain bootstrapping issues (i.e., where to place the driver if not the ESP, a file system the driver is supposed to be used for), and also reimplements a considerable component of the OS storage stack in firmware mode, which seems problematic.

So what to do instead? My recommendation would be to solve this via userspace tooling. If redundant disk support shall be implemented for the ESP, then create separate ESPs on all disks, and synchronize them on the file system level instead of the block level. Or in other words, the tools that install/update/manage kernels or boot loaders should be taught to maintain multiple ESPs instead of one. Copy the kernels/boot loader files to all of them, and remove them from all of them. Under the assumption that the goal of RAID is a more reliable system this should be the best way to achieve that, as it doesn’t pretend the firmware could do things it actually cannot do. Moreover it minimizes the complexity of the boot loader, shifting the syncing logic to userspace, where it’s typically easier to get right.

Addendum: Networked Boot

The discussion above focuses on booting up from a local disk. When thinking about networked boot I think two scenarios are particularly relevant:

  1. PXE-style network booting. I think in this mode of operation focus should be on directly booting a single UKI image instead of a boot loader. This sidesteps the whole issue of maintaining any boot partition at all, and simplifies the boot process greatly. In scenarios where this is not sufficient, and an interactive boot menu or other boot loader features are desired, it might be a good idea to take inspiration from the UKI concept, and build a single boot loader EFI binary (such as systemd-boot), and include the UKIs for the boot menu items and other resources inside it via PE sections. Or in other words, build a single boot loader binary that is “supercharged” and contains all auxiliary resources in its own PE sections. (Note: this does not exist, it’s an idea I intend to explore with systemd-boot). Benefit: a single file has to be downloaded via PXE/TFTP, not more. Disadvantage: unused resources are downloaded unnecessarily. Either way: in this context there is no local storage, and the ESP/XBOOTLDR discussion above is without relevance.

  2. Initrd-style network booting. In this scenario the boot loader and kernel/initrd (better: UKI) are available on a local disk. The initrd then configures the network and transitions to a network share or file system on a network block device for the root file system. In this case the discussion above applies, and in fact the ESP or XBOOTLDR partition would be the only partition available locally on disk.

And this is all I have for today.


Brave New Trusted Boot World

🔐 Brave New Trusted Boot World 🚀

This document looks at the boot process of general purpose Linux distributions. It covers the status quo and how we envision Linux boot to work in the future with a focus on robustness and simplicity.

This document will assume that the reader has comprehensive familiarity with TPM 2.0 security chips and their capabilities (e.g., PCRs, measurements, SRK), boot loaders, the shim binary, Linux, initrds, UEFI Firmware, PE binaries, and SecureBoot.

Problem Description

Status quo ante of the boot logic on typical Linux distributions:

  • Most popular Linux distributions generate initrds locally, and they are unsigned, thus not protected through SecureBoot (since that would require local SecureBoot key enrollment, which is generally not done), nor TPM PCRs.

  • Boot chain is typically Firmware → shimgrub → Linux kernel → initrd (dracut or similar) → root file system

  • Firmware’s UEFI SecureBoot protects shim, shim’s key management protects grub and kernel. No code signing protects initrd. initrd acquires the key for encrypted root fs from the user (or TPM/FIDO2/PKCS11).

  • shim/grub/kernel is measured into TPM PCR 4, among other stuff

  • EFI TPM event log reports measured data into TPM PCRs, and can be used to reconstruct and validate state of TPM PCRs from the used resources.

  • No userspace components are typically measured, except for what IMA measures

  • New kernels require locally generating new boot loader scripts and generating a new initrd each time. OS updates thus mean fragile generation of multiple resources and copying multiple files into the boot partition.

Problems with the status quo ante:

  • initrd typically unlocks root file system encryption, but is not protected whatsoever, and trivial to attack and modify offline

  • OS updates are brittle: PCR values of grub are very hard to pre-calculate, as grub measures chosen control flow path, not just code images. PCR values vary wildly, and OS provided resources are not measured into separate PCRs. Grub’s PCR measurements might be useful up to a point to reason about the boot after the fact, for the most basic remote attestation purposes, but useless for calculating them ahead of time during the OS build process (which would be desirable to be able to bind secrets to future expected PCR state, for example to bind secrets to an OS in a way that it remain accessible even after that OS is updated).

  • Updates of a boot loader are not robust, require multi-file updates of ESP and boot partition, and regeneration of boot scripts

  • No rollback protection (no way to cryptographically invalidate access to TPM-bound secrets on OS updates)

  • Remote attestation of running software is needlessly complex since initrds are generated locally and thus basically are guaranteed to vary on each system.

  • Locking resources maintained by arbitrary user apps to TPM state (PCRs) is not realistic for general purpose systems, since PCRs will change on every OS update, and there’s no mechanism to re-enroll each such resource before every OS update, and remove the old enrollment after the update.

  • There is no concept to cryptographically invalidate/revoke secrets for an older OS version once updated to a new OS version. An attacker thus can always access the secrets generated on old OSes if they manage to exploit an old version of the OS — even if a newer version already has been deployed.

Goals of the new design:

  • Provide a fully signed execution path from firmware to userspace, no exceptions

  • Provide a fully measured execution path from firmware to userspace, no exceptions

  • Separate out TPM PCRs assignments, by “owner” of measured resources, so that resources can be bound to them in a fine-grained fashion.

  • Allow easy pre-calculation of expected PCR values based on booted kernel/initrd, configuration, local identity of the system

  • Rollback protection

  • Simple & robust updates: one updated file per concept

  • Updates without requiring re-enrollment/local preparation of the TPM-protected resources (no more “brittle” PCR hashes that must be propagated into every TPM-protected resource on each OS update)

  • System ready for easy remote attestation, to prove validity of booted OS, configuration and local identity

  • Ability to bind secrets to specific phases of the boot, e.g. the root fs encryption key should be retrievable from the TPM only in the initrd, but not after the host transitioned into the root fs.

  • Reasonably secure, automatic, unattended unlocking of disk encryption secrets should be possible.

  • “Democratize” use of PCR policies by defining PCR register meanings, and making binding to them robust against updates, so that external projects can safely and securely bind their own data to them (or use them for remote attestation) without risking breakage whenever the OS is updated.

  • Build around TPM 2.0 (with graceful fallback for TPM-less systems if desired, but TPM 1.2 support is out of scope)

Considered attack scenarios and considerations:

  • Evil Maid: neither online nor offline (i.e. “at rest”), physical access to a storage device should enable an attacker to read the user’s plaintext data on disk (confidentiality); neither online nor offline, physical access to a storage device should allow undetected modification/backdooring of user data or OS (integrity), or exfiltration of secrets.

  • TPMs are assumed to be reasonably “secure”, i.e. can securely store/encrypt secrets. Communication to TPM is not “secure” though and must be protected on the wire.

  • Similar, the CPU is assumed to be reasonably “secure”

  • SecureBoot is assumed to be reasonably “secure” to permit validated boot up to and including shim+boot loader+kernel (but see discussion below)

  • All user data must be encrypted and authenticated. All vendor and administrator data must be authenticated.

  • It is assumed all software involved regularly contains vulnerabilities and requires frequent updates to address them, plus regular revocation of old versions.

  • It is further assumed that key material used for signing code by the OS vendor can reasonably be kept secure (via use of HSM, and similar, where secret key information never leaves the signing hardware) and does not require frequent roll-over.

Proposed Construction

Central to the proposed design is the concept of a Unified Kernel Image (UKI). These UKIs are the combination of a Linux kernel image, and initrd, a UEFI boot stub program (and further resources, see below) into one single UEFI PE file that can either be directly invoked by the UEFI firmware (which is useful in particular in some cloud/Confidential Computing environments) or through a boot loader (which is generally useful to implement support for multiple kernel versions, with interactive or automatic selection of image to boot into, potentially with automatic fallback management to increase robustness).

UKI Components

Specifically, UKIs typically consist of the following resources:

  1. An UEFI boot stub that is a small piece of code still running in UEFI mode and that transitions into the Linux kernel included in the UKI (e.g., as implemented in sd-stub, see below)

  2. The Linux kernel to boot in the .linux PE section

  3. The initrd that the kernel shall unpack and invoke in the .initrd PE section

  4. A kernel command line string, in the .cmdline PE section

  5. Optionally, information describing the OS this kernel is intended for, in the .osrel PE section (derived from /etc/os-release of the booted OS). This is useful for presentation of the UKI in the boot loader menu, and ordering it against other entries, using the included version information.

  6. Optionally, information describing kernel release information (i.e. uname -r output) in the .uname PE section. This is also useful for presentation of the UKI in the boot loader menu, and ordering it against other entries.

  7. Optionally, a boot splash to bring to screen before transitioning into the Linux kernel in the .splash PE section

  8. Optionally, a compiled Devicetree database file, for systems which need it, in the .dtb PE section

  9. Optionally, the public key in PEM format that matches the signatures of the .pcrsig PE section (see below), in a .pcrpkey PE section.

  10. Optionally, a JSON file encoding expected PCR 11 hash values seen from userspace once the UKI has booted up, along with signatures of these expected PCR 11 hash values, matching a specific public key in the .pcrsig PE section. (Note: we use plural for “values” and “signatures” here, as this JSON file will typically carry a separate value and signature for each PCR bank for PCR 11, i.e. one pair of value and signature for the SHA1 bank, and another pair for the SHA256 bank, and so on. This ensures when enrolling or unlocking a TPM-bound secret we’ll always have a signature around matching the banks available locally (after all, which banks the local hardware supports is up to the hardware). For the sake of simplifying this already overly complex topic, we’ll pretend in the rest of the text there was only one PCR signature per UKI we have to care about, even if this is not actually the case.)

Given UKIs are regular UEFI PE files, they can thus be signed as one for SecureBoot, protecting all of the individual resources listed above at once, and their combination. Standard Linux tools such as sbsigntool and pesign can be used to sign UKI files.

UKIs wrap all of the above data in a single file, hence all of the above components can be updated in one go through single file atomic updates, which is useful given that the primary expected storage place for these UKIs is the UEFI System Partition (ESP), which is a vFAT file system, with its limited data safety guarantees.

UKIs can be generated via a single, relatively simple objcopy invocation, that glues the listed components together, generating one PE binary that then can be signed for SecureBoot. (For details on building these, see below.)

Note that the primary location to place UKIs in is the EFI System Partition (or an otherwise firmware accessible file system). This typically means a VFAT file system of some form. Hence an effective UKI size limit of 4GiB is in place, as that’s the largest file size a FAT32 file system supports.

Basic UEFI Stub Execution Flow

The mentioned UEFI stub program will execute the following operations in UEFI mode before transitioning into the Linux kernel that is included in its .linux PE section:

  1. The PE sections listed are searched for in the invoked UKI the stub is part of, and superficially validated (i.e. general file format is in order).

  2. All PE sections listed above of the invoked UKI are measured into TPM PCR 11. This TPM PCR is expected to be all zeroes before the UKI initializes. Pre-calculation is thus very straight-forward if the resources included in the PE image are known. (Note: as a single exception the .pcrsig PE section is excluded from this measurement, as it is supposed to carry the expected result of the measurement, and thus cannot also be input to it, see below for further details about this section.)

  3. If the .splash PE section is included in the UKI it is brought onto the screen

  4. If the .dtb PE section is included in the UKI it is activated using the Devicetree UEFI “fix-up” protocol

  5. If a command line was passed from the boot loader to the UKI executable it is discarded if SecureBoot is enabled and the command line from the .cmdline used. If SecureBoot is disabled and a command line was passed it is used in place of the one from .cmdline. Either way the used command line is measured into TPM PCR 12. (This of course removes any flexibility of control of the kernel command line of the local user. In many scenarios this is probably considered beneficial, but in others it is not, and some flexibility might be desired. Thus, this concept probably needs to be extended sooner or later, to allow more flexible kernel command line policies to be enforced via definitions embedded into the UKI. For example: allowing definition of multiple kernel command lines the user/boot menu can select one from; allowing additional allowlisted parameters to be specified; or even optionally allowing any verification of the kernel command line to be turned off even in SecureBoot mode. It would then be up to the builder of the UKI to decide on the policy of the kernel command line.)

  6. It will set a couple of volatile EFI variables to inform userspace about executed TPM PCR measurements (and which PCR registers were used), and other execution properties. (For example: the EFI variable StubPcrKernelImage in the 4a67b082-0a4c-41cf-b6c7-440b29bb8c4f vendor namespace indicates the PCR register used for the UKI measurement, i.e. the value “11”).

  7. An initrd cpio archive is dynamically synthesized from the .pcrsig and .pcrpkey PE section data (this is later passed to the invoked Linux kernel as additional initrd, to be overlaid with the main initrd from the .initrd section). These files are later available in the /.extra/ directory in the initrd context.

  8. The Linux kernel from the .linux PE section is invoked with with a combined initrd that is composed from the blob from the .initrd PE section, the dynamically generated initrd containing the .pcrsig and .pcrpkey PE sections, and possibly some additional components like sysexts or syscfgs.

TPM PCR Assignments

In the construction above we take possession of two PCR registers previously unused on generic Linux distributions:

  • TPM PCR 11 shall contain measurements of all components of the UKI (with exception of the .pcrsig PE section, see above). This PCR will also contain measurements of the boot phase once userspace takes over (see below).

  • TPM PCR 12 shall contain measurements of the used kernel command line. (Plus potentially other forms of parameterization/configuration passed into the UKI, not discussed in this document)

On top of that we intend to define two more PCR registers like this:

  • TPM PCR 15 shall contain measurements of the volume encryption key of the root file system of the OS.

  • [TPM PCR 13 shall contain measurements of additional extension images for the initrd, to enable a modularized initrd – not covered by this document]

(See the Linux TPM PCR Registry for an overview how these four PCRs fit into the list of Linux PCR assignments.)

For all four PCRs the assumption is that they are zero before the UKI initializes, and only the data that the UKI and the OS measure into them is included. This makes pre-calculating them straightforward: given a specific set of UKI components, it is immediately clear what PCR values can be expected in PCR 11 once the UKI booted up. Given a kernel command line (and other parameterization/configuration) it is clear what PCR values are expected in PCR 12.

Note that these four PCRs are defined by the conceptual “owner” of the resources measured into them. PCR 11 only contains resources the OS vendor controls. Thus it is straight-forward for the OS vendor to pre-calculate and then cryptographically sign the expected values for PCR 11. The PCR 11 values will be identical on all systems that run the same version of the UKI. PCR 12 only contains resources the administrator controls, thus the administrator can pre-calculate PCR values, and they will be correct on all instances of the OS that use the same parameters/configuration. PCR 15 only contains resources inherently local to the local system, i.e. the cryptographic key material that encrypts the root file system of the OS.

Separating out these three roles does not imply these actually need to be separate when used. However the assumption is that in many popular environments these three roles should be separate.

By separating out these PCRs by the owner’s role, it becomes straightforward to remotely attest, individually, on the software that runs on a node (PCR 11), the configuration it uses (PCR 12) or the identity of the system (PCR 15). Moreover, it becomes straightforward to robustly and securely encrypt data so that it can only be unlocked on a specific set of systems that share the same OS, or the same configuration, or have a specific identity – or a combination thereof.

Note that the mentioned PCRs are so far not typically used on generic Linux-based operating systems, to our knowledge. Windows uses them, but given that Windows and Linux should typically not be included in the same boot process this should be unproblematic, as Windows’ use of these PCRs should thus not conflict with ours.

To summarize:

PCR Purpose Owner Expected Value before UKI boot Pre-Calculable
11 Measurement of UKI components and boot phases OS Vendor Zero Yes
(at UKI build time)
12 Measurement of kernel command line, additional kernel runtime configuration such as systemd credentials, systemd syscfg images Administrator Zero Yes
(when system configuration is assembled)
13 System Extension Images of initrd
(and possibly more)
(Administrator) Zero Yes
15 Measurement of root file system volume key
(Possibly later more: measurement of root file system UUIDs and labels and of the machine ID /etc/machine-id)
Local System Zero Yes
(after first boot once ll such IDs are determined)

Signature Keys

In the model above in particular two sets of private/public key pairs are relevant:

  • The SecureBoot key to sign the UKI PE executable with. This controls permissible choices of OS/kernel

  • The key to sign the expected PCR 11 values with. Signatures made with this key will end up in the .pcrsig PE section. The public key part will end up in the .pcrpkey PE section.

Typically the key pair for the PCR 11 signatures should be chosen with a narrow focus, reused for exactly one specific OS (e.g. “Fedora Desktop Edition”) and the series of UKIs that belong to it (all the way through all the versions of the OS). The SecureBoot signature key can be used with a broader focus, if desired. By keeping the PCR 11 signature key narrow in focus one can ensure that secrets bound to the signature key can only be unlocked on the narrow set of UKIs desired.

TPM Policy Use

Depending on the intended access policy to a resource protected by the TPM, one or more of the PCRs described above should be selected to bind TPM policy to.

For example, the root file system encryption key should likely be bound to TPM PCR 11, so that it can only be unlocked if a specific set of UKIs is booted (it should then, once acquired, be measured into PCR 15, as discussed above, so that later TPM objects can be bound to it, further down the chain). With the model described above this is reasonably straight-forward to do:

  • When userspace wants to bind disk encryption to a specific series of UKIs (“enrollment”), it looks for the public key passed to the initrd in the /.extra/ directory (which as discussed above originates in the .pcrpkey PE section of the UKI). The relevant userspace component (e.g. systemd) is then responsible for generating a random key to be used as symmetric encryption key for the storage volume (let’s call it disk encryption key _here, DEK_). The TPM is then used to encrypt (“seal”) the DEK with its internal Storage Root Key (TPM SRK). A TPM2 policy is bound to the encrypted DEK. The policy enforces that the DEK may only be decrypted if a valid signature is provided that matches the state of PCR 11 and the public key provided in the /.extra/ directory of the initrd. The plaintext DEK key is passed to the kernel to implement disk encryption (e.g. LUKS/dm-crypt). (Alternatively, hardware disk encryption can be used too, i.e. Intel MKTME, AMD SME or even OPAL, all of which are outside of the scope of this document.) The TPM-encrypted version of the DEK which the TPM returned is written to the encrypted volume’s superblock.

  • When userspace wants to unlock disk encryption on a specific UKI, it looks for the signature data passed to the initrd in the /.extra/ directory (which as discussed above originates in the .pcrsig PE section of the UKI). It then reads the encrypted version of the DEK from the superblock of the encrypted volume. The signature and the encrypted DEK are then passed to the TPM. The TPM then checks if the current PCR 11 state matches the supplied signature from the .pcrsig section and the public key used during enrollment. If all checks out it decrypts (“unseals”) the DEK and passes it back to the OS, where it is then passed to the kernel which implements the symmetric part of disk encryption.

Note that in this scheme the encrypted volume’s DEK is not bound to specific literal PCR hash values, but to a public key which is expected to sign PCR hash values.

Also note that the state of PCR 11 only matters during unlocking. It is not used or checked when enrolling.

In this scenario:

  • Input to the TPM part of the enrollment process are the TPM’s internal SRK, the plaintext DEK provided by the OS, and the public key later used for signing expected PCR values, also provided by the OS. – Output is the encrypted (“sealed”) DEK.

  • Input to the TPM part of the unlocking process are the TPM’s internal SRK, the current TPM PCR 11 values, the public key used during enrollment, a signature that matches both these PCR values and the public key, and the encrypted DEK. – Output is the plaintext (“unsealed”) DEK.

Note that sealing/unsealing is done entirely on the TPM chip, the host OS just provides the inputs (well, only the inputs that the TPM chip doesn’t know already on its own), and receives the outputs. With the exception of the plaintext DEK, none of the inputs/outputs are sensitive, and can safely be stored in the open. On the wire the plaintext DEK is protected via TPM parameter encryption (not discussed in detail here because though important not in scope for this document).

TPM PCR 11 is the most important of the mentioned PCRs, and its use is thus explained in detail here. The other mentioned PCRs can be used in similar ways, but signatures/public keys must be provided via other means.

This scheme builds on the functionality Linux’ LUKS2 functionality provides, i.e. key management supporting multiple slots, and the ability to embed arbitrary metadata in the encrypted volume’s superblock. Note that this means the TPM2-based logic explained here doesn’t have to be the only way to unlock an encrypted volume. For example, in many setups it is wise to enroll both this TPM-based mechanism and an additional “recovery key” (i.e. a high-entropy computer generated passphrase the user can provide manually in case they lose access to the TPM and need to access their data), of which either can be used to unlock the volume.

Boot Phases

Secrets needed during boot-up (such as the root file system encryption key) should typically not be accessible anymore afterwards, to protect them from access if a system is attacked during runtime. To implement this the scheme above is extended in one way: at certain milestones of the boot process additional fixed “words” should be measured into PCR 11. These milestones are placed at conceptual security boundaries, i.e. whenever code transitions from a higher privileged context to a less privileged context.

Specifically:

  • When the initrd initializes (“initrd-enter”)

  • When the initrd transitions into the root file system (“initrd-leave”)

  • When the early boot phase of the OS on the root file system has completed, i.e. all storage and file systems have been set up and mounted, immediately before regular services are started (“sysinit”)

  • When the OS on the root file system completed the boot process far enough to allow unprivileged users to log in (“complete”)

  • When the OS begins shut down (“shutdown”)

  • When the service manager is mostly finished with shutting down and is about to pass control to the final phase of the shutdown logic (“final”)

By measuring these additional words into PCR 11 the distinct phases of the boot process can be distinguished in a relatively straight-forward fashion and the expected PCR values in each phase can be determined.

The phases are measured into PCR 11 (as opposed to some other PCR) mostly because available PCRs are scarce, and the boot phases defined are typically specific to a chosen OS, and hence fit well with the other data measured into PCR 11: the UKI which is also specific to the OS. The OS vendor generates both the UKI and defines the boot phases, and thus can safely and reliably pre-calculate/sign the expected PCR values for each phase of the boot.

Revocation/Rollback Protection

In order to secure secrets stored at rest, in particular in environments where unattended decryption shall be possible, it is essential that an attacker cannot use old, known-buggy – but properly signed – versions of software to access them.

Specifically, if disk encryption is bound to an OS vendor (via UKIs that include expected PCR values, signed by the vendor’s public key) there must be a mechanism to lock out old versions of the OS or UKI from accessing TPM based secrets once it is determined that the old version is vulnerable.

To implement this we propose making use of one of the “counters” TPM 2.0 devices provide: integer registers that are persistent in the TPM and can only be increased on request of the OS, but never be decreased. When sealing resources to the TPM, a policy may be declared to the TPM that restricts how the resources can later be unlocked: here we use one that requires that along with the expected PCR values (as discussed above) a counter integer range is provided to the TPM chip, along with a suitable signature covering both, matching the public key provided during sealing. The sealing/unsealing mechanism described above is thus extended: the signature passed to the TPM during unsealing now covers both the expected PCR values and the expected counter range. To be able to use a signature associated with an UKI provided by the vendor to unseal a resource, the counter thus must be at least increased to the lower end of the range the signature is for. By doing so the ability is lost to unseal the resource for signatures associated with older versions of the UKI, because their upper end of the range disables access once the counter has been increased far enough. By carefully choosing the upper and lower end of the counter range whenever the PCR values for an UKI shall be signed it is thus possible to ensure that updates can invalidate prior versions’ access to resources. By placing some space between the upper and lower end of the range it is possible to allow a controlled level of fallback UKI support, with clearly defined milestones where fallback to older versions of an UKI is not permitted anymore.

Example: a hypothetical distribution FooOS releases a regular stream of UKI kernels 5.1, 5.2, 5.3, … It signs the expected PCR values for these kernels with a key pair it maintains in a HSM. When signing UKI 5.1 it includes information directed at the TPM in the signed data declaring that the TPM counter must be above 100, and below 120, in order for the signature to be used. Thus, when the UKI is booted up and used for unlocking an encrypted volume the unlocking code must first increase the counter to 100 if needed, as the TPM will otherwise refuse unlocking the volume. The next release of the UKI, i.e. UKI 5.2 is a feature release, i.e. reverting back to the old kernel locally is acceptable. It thus does not increase the lower bound, but it increases the upper bound for the counter in the signature payload, thus encoding a valid range 100…121 in the signed payload. Now a major security vulnerability is discovered in UKI 5.1. A new UKI 5.3 is prepared that fixes this issue. It is now essential that UKI 5.1 can no longer be used to unlock the TPM secrets. Thus UKI 5.3 will bump the lower bound to 121, and increase the upper bound by one, thus allowing a range 121…122. Or in other words: for each new UKI release the signed data shall include a counter range declaration where the upper bound is increased by one. The lower range is left as-is between releases, except when an old version shall be cut off, in which case it is bumped to one above the upper bound used in that release.

UKI Generation

As mentioned earlier, UKIs are the combination of various resources into one PE file. For most of these individual components there are pre-existing tools to generate the components. For example the included kernel image can be generated with the usual Linux kernel build system. The initrd included in the UKI can be generated with existing tools such as dracut and similar. Once the basic components (.linux, .initrd, .cmdline, .splash, .dtb, .osrel, .uname) have been acquired the combination process works roughly like this:

  1. The expected PCR 11 hashes (and signatures for them) for the UKI are calculated. The tool for that takes all basic UKI components and a signing key as input, and generates a JSON object as output that includes both the literal expected PCR hash values and a signature for them. (For all selected TPM2 banks)

  2. The EFI stub binary is now combined with the basic components, the generated JSON PCR signature object from the first step (in the .pcrsig section) and the public key for it (in the .pcrpkey section). This is done via a simple “objcopy” invocation resulting in a single UKI PE binary.

  3. The resulting EFI PE binary is then signed for SecureBoot (via a tool such as sbsign or similar).

Note that the UKI model implies pre-built initrds. How to generate these (and securely extend and parameterize them) is outside of the scope of this document, but a related document will be provided highlighting these concepts.

Protection Coverage of SecureBoot Signing and PCRs

The scheme discussed here touches both SecureBoot code signing and TPM PCR measurements. These two distinct mechanisms cover separate parts of the boot process.

Specifically:

  • Firmware/Shim SecureBoot signing covers bootloader and UKI

  • TPM PCR 11 covers the UKI components and boot phase

  • TPM PCR 12 covers admin configuration

  • TPM PCR 15 covers the local identity of the host

Note that this means SecureBoot coverage ends once the system transitions from the initrd into the root file system. It is assumed that trust and integrity have been established before this transition by some means, for example LUKS/dm-crypt/dm-integrity, ideally bound to PCR 11 (i.e. UKI and boot phase).

A robust and secure update scheme for PCR 11 (i.e. UKI) has been described above, which allows binding TPM-locked resources to a UKI. For PCR 12 no such scheme is currently designed, but might be added later (use case: permit access to certain secrets only if the system runs with configuration signed by a specific set of keys). Given that resources measured into PCR 15 typically aren’t updated (or if they are updated loss of access to other resources linked to them is desired) no update scheme should be necessary for it.

This document focuses on the three PCRs discussed above. Disk encryption and other userspace may choose to also bind to other PCRs. However, doing so means the PCR brittleness issue returns that this design is supposed to remove. PCRs defined by the various firmware UEFI/TPM specifications generally do not know any concept for signatures of expected PCR values.

It is known that the industry-adopted SecureBoot signing keys are too broad to act as more than a denylist for known bad code. It is thus probably a good idea to enroll vendor SecureBoot keys wherever possible (e.g. in environments where the hardware is very well known, and VM environments), to raise the bar on preparing rogue UKI-like PE binaries that will result in PCR values that match expectations but actually contain bad code. Discussion about that is however outside of the scope of this document.

Whole OS embedded in the UKI

The above is written under the assumption that the UKI embeds an initrd whose job it is to set up the root file system: find it, validate it, cryptographically unlock it and similar. Once the root file system is found, the system transitions into it.

While this is the traditional design and likely what most systems will use, it is also possible to embed a regular root file system into the UKI and avoid any transition to an on-disk root file system. In this mode the whole OS would be encapsulated in the UKI, and signed/measured as one. In such a scenario the whole of the OS must be loaded into RAM and remain there, which typically restricts the general usability of such an approach. However, for specific purposes this might be the design of choice, for example to implement self-sufficient recovery or provisioning systems.

Proposed Implementations & Current Status

The toolset for most of the above is already implemented in systemd and related projects in one way or another. Specifically:

  1. The systemd-stub (or short: sd-stub) component implements the discussed UEFI stub program

  2. The systemd-measure tool can be used to pre-calculate expected PCR 11 values given the UKI components and can sign the result, as discussed in the UKI Image Generation section above.

  3. The systemd-cryptenroll and systemd-cryptsetup tools can be used to bind a LUKS2 encrypted file system volume to a TPM and PCR 11 public key/signatures, according to the scheme described above. (The two components also implement a “recovery key” concept, as discussed above)

  4. The systemd-pcrphase component measures specific words into PCR 11 at the discussed phases of the boot process.

  5. The systemd-creds tool may be used to encrypt/decrypt data objects called “credentials” that can be passed into services and booted systems, and are automatically decrypted (if needed) immediately before service invocation. Encryption is typically bound to the local TPM, to ensure the data cannot be recovered elsewhere.

Note that systemd-stub (i.e. the UEFI code glued into the UKI) is distinct from systemd-boot (i.e. the UEFI boot loader than can manage multiple UKIs and other boot menu items and implements automatic fallback, an interactive menu and a programmatic interface for the OS among other things). One can be used without the other – both sd-stub without sd-boot and vice versa – though they integrate nicely if used in combination.

Note that the mechanisms described are relatively generic, and can be implemented and be consumed in other software too, systemd should be considered a reference implementation, though one that found comprehensive adoption across Linux distributions.

Some concepts discussed above are currently not implemented. Specifically:

  1. The rollback protection logic is currently not implemented.

  2. The mentioned measurement of the root file system volume key to PCR 15 is implemented, but not merged into the systemd main branch yet.

The UAPI Group

We recently started a new group for discussing concepts and specifications of basic OS components, including UKIs as described above. It's called the UAPI Group. Please have a look at the various documents and specifications already available there, and expect more to come. Contributions welcome!

Glossary

TPM

Trusted Platform Module; a security chip found in many modern systems, both physical systems and increasingly also in virtualized environments. Traditionally a discrete chip on the mainboard but today often implemented in firmware, and lately directly in the CPU SoC.

PCR

Platform Configuration Register; a set of registers on a TPM that are initialized to zero at boot. The firmware and OS can “extend” these registers with hashes of data used during the boot process and afterwards. “Extension” means the supplied data is first cryptographically hashed. The resulting hash value is then combined with the previous value of the PCR and the combination hashed again. The result will become the new value of the PCR. By doing this iteratively for all parts of the boot process (always with the data that will be used next during the boot process) a concept of “Measured Boot” can be implemented: as long as every element in the boot chain measures (i.e. extends into the PCR) the next part of the boot like this, the resulting PCR values will prove cryptographically that only a certain set of boot components can have been used to boot up. A standards compliant TPM usually has 24 PCRs, but more than half of those are already assigned specific meanings by the firmware. Some of the others may be used by the OS, of which we use four in the concepts discussed in this document.

Measurement

The act of “extending” a PCR with some data object.

SRK

Storage Root Key; a special cryptographic key generated by a TPM that never leaves the TPM, and can be used to encrypt/decrypt data passed to the TPM.

UKI

Unified Kernel Image; the concept this document is about. A combination of kernel, initrd and other resources. See above.

SecureBoot

A mechanism where every software component involved in the boot process is cryptographically signed and checked against a set of public keys stored in the mainboard hardware, implemented in firmware, before it is used.

Measured Boot

A boot process where each component measures (i.e., hashes and extends into a TPM PCR, see above) the next component it will pass control to before doing so. This serves two purposes: it can be used to bind security policy for encrypted secrets to the resulting PCR values (or signatures thereof, see above), and it can be used to reason about used software after the fact, for example for the purpose of remote attestation.

initrd

Short for “initial RAM disk”, which – strictly speaking – is a misnomer today, because no RAM disk is anymore involved, but a tmpfs file system instance. Also known as “initramfs”, which is also misleading, given the file system is not ramfs anymore, but tmpfs (both of which are in-memory file systems on Linux, with different semantics). The initrd is passed to the Linux kernel and is basically a file system tree in cpio archive. The kernel unpacks the image into a tmpfs (i.e., into an in-memory file system), and then executes a binary from it. It thus contains the binaries for the first userspace code the kernel invokes. Typically, the initrd’s job is to find the actual root file system, unlock it (if encrypted), and transition into it.

UEFI

Short for “Unified Extensible Firmware Interface”, it is a widely adopted standard for PC firmware, with native support for SecureBoot and Measured Boot.

EFI

More or less synonymous to UEFI, IRL.

Shim

A boot component originating in the Linux world, which in a way extends the public key database SecureBoot maintains (which is under control from Microsoft) with a second layer (which is under control of the Linux distributions and of the owner of the physical device).

PE

Portable Executable; a file format for executable binaries, originally from the Windows world, but also used by UEFI firmware. PE files may contain code and data, categorized in labeled “sections”

ESP

EFI System Partition; a special partition on a storage medium that the firmware is able to look for UEFI PE binaries in to execute at boot.

HSM

Hardware Security Module; a piece of hardware that can generate and store secret cryptographic keys, and execute operations with them, without the keys leaving the hardware (though this is configurable). TPMs can act as HSMs.

DEK

Disk Encryption Key; an asymmetric cryptographic key used for unlocking disk encryption, i.e. passed to LUKS/dm-crypt for activating an encrypted storage volume.

LUKS2

Linux Unified Key Setup Version 2; a specification for a superblock for encrypted volumes widely used on Linux. LUKS2 is the default on-disk format for the cryptsetup suite of tools. It provides flexible key management with multiple independent key slots and allows embedding arbitrary metadata in a JSON format in the superblock.

Thanks

I’d like to thank Alain Gefflaut, Anna Trikalinou, Christian Brauner, Daan de Meyer, Luca Boccassi, Zbigniew Jędrzejewski-Szmek for reviewing this text.


Fitting Everything Together

TLDR: Hermetic /usr/ is awesome; let's popularize image-based OSes with modernized security properties built around immutability, SecureBoot, TPM2, adaptability, auto-updating, factory reset, uniformity – built from traditional distribution packages, but deployed via images.

Over the past years, systemd gained a number of components for building Linux-based operating systems. While these components individually have been adopted by many distributions and products for specific purposes, we did not publicly communicate a broader vision of how they should all fit together in the long run. In this blog story I hope to provide that from my personal perspective, i.e. explain how I personally would build an OS and where I personally think OS development with Linux should go.

I figure this is going to be a longer blog story, but I hope it will be equally enlightening. Please understand though that everything I write about OS design here is my personal opinion, and not one of my employer.

For the last 12 years or so I have been working on Linux OS development, mostly around systemd. In all those years I had a lot of time thinking about the Linux platform, and specifically traditional Linux distributions and their strengths and weaknesses. I have seen many attempts to reinvent Linux distributions in one way or another, to varying success. After all this most would probably agree that the traditional RPM or dpkg/apt-based distributions still define the Linux platform more than others (for 25+ years now), even though some Linux-based OSes (Android, ChromeOS) probably outnumber the installations overall.

And over all those 12 years I kept wondering, how would I actually build an OS for a system or for an appliance, and what are the components necessary to achieve that. And most importantly, how can we make these components generic enough so that they are useful in generic/traditional distributions too, and in other use cases than my own.

The Project

Before figuring out how I would build an OS it's probably good to figure out what type of OS I actually want to build, what purpose I intend to cover. I think a desktop OS is probably the most interesting. Why is that? Well, first of all, I use one of these for my job every single day, so I care immediately, it's my primary tool of work. But more importantly: I think building a desktop OS is one of the most complex overall OS projects you can work on, simply because desktops are so much more versatile and variable than servers or embedded devices. If one figures out the desktop case, I think there's a lot more to learn from, and reuse in the server or embedded case, then going the other way. After all, there's a reason why so much of the widely accepted Linux userspace stack comes from people with a desktop background (including systemd, BTW).

So, let's see how I would build a desktop OS. If you press me hard, and ask me why I would do that given that ChromeOS already exists and more or less is a Linux desktop OS: there's plenty I am missing in ChromeOS, but most importantly, I am lot more interested in building something people can easily and naturally rebuild and hack on, i.e. Google-style over-the-wall open source with its skewed power dynamic is not particularly attractive to me. I much prefer building this within the framework of a proper open source community, out in the open, and basing all this strongly on the status quo ante, i.e. the existing distributions. I think it is crucial to provide a clear avenue to build a modern OS based on the existing distribution model, if there shall ever be a chance to make this interesting for a larger audience.

(Let me underline though: even though I am going to focus on a desktop here, most of this is directly relevant for servers as well, in particular container host OSes and suchlike, or embedded devices, e.g. car IVI systems and so on.)

Design Goals

  1. First and foremost, I think the focus must be on an image-based design rather than a package-based one. For robustness and security it is essential to operate with reproducible, immutable images that describe the OS or large parts of it in full, rather than operating always with fine-grained RPM/dpkg style packages. That's not to say that packages are not relevant (I actually think they matter a lot!), but I think they should be less of a tool for deploying code but more one of building the objects to deploy. A different way to see this: any OS built like this must be easy to replicate in a large number of instances, with minimal variability. Regardless if we talk about desktops, servers or embedded devices: focus for my OS should be on "cattle", not "pets", i.e that from the start it's trivial to reuse the well-tested, cryptographically signed combination of software over a large set of devices the same way, with a maximum of bit-exact reuse and a minimum of local variances.

  2. The trust chain matters, from the boot loader all the way to the apps. This means all code that is run must be cryptographically validated before it is run. All storage must be cryptographically protected: public data must be integrity checked; private data must remain confidential.

    This is in fact where big distributions currently fail pretty badly. I would go as far as saying that SecureBoot on Linux distributions is mostly security theater at this point, if you so will. That's because the initrd that unlocks your FDE (i.e. the cryptographic concept that protects the rest of your system) is not signed or protected in any way. It's trivial to modify for an attacker with access to your hard disk in an undetectable way, and collect your FDE passphrase. The involved bureaucracy around the implementation of UEFI SecureBoot of the big distributions is to a large degree pointless if you ask me, given that once the kernel is assumed to be in a good state, as the next step the system invokes completely unsafe code with full privileges.

    This is a fault of current Linux distributions though, not of SecureBoot in general. Other OSes use this functionality in more useful ways, and we should correct that too.

  3. Pretty much the same thing: offline security matters. I want my data to be reasonably safe at rest, i.e. cryptographically inaccessible even when I leave my laptop in my hotel room, suspended.

  4. Everything should be cryptographically measured, so that remote attestation is supported for as much software shipped on the OS as possible.

  5. Everything should be self descriptive, have single sources of truths that are closely attached to the object itself, instead of stored externally.

  6. Everything should be self-updating. Today we know that software is never bug-free, and thus requires a continuous update cycle. Not only the OS itself, but also any extensions, services and apps running on it.

  7. Everything should be robust in respect to aborted OS operations, power loss and so on. It should be robust towards hosed OS updates (regardless if the download process failed, or the image was buggy), and not require user interaction to recover from them.

  8. There must always be a way to put the system back into a well-defined, guaranteed safe state ("factory reset"). This includes that all sensitive data from earlier uses becomes cryptographically inaccessible.

  9. The OS should enforce clear separation between vendor resources, system resources and user resources: conceptually and when it comes to cryptographical protection.

  10. Things should be adaptive: the system should come up and make the best of the system it runs on, adapt to the storage and hardware. Moreover, the system should support execution on bare metal equally well as execution in a VM environment and in a container environment (i.e. systemd-nspawn).

  11. Things should not require explicit installation. i.e. every image should be a live image. For installation it should be sufficient to dd an OS image onto disk. Thus, strong focus on "instantiate on first boot", rather than "instantiate before first boot".

  12. Things should be reasonably minimal. The image the system starts its life with should be quick to download, and not include resources that can as well be created locally later.

  13. System identity, local cryptographic keys and so on should be generated locally, not be pre-provisioned, so that there's no leak of sensitive data during the transport onto the system possible.

  14. Things should be reasonably democratic and hackable. It should be easy to fork an OS, to modify an OS and still get reasonable cryptographic protection. Modifying your OS should not necessarily imply that your "warranty is voided" and you lose all good properties of the OS, if you so will.

  15. Things should be reasonably modular. The privileged part of the core OS must be extensible, including on the individual system. It's not sufficient to support extensibility just through high-level UI applications.

  16. Things should be reasonably uniform, i.e. ideally the same formats and cryptographic properties are used for all components of the system, regardless if for the host OS itself or the payloads it receives and runs.

  17. Even taking all these goals into consideration, it should still be close to traditional Linux distributions, and take advantage of what they are really good at: integration and security update cycles.

Now that we know our goals and requirements, let's start designing the OS along these lines.

Hermetic /usr/

First of all the OS resources (code, data files, …) should be hermetic in an immutable /usr/. This means that a /usr/ tree should carry everything needed to set up the minimal set of directories and files outside of /usr/ to make the system work. This /usr/ tree can then be mounted read-only into the writable root file system that then will eventually carry the local configuration, state and user data in /etc/, /var/ and /home/ as usual.

Thankfully, modern distributions are surprisingly close to working without issues in such a hermetic context. Specifically, Fedora works mostly just fine: it has adopted the /usr/ merge and the declarative systemd-sysusers and systemd-tmpfiles components quite comprehensively, which means the directory trees outside of /usr/ are automatically generated as needed if missing. In particular /etc/passwd and /etc/group (and related files) are appropriately populated, should they be missing entries.

In my model a hermetic OS is hence comprehensively defined within /usr/: combine the /usr/ tree with an empty, otherwise unpopulated root file system, and it will boot up successfully, automatically adding the strictly necessary files, and resources that are necessary to boot up.

Monopolizing vendor OS resources and definitions in an immutable /usr/ opens multiple doors to us:

  • We can apply dm-verity to the whole /usr/ tree, i.e. guarantee structural, cryptographic integrity on the whole vendor OS resources at once, with full file system metadata.

  • We can implement updates to the OS easily: by implementing an A/B update scheme on the /usr/ tree we can update the OS resources atomically and robustly, while leaving the rest of the OS environment untouched.

  • We can implement factory reset easily: erase the root file system and reboot. The hermetic OS in /usr/ has all the information it needs to set up the root file system afresh — exactly like in a new installation.

Initial Look at the Partition Table

So let's have a look at a suitable partition table, taking a hermetic /usr/ into account. Let's conceptually start with a table of four entries:

  1. An UEFI System Partition (required by firmware to boot)

  2. Immutable, Verity-protected, signed file system with the /usr/ tree in version A

  3. Immutable, Verity-protected, signed file system with the /usr/ tree in version B

  4. A writable, encrypted root file system

(This is just for initial illustration here, as we'll see later it's going to be a bit more complex in the end.)

The Discoverable Partitions Specification provides suitable partition types UUIDs for all of the above partitions. Which is great, because it makes the image self-descriptive: simply by looking at the image's GPT table we know what to mount where. This means we do not need a manual /etc/fstab, and a multitude of tools such as systemd-nspawn and similar can operate directly on the disk image and boot it up.

Booting

Now that we have a rough idea how to organize the partition table, let's look a bit at how to boot into that. Specifically, in my model "unified kernels" are the way to go, specifically those implementing Boot Loader Specification Type #2. These are basically kernel images that have an initial RAM disk attached to them, as well as a kernel command line, a boot splash image and possibly more, all wrapped into a single UEFI PE binary. By combining these into one we achieve two goals: they become extremely easy to update (i.e. drop in one file, and you update kernel+initrd) and more importantly, you can sign them as one for the purpose of UEFI SecureBoot.

In my model, each version of such a kernel would be associated with exactly one version of the /usr/ tree: both are always updated at the same time. An update then becomes relatively simple: drop in one new /usr/ file system plus one kernel, and the update is complete.

The boot loader used for all this would be systemd-boot, of course. It's a very simple loader, and implements the aforementioned boot loader specification. This means it requires no explicit configuration or anything: it's entirely sufficient to drop in one such unified kernel file, and it will be picked up, and be made a candidate to boot into.

You might wonder how to configure the root file system to boot from with such a unified kernel that contains the kernel command line and is signed as a whole and thus immutable. The idea here is to use the usrhash= kernel command line option implemented by systemd-veritysetup-generator and systemd-fstab-generator. It does two things: it will search and set up a dm-verity volume for the /usr/ file system, and then mount it. It takes the root hash value of the dm-verity Merkle tree as the parameter. This hash is then also used to find the /usr/ partition in the GPT partition table, under the assumption that the partition UUIDs are derived from it, as per the suggestions in the discoverable partitions specification (see above).

systemd-boot (if not told otherwise) will do a version sort of the kernel image files it finds, and then automatically boot the newest one. Picking a specific kernel to boot will also fixate which version of the /usr/ tree to boot into, because — as mentioned — the Verity root hash of it is built into the kernel command line the unified kernel image contains.

In my model I'd place the kernels directly into the UEFI System Partition (ESP), in order to simplify things. (systemd-boot also supports reading them from a separate boot partition, but let's not complicate things needlessly, at least for now.)

So, with all this, we now already have a boot chain that goes something like this: once the boot loader is run, it will pick the newest kernel, which includes the initial RAM disk and a secure reference to the /usr/ file system to use. This is already great. But a /usr/ alone won't make us happy, we also need a root file system. In my model, that file system would be writable, and the /etc/ and /var/ hierarchies would be located directly on it. Since these trees potentially contain secrets (SSH keys, …) the root file system needs to be encrypted. We'll use LUKS2 for this, of course. In my model, I'd bind this to the TPM2 chip (for compatibility with systems lacking one, we can find a suitable fallback, which then provides weaker guarantees, see below). A TPM2 is a security chip available in most modern PCs. Among other things it contains a persistent secret key that can be used to encrypt data, in a way that only if you possess access to it and can prove you are using validated software you can decrypt it again. The cryptographic measuring I mentioned earlier is what allows this to work. But … let's not get lost too much in the details of TPM2 devices, that'd be material for a novel, and this blog story is going to be way too long already.

What does using a TPM2 bound key for unlocking the root file system get us? We can encrypt the root file system with it, and you can only read or make changes to the root file system if you also possess the TPM2 chip and run our validated version of the OS. This protects us against an evil maid scenario to some level: an attacker cannot just copy the hard disk of your laptop while you leave it in your hotel room, because unless the attacker also steals the TPM2 device it cannot be decrypted. The attacker can also not just modify the root file system, because such changes would be detected on next boot because they aren't done with the right cryptographic key.

So, now we have a system that already can boot up somewhat completely, and run userspace services. All code that is run is verified in some way: the /usr/ file system is Verity protected, and the root hash of it is included in the kernel that is signed via UEFI SecureBoot. And the root file system is locked to the TPM2 where the secret key is only accessible if our signed OS + /usr/ tree is used.

(One brief intermission here: so far all the components I am referencing here exist already, and have been shipped in systemd and other projects already, including the TPM2 based disk encryption. There's one thing missing here however at the moment that still needs to be developed (happy to take PRs!): right now TPM2 based LUKS2 unlocking is bound to PCR hash values. This is hard to work with when implementing updates — what we'd need instead is unlocking by signatures of PCR hashes. TPM2 supports this, but we don't support it yet in our systemd-cryptsetup + systemd-cryptenroll stack.)

One of the goals mentioned above is that cryptographic key material should always be generated locally on first boot, rather than pre-provisioned. This of course has implications for the encryption key of the root file system: if we want to boot into this system we need the root file system to exist, and thus a key already generated that it is encrypted with. But where precisely would we generate it if we have no installer which could generate while installing (as it is done in traditional Linux distribution installers). My proposed solution here is to use systemd-repart, which is a declarative, purely additive repartitioner. It can run from the initrd to create and format partitions on boot, before transitioning into the root file system. It can also format the partitions it creates and encrypt them, automatically enrolling an TPM2-bound key.

So, let's revisit the partition table we mentioned earlier. Here's what in my model we'd actually ship in the initial image:

  1. An UEFI System Partition (ESP)

  2. An immutable, Verity-protected, signed file system with the /usr/ tree in version A

And that's already it. No root file system, no B /usr/ partition, nothing else. Only two partitions are shipped: the ESP with the systemd-boot loader and one unified kernel image, and the A version of the /usr/ partition. Then, on first boot systemd-repart will notice that the root file system doesn't exist yet, and will create it, encrypt it, and format it, and enroll the key into the TPM2. It will also create the second /usr/ partition (B) that we'll need for later A/B updates (which will be created empty for now, until the first update operation actually takes place, see below). Once done the initrd will combine the fresh root file system with the shipped /usr/ tree, and transition into it. Because the OS is hermetic in /usr/ and contains all the systemd-tmpfiles and systemd-sysuser information it can then set up the root file system properly and create any directories and symlinks (and maybe a few files) necessary to operate.

Besides the fact that the root file system's encryption keys are generated on the system we boot from and never leave it, it is also pretty nice that the root file system will be sized dynamically, taking into account the physical size of the backing storage. This is perfect, because on first boot the image will automatically adapt to what it has been dd'ed onto.

Factory Reset

This is a good point to talk about the factory reset logic, i.e. the mechanism to place the system back into a known good state. This is important for two reasons: in our laptop use case, once you want to pass the laptop to someone else, you want to ensure your data is fully and comprehensively erased. Moreover, if you have reason to believe your device was hacked you want to revert the device to a known good state, i.e. ensure that exploits cannot persist. systemd-repart already has a mechanism for it. In the declarations of the partitions the system should have, entries may be marked to be candidates for erasing on factory reset. The actual factory reset is then requested by one of two means: by specifying a specific kernel command line option (which is not too interesting here, given we lock that down via UEFI SecureBoot; but then again, one could also add a second kernel to the ESP that is identical to the first, with only different that it lists this command line option: thus when the user selects this entry it will initiate a factory reset) — and via an EFI variable that can be set and is honoured on the immediately following boot. So here's how a factory reset would then go down: once the factory reset is requested it's enough to reboot. On the subsequent boot systemd-repart runs from the initrd, where it will honour the request and erase the partitions marked for erasing. Once that is complete the system is back in the state we shipped the system in: only the ESP and the /usr/ file system will exist, but the root file system is gone. And from here we can continue as on the original first boot: create a new root file system (and any other partitions), and encrypt/set it up afresh.

So now we have a nice setup, where everything is either signed or encrypted securely. The system can adapt to the system it is booted on automatically on first boot, and can easily be brought back into a well defined state identical to the way it was shipped in.

Modularity

But of course, such a monolithic, immutable system is only useful for very specific purposes. If /usr/ can't be written to, – at least in the traditional sense – one cannot just go and install a new software package that one needs. So here two goals are superficially conflicting: on one hand one wants modularity, i.e. the ability to add components to the system, and on the other immutability, i.e. that precisely this is prohibited.

So let's see what I propose as a middle ground in my model. First, what's the precise use case for such modularity? I see a couple of different ones:

  1. For some cases it is necessary to extend the system itself at the lowest level, so that the components added in extend (or maybe even replace) the resources shipped in the base OS image, so that they live in the same namespace, and are subject to the same security restrictions and privileges. Exposure to the details of the base OS and its interface for this kind of modularity is at the maximum.

    Example: a module that adds a debugger or tracing tools into the system. Or maybe an optional hardware driver module.

  2. In other cases, more isolation is preferable: instead of extending the system resources directly, additional services shall be added in that bring their own files, can live in their own namespace (but with "windows" into the host namespaces), however still are system components, and provide services to other programs, whether local or remote. Exposure to the details of the base OS for this kind of modularity is restricted: it mostly focuses on the ability to consume and provide IPC APIs from/to the system. Components of this type can still be highly privileged, but the level of integration is substantially smaller than for the type explained above.

    Example: a module that adds a specific VPN connection service to the OS.

  3. Finally, there's the actual payload of the OS. This stuff is relatively isolated from the OS and definitely from each other. It mostly consumes OS APIs, and generally doesn't provide OS APIs. This kind of stuff runs with minimal privileges, and in its own namespace of concepts.

    Example: a desktop app, for reading your emails.

Of course, the lines between these three types of modules are blurry, but I think distinguishing them does make sense, as I think different mechanisms are appropriate for each. So here's what I'd propose in my model to use for this.

  1. For the system extension case I think the systemd-sysext images are appropriate. This tool operates on system extension images that are very similar to the host's disk image: they also contain a /usr/ partition, protected by Verity. However, they just include additions to the host image: binaries that extend the host. When such a system extension image is activated, it is merged via an immutable overlayfs mount into the host's /usr/ tree. Thus any file shipped in such a system extension will suddenly appear as if it was part of the host OS itself. For optional components that should be considered part of the OS more or less this is a very simple and powerful way to combine an immutable OS with an immutable extension. Note that most likely extensions for an OS matching this tool should be built at the same time within the same update cycle scheme as the host OS itself. After all, the files included in the extensions will have dependencies on files in the system OS image, and care must be taken that these dependencies remain in order.

  2. For adding in additional somewhat isolated system services in my model, Portable Services are the proposed tool of choice. Portable services are in most ways just like regular system services; they could be included in the system OS image or an extension image. However, portable services use RootImage= to run off separate disk images, thus within their own namespace. Images set up this way have various ways to integrate into the host OS, as they are in most ways regular system services, which just happen to bring their own directory tree. Also, unlike regular system services, for them sandboxing is opt-out rather than opt-in. In my model, here too the disk images are Verity protected and thus immutable. Just like the host OS they are GPT disk images that come with a /usr/ partition and Verity data, along with signing.

  3. Finally, the actual payload of the OS, i.e. the apps. To be useful in real life here it is important to hook into existing ecosystems, so that a large set of apps are available. Given that on Linux flatpak (or on servers OCI containers) are the established format that pretty much won they are probably the way to go. That said, I think both of these mechanisms have relatively weak properties, in particular when it comes to security, since immutability/measurements and similar are not provided. This means, unlike for system extensions and portable services a complete trust chain with attestation and per-app cryptographically protected data is much harder to implement sanely.

What I'd like to underline here is that the main system OS image, as well as the system extension images and the portable service images are put together the same way: they are GPT disk images, with one immutable file system and associated Verity data. The latter two should also contain a PKCS#7 signature for the top-level Verity hash. This uniformity has many benefits: you can use the same tools to build and process these images, but most importantly: by using a single way to validate them throughout the stack (i.e. Verity, in the latter cases with PKCS#7 signatures), validation and measurement is straightforward. In fact it's so obvious that we don't even have to implement it in systemd: the kernel has direct support for this Verity signature checking natively already (IMA).

So, by composing a system at runtime from a host image, extension images and portable service images we have a nicely modular system where every single component is cryptographically validated on every single IO operation, and every component is measured, in its entire combination, directly in the kernel's IMA subsystem.

(Of course, once you add the desktop apps or OCI containers on top, then these properties are lost further down the chain. But well, a lot is already won, if you can close the chain that far down.)

Note that system extensions are not designed to replicate the fine grained packaging logic of RPM/dpkg. Of course, systemd-sysext is a generic tool, so you can use it for whatever you want, but there's a reason it does not bring support for a dependency language: the goal here is not to replicate traditional Linux packaging (we have that already, in RPM/dpkg, and I think they are actually OK for what they do) but to provide delivery of larger, coarser sets of functionality, in lockstep with the underlying OS' life-cycle and in particular with no interdependencies, except on the underlying OS.

Also note that depending on the use case it might make sense to also use system extensions to modularize the initrd step. This is probably less relevant for a desktop OS, but for server systems it might make sense to package up support for specific complex storage in a systemd-sysext system extension, which can be applied to the initrd that is built into the unified kernel. (In fact, we have been working on implementing signed yet modular initrd support to general purpose Fedora this way.)

Note that portable services are composable from system extension too, by the way. This makes them even more useful, as you can share a common runtime between multiple portable service, or even use the host image as common runtime for portable services. In this model a common runtime image is shared between one or more system extensions, and composed at runtime via an overlayfs instance.

More Modularity: Secondary OS Installs

Having an immutable, cryptographically locked down host OS is great I think, and if we have some moderate modularity on top, that's also great. But oftentimes it's useful to be able to depart/compromise for some specific use cases from that, i.e. provide a bridge for example to allow workloads designed around RPM/dpkg package management to coexist reasonably nicely with such an immutable host.

For this purpose in my model I'd propose using systemd-nspawn containers. The containers are focused on OS containerization, i.e. they allow you to run a full OS with init system and everything as payload (unlike for example Docker containers which focus on a single service, and where running a full OS in it is a mess).

Running systemd-nspawn containers for such secondary OS installs has various nice properties. One of course is that systemd-nspawn supports the same level of cryptographic image validation that we rely on for the host itself. Thus, to some level the whole OS trust chain is reasonably recursive if desired: the firmware validates the OS, and the OS can validate a secondary OS installed within it. In fact, we can run our trusted OS recursively on itself and get similar security guarantees! Besides these security aspects, systemd-nspawn also has really nice properties when it comes to integration with the host. For example the --bind-user= permits binding a host user record and their directory into a container as a simple one step operation. This makes it extremely easy to have a single user and $HOME but share it concurrently with the host and a zoo of secondary OSes in systemd-nspawn containers, which each could run different distributions even.

Developer Mode

Superficially, an OS with an immutable /usr/ appears much less hackable than an OS where everything is writable. Moreover, an OS where everything must be signed and cryptographically validated makes it hard to insert your own code, given you are unlikely to possess access to the signing keys.

To address this issue other systems have supported a "developer" mode: when entered the security guarantees are disabled, and the system can be freely modified, without cryptographic validation. While that's a great concept to have I doubt it's what most developers really want: the cryptographic properties of the OS are great after all, it sucks having to give them up once developer mode is activated.

In my model I'd thus propose two different approaches to this problem. First of all, I think there's value in allowing users to additively extend/override the OS via local developer system extensions. With this scheme the underlying cryptographic validation would remain in tact, but — if this form of development mode is explicitly enabled – the developer could add in more resources from local storage, that are not tied to the OS builder's chain of trust, but a local one (i.e. simply backed by encrypted storage of some form).

The second approach is to make it easy to extend (or in fact replace) the set of trusted validation keys, with local ones that are under the control of the user, in order to make it easy to operate with kernel, OS, extension, portable service or container images signed by the local developer without involvement of the OS builder. This is relatively easy to do for components down the trust chain, i.e. the elements further up the chain should optionally allow additional certificates to allow validation with.

(Note that systemd currently has no explicit support for a "developer" mode like this. I think we should add that sooner or later however.)

Democratizing Code Signing

Closely related to the question of developer mode is the question of code signing. If you ask me, the status quo of UEFI SecureBoot code signing in the major Linux distributions is pretty sad. The work to get stuff signed is massive, but in effect it delivers very little in return: because initrds are entirely unprotected, and reside on partitions lacking any form of cryptographic integrity protection any attacker can trivially easily modify the boot process of any such Linux system and freely collected FDE passphrases entered. There's little value in signing the boot loader and kernel in a complex bureaucracy if it then happily loads entirely unprotected code that processes the actually relevant security credentials: the FDE keys.

In my model, through use of unified kernels this important gap is closed, hence UEFI SecureBoot code signing becomes an integral part of the boot chain from firmware to the host OS. Unfortunately, code signing – and having something a user can locally hack, is to some level conflicting. However, I think we can improve the situation here, and put more emphasis on enrolling developer keys in the trust chain easily. Specifically, I see one relevant approach here: enrolling keys directly in the firmware is something that we should make less of a theoretical exercise and more something we can realistically deploy. See this work in progress making this more automatic and eventually safe. Other approaches are thinkable (including some that build on existing MokManager infrastructure), but given the politics involved, are harder to conclusively implement.

Running the OS itself in a container

What I explain above is put together with running on a bare metal system in mind. However, one of the stated goals is to make the OS adaptive enough to also run in a container environment (specifically: systemd-nspawn) nicely. Booting a disk image on bare metal or in a VM generally means that the UEFI firmware validates and invokes the boot loader, and the boot loader invokes the kernel which then transitions into the final system. This is different for containers: here the container manager immediately calls the init system, i.e. PID 1. Thus the validation logic must be different: cryptographic validation must be done by the container manager. In my model this is solved by shipping the OS image not only with a Verity data partition (as is already necessary for the UEFI SecureBoot trust chain, see above), but also with another partition, containing a PKCS#7 signature of the root hash of said Verity partition. This of course is exactly what I propose for both the system extension and portable service image. Thus, in my model the images for all three uses are put together the same way: an immutable /usr/ partition, accompanied by a Verity partition and a PKCS#7 signature partition. The OS image itself then has two ways "into" the trust chain: either through the signed unified kernel in the ESP (which is used for bare metal and VM boots) or by using the PKCS#7 signature stored in the partition (which is used for container/systemd-nspawn boots).

Parameterizing Kernels

A fully immutable and signed OS has to establish trust in the user data it makes use of before doing so. In the model I describe here, for /etc/ and /var/ we do this via disk encryption of the root file system (in combination with integrity checking). But the point where the root file system is mounted comes relatively late in the boot process, and thus cannot be used to parameterize the boot itself. In many cases it's important to be able to parameterize the boot process however.

For example, for the implementation of the developer mode indicated above it's useful to be able to pass this fact safely to the initrd, in combination with other fields (e.g. hashed root password for allowing in-initrd logins for debug purposes). After all, if the initrd is pre-built by the vendor and signed as whole together with the kernel it cannot be modified to carry such data directly (which is in fact how parameterizing of the initrd to a large degree was traditionally done).

In my model this is achieved through system credentials, which allow passing parameters to systems (and services for the matter) in an encrypted and authenticated fashion, bound to the TPM2 chip. This means that we can securely pass data into the initrd so that it can be authenticated and decrypted only on the system it is intended for and with the unified kernel image it was intended for.

Swap

In my model the OS would also carry a swap partition. For the simple reason that only then systemd-oomd.service can provide the best results. Also see In defence of swap: common misconceptions

Updating Images

We have a rough idea how the system shall be organized now, let's next focus on the deployment cycle: software needs regular update cycles, and software that is not updated regularly is a security problem. Thus, I am sure that any modern system must be automatically updated, without this requiring avoidable user interaction.

In my model, this is the job for systemd-sysupdate. It's a relatively simple A/B image updater: it operates either on partitions, on regular files in a directory, or on subdirectories in a directory. Each entry has a version (which is encoded in the GPT partition label for partitions, and in the filename for regular files and directories): whenever an update is initiated the oldest version is erased, and the newest version is downloaded.

With the setup described above a system update becomes a really simple operation. On each update the systemd-sysupdate tool downloads a /usr/ file system partition, an accompanying Verity partition, a PKCS#7 signature partition, and drops it into the host's partition table (where it possibly replaces the oldest version so far stored there). Then it downloads a unified kernel image and drops it into the EFI System Partition's /EFI/Linux (as per Boot Loader Specification; possibly erase the oldest such file there). And that's already the whole update process: four files are downloaded from the server, unpacked and put in the most straightforward of ways into the partition table or file system. Unlike in other OS designs there's no mechanism required to explicitly switch to the newer version, the aforementioned systemd-boot logic will automatically pick the newest kernel once it is dropped in.

Above we talked a lot about modularity, and how to put systems together as a combination of a host OS image, system extension images for the initrd and the host, portable service images and systemd-nspawn container images. I already emphasized that these image files are actually always the same: GPT disk images with partition definitions that match the Discoverable Partition Specification. This comes very handy when thinking about updating: we can use the exact same systemd-sysupdate tool for updating these other images as we use for the host image. The uniformity of the on-disk format allows us to update them uniformly too.

Boot Counting + Assessment

Automatic OS updates do not come without risks: if they happen automatically, and an update goes wrong this might mean your system might be automatically updated into a brick. This of course is less than ideal. Hence it is essential to address this reasonably automatically. In my model, there's systemd's Automatic Boot Assessment for that. The mechanism is simple: whenever a new unified kernel image is dropped into the system it will be stored with a small integer counter value included in the filename. Whenever the unified kernel image is selected for booting by systemd-boot, it is decreased by one. Once the system booted up successfully (which is determined by userspace) the counter is removed from the file name (which indicates "this entry is known to work"). If the counter ever hits zero, this indicates that it tried to boot it a couple of times, and each time failed, thus is apparently "bad". In this case systemd-boot will not consider the kernel anymore, and revert to the next older (that doesn't have a counter of zero).

By sticking the boot counter into the filename of the unified kernel we can directly attach this information to the kernel, and thus need not concern ourselves with cleaning up secondary information about the kernel when the kernel is removed. Updating with a tool like systemd-sysupdate remains a very simple operation hence: drop one old file, add one new file.

Picking the Newest Version

I already mentioned that systemd-boot automatically picks the newest unified kernel image to boot, by looking at the version encoded in the filename. This is done via a simple strverscmp() call (well, truth be told, it's a modified version of that call, different from the one implemented in libc, because real-life package managers use more complex rules for comparing versions these days, and hence it made sense to do that here too). The concept of having multiple entries of some resource in a directory, and picking the newest one automatically is a powerful concept, I think. It means adding/removing new versions is extremely easy (as we discussed above, in systemd-sysupdate context), and allows stateless determination of what to use.

If systemd-boot can do that, what about system extension images, portable service images, or systemd-nspawn container images that do not actually use systemd-boot as the entrypoint? All these tools actually implement the very same logic, but on the partition level: if multiple suitable /usr/ partitions exist, then the newest is determined by comparing the GPT partition label of them.

This is in a way the counterpart to the systemd-sysupdate update logic described above: we always need a way to determine which partition to actually then use after the update took place: and this becomes very easy each time: enumerate possible entries, pick the newest as per the (modified) strverscmp() result.

Home Directory Management

In my model the device's users and their home directories are managed by systemd-homed. This means they are relatively self-contained and can be migrated easily between devices. The numeric UID assignment for each user is done at the moment of login only, and the files in the home directory are mapped as needed via a uidmap mount. It also allows us to protect the data of each user individually with a credential that belongs to the user itself. i.e. instead of binding confidentiality of the user's data to the system-wide full-disk-encryption each user gets their own encrypted home directory where the user's authentication token (password, FIDO2 token, PKCS#11 token, recovery key…) is used as authentication and decryption key for the user's data. This brings a major improvement for security as it means the user's data is cryptographically inaccessible except when the user is actually logged in.

It also allows us to correct another major issue with traditional Linux systems: the way how data encryption works during system suspend. Traditionally on Linux the disk encryption credentials (e.g. LUKS passphrase) is kept in memory also when the system is suspended. This is a bad choice for security, since many (most?) of us probably never turn off their laptop but suspend it instead. But if the decryption key is always present in unencrypted form during the suspended time, then it could potentially be read from there by a sufficiently equipped attacker.

By encrypting the user's home directory with the user's authentication token we can first safely "suspend" the home directory before going to the system suspend state (i.e. flush out the cryptographic keys needed to access it). This means any process currently accessing the home directory will be frozen for the time of the suspend, but that's expected anyway during a system suspend cycle. Why is this better than the status quo ante? In this model the home directory's cryptographic key material is erased during suspend, but it can be safely reacquired on resume, from system code. If the system is only encrypted as a whole however, then the system code itself couldn't reauthenticate the user, because it would be frozen too. By separating home directory encryption from the root file system encryption we can avoid this problem.

Partition Setup

So we discussed the organization of the partitions OS images multiple times in the above, each time focusing on a specific aspect. Let's now summarize how this should look like all together.

In my model, the initial, shipped OS image should look roughly like this:

  • (1) An UEFI System Partition, with systemd-boot as boot loader and one unified kernel
  • (2) A /usr/ partition (version "A"), with a label fooOS_0.7 (under the assumption we called our project fooOS and the image version is 0.7).
  • (3) A Verity partition for the /usr/ partition (version "A"), with the same label
  • (4) A partition carrying the Verity root hash for the /usr/ partition (version "A"), along with a PKCS#7 signature of it, also with the same label

On first boot this is augmented by systemd-repart like this:

  • (5) A second /usr/ partition (version "B"), initially with a label _empty (which is the label systemd-sysupdate uses to mark partitions that currently carry no valid payload)
  • (6) A Verity partition for that (version "B"), similar to the above case, also labelled _empty
  • (7) And ditto a Verity root hash partition with a PKCS#7 signature (version "B"), also labelled _empty
  • (8) A root file system, encrypted and locked to the TPM2
  • (9) A home file system, integrity protected via a key also in TPM2 (encryption is unnecessary, since systemd-homed adds that on its own, and it's nice to avoid duplicate encryption)
  • (10) A swap partition, encrypted and locked to the TPM2

Then, on the first OS update the partitions 5, 6, 7 are filled with a new version of the OS (let's say 0.8) and thus get their label updated to fooOS_0.8. After a boot, this version is active.

On a subsequent update the three partitions fooOS_0.7 get wiped and replaced by fooOS_0.9 and so on.

On factory reset, the partitions 8, 9, 10 are deleted, so that systemd-repart recreates them, using a new set of cryptographic keys.

Here's a graphic that hopefully illustrates the partition stable from shipped image, through first boot, multiple update cycles and eventual factory reset:

Partitions Overview

Trust Chain

So let's summarize the intended chain of trust (for bare metal/VM boots) that ensures every piece of code in this model is signed and validated, and any system secret is locked to TPM2.

  1. First, firmware (or possibly shim) authenticates systemd-boot.

  2. Once systemd-boot picks a unified kernel image to boot, it is also authenticated by firmware/shim.

  3. The unified kernel image contains an initrd, which is the first userspace component that runs. It finds any system extensions passed into the initrd, and sets them up through Verity. The kernel will validate the Verity root hash signature of these system extension images against its usual keyring.

  4. The initrd also finds credentials passed in, then securely unlocks (which means: decrypts + authenticates) them with a secret from the TPM2 chip, locked to the kernel image itself.

  5. The kernel image also contains a kernel command line which contains a usrhash= option that pins the root hash of the /usr/ partition to use.

  6. The initrd then unlocks the encrypted root file system, with a secret bound to the TPM2 chip.

  7. The system then transitions into the main system, i.e. the combination of the Verity protected /usr/ and the encrypted root files system. It then activates two more encrypted (and/or integrity protected) volumes for /home/ and swap, also with a secret tied to the TPM2 chip.

Here's an attempt to illustrate the above graphically:

Trust Chain

This is the trust chain of the basic OS. Validation of system extension images, portable service images, systemd-nspawn container images always takes place the same way: the kernel validates these Verity images along with their PKCS#7 signatures against the kernel's keyring.

File System Choice

In the above I left the choice of file systems unspecified. For the immutable /usr/ partitions squashfs might be a good candidate, but any other that works nicely in a read-only fashion and generates reproducible results is a good choice, too. The home directories as managed by systemd-homed should certainly use btrfs, because it's the only general purpose file system supporting online grow and shrink, which systemd-homed can take benefit of, to manage storage.

For the root file system btrfs is likely also the best idea. That's because we intend to use LUKS/dm-crypt underneath, which by default only provides confidentiality, not authenticity of the data (unless combined with dm-integrity). Since btrfs (unlike xfs/ext4) does full data checksumming it's probably the best choice here, since it means we don't have to use dm-integrity (which comes at a higher performance cost).

OS Installation vs. OS Instantiation

In the discussion above a lot of focus was put on setting up the OS and completing the partition layout and such on first boot. This means installing the OS becomes as simple as dd-ing (i.e. "streaming") the shipped disk image into the final HDD medium. Simple, isn't it?

Of course, such a scheme is just too simple for many setups in real life. Whenever multi-boot is required (i.e. co-installing an OS implementing this model with another unrelated one), dd-ing a disk image onto the HDD is going to overwrite user data that was supposed to be kept around.

In order to cover for this case, in my model, we'd use systemd-repart (again!) to allow streaming the source disk image into the target HDD in a smarter, additive way. The tool after all is purely additive: it will add in partitions or grow them if they are missing or too small. systemd-repart already has all the necessary provisions to not only create a partition on the target disk, but also copy blocks from a raw installer disk. An install operation would then become a two stop process: one invocation of systemd-repart that adds in the /usr/, its Verity and the signature partition to the target medium, populated with a copy of the same partition of the installer medium. And one invocation of bootctl that installs the systemd-boot boot loader in the ESP. (Well, there's one thing missing here: the unified OS kernel also needs to be dropped into the ESP. For now, this can be done with a simple cp call. In the long run, this should probably be something bootctl can do as well, if told so.)

So, with this scheme we have a simple scheme to cover all bases: we can either just dd an image to disk, or we can stream an image onto an existing HDD, adding a couple of new partitions and files to the ESP.

Of course, in reality things are more complex than that even: there's a good chance that the existing ESP is simply too small to carry multiple unified kernels. In my model, the way to address this is by shipping two slightly different systemd-repart partition definition file sets: the ideal case when the ESP is large enough, and a fallback case, where it isn't and where we then add in an addition XBOOTLDR partition (as per the Discoverable Partitions Specification). In that mode the ESP carries the boot loader, but the unified kernels are stored in the XBOOTLDR partition. This scenario is not quite as simple as the XBOOTLDR-less scenario described first, but is equally well supported in the various tools. Note that systemd-repart can be told size constraints on the partitions it shall create or augment, thus to implement this scheme it's enough to invoke the tool with the fallback partition scheme if invocation with the ideal scheme fails.

Either way: regardless how the partitions, the boot loader and the unified kernels ended up on the system's hard disk, on first boot the code paths are the same again: systemd-repart will be called to augment the partition table with the root file system, and properly encrypt it, as was already discussed earlier here. This means: all cryptographic key material used for disk encryption is generated on first boot only, the installer phase does not encrypt anything.

Live Systems vs. Installer Systems vs. Installed Systems

Traditionally on Linux three types of systems were common: "installed" systems, i.e. that are stored on the main storage of the device and are the primary place people spend their time in; "installer" systems which are used to install them and whose job is to copy and setup the packages that make up the installed system; and "live" systems, which were a middle ground: a system that behaves like an installed system in most ways, but lives on removable media.

In my model I'd like to remove the distinction between these three concepts as much as possible: each of these three images should carry the exact same /usr/ file system, and should be suitable to be replicated the same way. Once installed the resulting image can also act as an installer for another system, and so on, creating a certain "viral" effect: if you have one image or installation it's automatically something you can replicate 1:1 with a simple systemd-repart invocation.

Building Images According to this Model

The above explains how the image should look like and how its first boot and update cycle will modify it. But this leaves one question unanswered: how to actually build the initial image for OS instances according to this model?

Note that there's nothing too special about the images following this model: they are ultimately just GPT disk images with Linux file systems, following the Discoverable Partition Specification. This means you can use any set of tools of your choice that can put together GPT disk images for compliant images.

I personally would use mkosi for this purpose though. It's designed to generate compliant images, and has a rich toolset for SecureBoot and signed/Verity file systems already in place.

What is key here is that this model doesn't depart from RPM and dpkg, instead it builds on top of that: in this model they are excellent for putting together images on the build host, but deployment onto the runtime host does not involve individual packages.

I think one cannot underestimate the value traditional distributions bring, regarding security, integration and general polishing. The concepts I describe above are inherited from this, but depart from the idea that distribution packages are a runtime concept and make it a build-time concept instead.

Note that the above is pretty much independent from the underlying distribution.

Final Words

I have no illusions, general purpose distributions are not going to adopt this model as their default any time soon, and it's not even my goal that they do that. The above is my personal vision, and I don't expect people to buy into it 100%, and that's fine. However, what I am interested in is finding the overlaps, i.e. work with people who buy 50% into this vision, and share the components.

My goals here thus are to:

  1. Get distributions to move to a model where images like this can be built from the distribution easily. Specifically this means that distributions make their OS hermetic in /usr/.

  2. Find the overlaps, share components with other projects to revisit how distributions are put together. This is already happening, see systemd-tmpfiles and systemd-sysuser support in various distributions, but I think there's more to share.

  3. Make people interested in building actual real-world images based on general purpose distributions adhering to the model described above. I'd love a "GnomeBook" image with full trust properties, that is built from true Linux distros, such as Fedora or ArchLinux.

FAQ

  1. What about ostree? Doesn't ostree already deliver what this blog story describes?

    ostree is fine technology, but in respect to security and robustness properties it's not too interesting I think, because unlike image-based approaches it cannot really deliver integrity/robustness guarantees over the whole tree easily. To be able to trust an ostree setup you have to establish trust in the underlying file system first, and the complexity of the file system makes that challenging. To provide an effective offline-secure trust chain through the whole depth of the stack it is essential to cryptographically validate every single I/O operation. In an image-based model this is trivially easy, but in ostree model it's with current file system technology not possible and even if this is added in one way or another in the future (though I am not aware of anyone doing on-access file-based integrity that spans a whole hierarchy of files that was compatible with ostree's hardlink farm model) I think validation is still at too high a level, since Linux file system developers made very clear their implementations are not robust to rogue images. (There's this stuff planned, but doing structural authentication ahead of time instead of on access makes the idea to weak — and I'd expect too slow — in my eyes.)

    With my design I want to deliver similar security guarantees as ChromeOS does, but ostree is much weaker there, and I see no perspective of this changing. In a way ostree's integrity checks are similar to RPM's and enforced on download rather than on access. In the model I suggest above, it's always on access, and thus safe towards offline attacks (i.e. evil maid attacks). In today's world, I think offline security is absolutely necessary though.

    That said, ostree does have some benefits over the model described above: it naturally shares file system inodes if many of the modules/images involved share the same data. It's thus more space efficient on disk (and thus also in RAM/cache to some degree) by default. In my model it would be up to the image builders to minimize shipping overly redundant disk images, by making good use of suitably composable system extensions.

  2. What about configuration management?

    At first glance immutable systems and configuration management don't go that well together. However, do note, that in the model I propose above the root file system with all its contents, including /etc/ and /var/ is actually writable and can be modified like on any other typical Linux distribution. The only exception is /usr/ where the immutable OS is hermetic. That means configuration management tools should work just fine in this model – up to the point where they are used to install additional RPM/dpkg packages, because that's something not allowed in the model above: packages need to be installed at image build time and thus on the image build host, not the runtime host.

  3. What about non-UEFI and non-TPM2 systems?

    The above is designed around the feature set of contemporary PCs, and this means UEFI and TPM2 being available (simply because the PC is pretty much defined by the Windows platform, and current versions of Windows require both).

    I think it's important to make the best of the features of today's PC hardware, and then find suitable fallbacks on more limited hardware. Specifically this means: if there's desire to implement something like the this on non-UEFI or non-TPM2 hardware we should look for suitable fallbacks for the individual functionality, but generally try to add glue to the old systems so that conceptually they behave more like the new systems instead of the other way round. Or in other words: most of the above is not strictly tied to UEFI or TPM2, and for many cases already there are reasonably fallbacks in place for more limited systems. Of course, without TPM2 many of the security guarantees will be weakened.

  4. How would you name an OS built that way?

    I think a desktop OS built this way if it has the GNOME desktop should of course be called GnomeBook, to mimic the ChromeBook name. ;-)

    But in general, I'd call hermetic, adaptive, immutable OSes like this "particles".

How can you help?

  1. Help making Distributions Hermetic in /usr/!

    One of the core ideas of the approach described above is to make the OS hermetic in /usr/, i.e. make it carry a comprehensive description of what needs to be set up outside of it when instantiated. Specifically, this means that system users that are needed are declared in systemd-sysusers snippets, and skeleton files and directories are created via systemd-tmpfiles. Moreover additional partitions should be declared via systemd-repart drop-ins.

    At this point some distributions (such as Fedora) are (probably more by accident than on purpose) already mostly hermetic in /usr/, at least for the most basic parts of the OS. However, this is not complete: many daemons require to have specific resources set up in /var/ or /etc/ before they can work, and the relevant packages do not carry systemd-tmpfiles descriptions that add them if missing. So there are two ways you could help here: politically, it would be highly relevant to convince distributions that an OS that is hermetic in /usr/ is highly desirable and it's a worthy goal for packagers to get there. More specifically, it would be desirable if RPM/dpkg packages would ship with enough systemd-tmpfiles information so that configuration files the packages strictly need for operation are symlinked (or copied) from /usr/share/factory/ if they are missing (even better of course would be if packages from their upstream sources on would just work with an empty /etc/ and /var/, and create themselves what they need and default to good defaults in absence of configuration files).

    Note that distributions that adopted systemd-sysusers, systemd-tmpfiles and the /usr/ merge are already quite close to providing an OS that is hermetic in /usr/. These were the big, the major advancements: making the image fully hermetic should be less controversial – at least that's my guess.

    Also note that making the OS hermetic in /usr/ is not just useful in scenarios like the above. It also means that stuff like this and like this can work well.

  2. Fill in the gaps!

    I already mentioned a couple of missing bits and pieces in the implementation of the overall vision. In the systemd project we'd be delighted to review/merge any PRs that fill in the voids.

  3. Build your own OS like this!

    Of course, while we built all these building blocks and they have been adopted to various levels and various purposes in the various distributions, no one so far built an OS that puts things together just like that. It would be excellent if we had communities that work on building images like what I propose above. i.e. if you want to work on making a secure GnomeBook as I suggest above a reality that would be more than welcome.

    How could this look like specifically? Pick an existing distribution, write a set of mkosi descriptions plus some additional drop-in files, and then build this on some build infrastructure. While doing so, report the gaps, and help us address them.

Further Documentation of Used Components and Concepts

  1. systemd-tmpfiles
  2. systemd-sysusers
  3. systemd-boot
  4. systemd-stub
  5. systemd-sysext
  6. systemd-portabled, Portable Services Introduction
  7. systemd-repart
  8. systemd-nspawn
  9. systemd-sysupdate
  10. systemd-creds, System and Service Credentials
  11. systemd-homed
  12. Automatic Boot Assessment
  13. Boot Loader Specification
  14. Discoverable Partitions Specification
  15. Safely Building Images

Earlier Blog Stories Related to this Topic

  1. The Strange State of Authenticated Boot and Disk Encryption on Generic Linux Distributions
  2. The Wondrous World of Discoverable GPT Disk Images
  3. Unlocking LUKS2 volumes with TPM2, FIDO2, PKCS#11 Security Hardware on systemd 248
  4. Portable Services with systemd v239
  5. mkosi — A Tool for Generating OS Images

And that's all for now.


Testing my System Code in /usr/ Without Modifying /usr/

I recently blogged about how to run a volatile systemd-nspawn container from your host's /usr/ tree, for quickly testing stuff in your host environment, sharing your home drectory, but all that without making a single modification to your host, and on an isolated node.

The one-liner discussed in that blog story is great for testing during system software development. Let's have a look at another systemd tool that I regularly use to test things during systemd development, in a relatively safe environment, but still taking full benefit of my host's setup.

Since a while now, systemd has been shipping with a simple component called systemd-sysext. It's primary usecase goes something like this: on one hand OS systems with immutable /usr/ hierarchies are fantastic for security, robustness, updating and simplicity, but on the other hand not being able to quickly add stuff to /usr/ is just annoying.

systemd-sysext is supposed to bridge this contradiction: when invoked it will merge a bunch of "system extension" images into /usr/ (and /opt/ as a matter of fact) through the use of read-only overlayfs, making all files shipped in the image instantly and atomically appear in /usr/ during runtime — as if they always had been there. Now, let's say you are building your locked down OS, with an immutable /usr/ tree, and it comes without ability to log into, without debugging tools, without anything you want and need when trying to debug and fix something in the system. With systemd-sysext you could use a system extension image that contains all this, drop it into the system, and activate it with systemd-sysext so that it genuinely extends the host system.

(There are many other usecases for this tool, for example, you could build systems that way that at their base use a generic image, but by installing one or more system extensions get extended to with additional more specific functionality, or drivers, or similar. The tool is generic, use it for whatever you want, but for now let's not get lost in listing all the possibilites.)

What's particularly nice about the tool is that it supports automatically discovered dm-verity images, with signatures and everything. So you can even do this in a fully authenticated, measured, safe way. But I am digressing…

Now that we (hopefully) have a rough understanding what systemd-sysext is and does, let's discuss how specficially we can use this in the context of system software development, to safely use and test bleeding edge development code — built freshly from your project's build tree – in your host OS without having to risk that the host OS is corrupted or becomes unbootable by stuff that didn't quite yet work the way it was envisioned:

The images systemd-sysext merges into /usr/ can be of two kinds: disk images with a file system/verity/signature, or simple, plain directory trees. To make these images available to the tool, they can be placed or symlinked into /usr/lib/extensions/, /var/lib/extensions/, /run/extensions/ (and a bunch of others). So if we now install our freshly built development software into a subdirectory of those paths, then that's entirely sufficient to make them valid system extension images in the sense of systemd-sysext, and thus can be merged into /usr/ to try them out.

To be more specific: when I develop systemd itself, here's what I do regularly, to see how my new development version would behave on my host system. As preparation I checked out the systemd development git tree first of course, hacked around in it a bit, then built it with meson/ninja. And now I want to test what I just built:

sudo DESTDIR=/run/extensions/systemd-test meson install -C build --quiet --no-rebuild &&
        sudo systemd-sysext refresh --force

Explanation: first, we'll install my current build tree as a system extension into /run/extensions/systemd-test/. And then we apply it to the host via the systemd-sysext refresh command. This command will search for all installed system extension images in the aforementioned directories, then unmount (i.e. "unmerge") any previously merged dirs from /usr/ and then freshly mount (i.e. "merge") the new set of system extensions on top of /usr/. And just like that, I have installed my development tree of systemd into the host OS, and all that without actually modifying/replacing even a single file on the host at all. Nothing here actually hit the disk!

Note that all this works on any system really, it is not necessary that the underlying OS even is designed with immutability in mind. Just because the tool was developed with immutable systems in mind it doesn't mean you couldn't use it on traditional systems where /usr/ is mutable as well. In fact, my development box actually runs regular Fedora, i.e. is RPM-based and thus has a mutable /usr/ tree. As long as system extensions are applied the whole of /usr/ becomes read-only though.

Once I am done testing, when I want to revert to how things were without the image installed, it is sufficient to call:

sudo systemd-sysext unmerge

And there you go, all files my development tree generated are gone again, and the host system is as it was before (and /usr/ mutable again, in case one is on a traditional Linux distribution).

Also note that a reboot (regardless if a clean one or an abnormal shutdown) will undo the whole thing automatically, since we installed our build tree into /run/ after all, i.e. a tmpfs instance that is flushed on boot. And given that the overlayfs merge is a runtime thing, too, the whole operation was executed without any persistence. Isn't that great?

(You might wonder why I specified --force on the systemd-sysext refresh line earlier. That's because systemd-sysext actually does some minimal version compatibility checks when applying system extension images. For that it will look at the host's /etc/os-release file with /usr/lib/extension-release.d/extension-release.<name>, and refuse operaton if the image is not actually built for the host OS version. Here we don't want to bother with dropping that file in there, we know already that the extension image is compatible with the host, as we just built it on it. --force allows us to skip the version check.)

You might wonder: what about the combination of the idea from the previous blog story (regarding running container's off the host /usr/ tree) with system extensions? Glad you asked. Right now we have no support for this, but it's high on our TODO list (patches welcome, of course!). i.e. a new switch for systemd-nspawn called --system-extension= that would allow merging one or more such extensions into the container tree booted would be stellar. With that, with a single command I could run a container off my host OS but with a development version of systemd dropped in, all without any persistence. How awesome would that be?

(Oh, and in case you wonder, all of this only works with distributions that have completed the /usr/ merge. On legacy distributions that didn't do that and still place parts of /usr/ all over the hierarchy the above won't work, since merging /usr/ trees via overlayfs is pretty pointess if the OS is not hermetic in /usr/.)

And that's all for now. Happy hacking!


Running a Container off the Host /usr/

Apparently, in some parts of this world, the /usr/-merge transition is still ongoing. Let's take the opportunity to have a look at one specific way to take benefit of the /usr/-merge (and associated work) IRL.

I develop system-level software as you might know. Oftentimes I want to run my development code on my PC but be reasonably sure it cannot destroy or otherwise negatively affect my host system. Now I could set up a container tree for that, and boot into that. But often I am too lazy for that, I don't want to bother with a slow package manager setting up a new OS tree for me. So here's what I often do instead — and this only works because of the /usr/-merge.

I run a command like the following (without any preparatory work):

systemd-nspawn \
        --directory=/ \
        --volatile=yes \
        -U \
        --set-credential=passwd.hashed-password.root:$(mkpasswd mysecret) \
        --set-credential=firstboot.locale:C.UTF-8 \
        --bind-user=lennart \
        -b

And then I very quickly get a login prompt on a container that runs the exact same software as my host — but is also isolated from the host. I do not need to prepare any separate OS tree or anything else. It just works. And my host user lennart is just there, ready for me to log into.

So here's what these systemd-nspawn options specifically do:

  • --directory=/ tells systemd-nspawn to run off the host OS' file hierarchy. That smells like danger of course, running two OS instances off the same directory hierarchy. But don't be scared, because:

  • --volatile=yes enables volatile mode. Specifically this means what we configured with --directory=/ as root file system is slightly rearranged. Instead of mounting that tree as it is, we'll mount a tmpfs instance as actual root file system, and then mount the /usr/ subdirectory of the specified hierarchy into the /usr/ subdirectory of the container file hierarchy in read-only fashion – and only that directory. So now we have a container directory tree that is basically empty, but imports all host OS binaries and libraries into its /usr/ tree. All software installed on the host is also available in the container with no manual work. This mechanism only works because on /usr/-merged OSes vendor resources are monopolized at a single place: /usr/. It's sufficient to share that one directory with the container to get a second instance of the host OS running. Note that this means /etc/ and /var/ will be entirely empty initially when this second system boots up. Thankfully, forward looking distributions (such as Fedora) have adopted systemd-tmpfiles and systemd-sysusers quite pervasively, so that system users and files/directories required for operation are created automatically should they be missing. Thus, even though at boot the mentioned directories are initially empty, once the system is booted up they are sufficiently populated for things to just work.

  • -U means we'll enable user namespacing, in fully automatic mode. This does three things: it picks a free host UID range dynamically for the container, then sets up user namespacing for the container processes mapping host UID range to UIDs 0…65534 in the container. It then sets up a similar UID mapped mount on the /usr/ tree of the container. Net effect: file ownerships as set on the host OS tree appear as they belong to the very same users inside of the container environment, except that we use user namespacing for everything, and thus the users are actually neatly isolated from the host.

  • --set-credential=passwd.hashed-password.root:$(mkpasswd mysecret) passes a credential to the container. Credentials are bits of data that you can pass to systemd services and whole systems. They are actually awesome concepts (e.g. they support TPM2 authentication/encryption that just works!) but I am not going to go into details around that, given it's off-topic in this specific scenario. Here we just take benefit of the fact that systemd-sysusers looks for a credential called passwd.hashed-password.root to initialize the root password of the system from. We set it to mysecret. This means once the system is booted up we can log in as root and the supplied password. Yay. (Remember, /etc/ is initially empty on this container, and thus also carries no /etc/passwd or /etc/shadow, and thus has no root user record, and thus no root password.)

    mkpasswd is a tool then converts a plain text password into a UNIX hashed password, which is what this specific credential expects.

  • Similar, --set-credential=firstboot.locale:C.UTF-8 tells the systemd-firstboot service in the container to initialize /etc/locale.conf with this locale.

  • --bind-user=lennart binds the host user lennart into the container, also as user lennart. This does two things: it mounts the host user's home directory into the container. It also copies a minimal user record of the specified user into the container that nss-systemd then picks up and includes in the regular user database. This means, once the container is booted up I can log in as lennart with my regular password, and once I logged in I will see my regular host home directory, and can make changes to it. Yippieh! (This does a couple of more things, such as UID mapping and things, but let's not get lost in too much details.)

So, if I run this, I will very quickly get a login prompt, where I can log into as my regular user. I have full access to my host home directory, but otherwise everything is nicely isolated from the host, and changes outside of the home directory are either prohibited or are volatile, i.e. go to a tmpfs instance whose lifetime is bound to the container's lifetime: when I shut down the container I just started, then any changes outside of my user's home directory are lost.

Note that while here I use --volatile=yes in combination with --directory=/ you can actually use it on any OS hierarchy, i.e. just about any directory that contains OS binaries.

Similar, the --bind-user= stuff works with any OS hierarchy too (but do note that only systemd 249 and newer will pick up the user records passed to the container that way, i.e. this requires at least v249 both on the host and in the container to work).

Or in short: the possibilities are endless!

Requirements

For this all to work, you need:

  1. A recent kernel (5.15 should suffice, as it brings UID mapped mounts for the most common file systems, so that -U and --bind-user= can work well.)

  2. A recent systemd (249 should suffice, which brings --bind-user=, and a -U switch backed by UID mapped mounts).

  3. A distribution that adopted the /usr/-merge, systemd-tmpfiles and systemd-sysusers so that the directory hierarchy and user databases are automatically populated when empty at boot. (Fedora 35 should suffice.)

Limitations

While a lot of today's software actually out of the box works well on systems that come up with an unpopulated /etc/ and /var/, and either fall back to reasonable built-in defaults, or deploy systemd-tmpfiles to create what is missing, things aren't perfect: some software typically installed an desktop OSes will fail to start when invoked in such a container, and be visible as ugly failed services, but it won't stop me from logging in and using the system for what I want to use it. It would be excellent to get that fixed, though. This can either be fixed in the relevant software upstream (i.e. if opening your configuration file fails with ENOENT, then just default to reasonable defaults), or in the distribution packaging (i.e. add a tmpfiles.d/ file that copies or symlinks in skeleton configuration from /usr/share/factory/etc/ via the C or L line types).

And then there's certain software dealing with hardware management and similar that simply cannot work in a container (as device APIs on Linux are generally not virtualized for containers) reasonably. It would be excellent if software like that would be updated to carry ConditionVirtualization=!container or ConditionPathIsReadWrite=/sys conditionalization in their unit files, so that it is automatically – cleanly – skipped when executed in such a container environment.

And that's all for now.


Authenticated Boot and Disk Encryption on Linux

The Strange State of Authenticated Boot and Disk Encryption on Generic Linux Distributions

TL;DR: Linux has been supporting Full Disk Encryption (FDE) and technologies such as UEFI SecureBoot and TPMs for a long time. However, the way they are set up by most distributions is not as secure as they should be, and in some ways quite frankly weird. In fact, right now, your data is probably more secure if stored on current ChromeOS, Android, Windows or MacOS devices, than it is on typical Linux distributions.

Generic Linux distributions (i.e. Debian, Fedora, Ubuntu, …) adopted Full Disk Encryption (FDE) more than 15 years ago, with the LUKS/cryptsetup infrastructure. It was a big step forward to a more secure environment. Almost ten years ago the big distributions started adding UEFI SecureBoot to their boot process. Support for Trusted Platform Modules (TPMs) has been added to the distributions a long time ago as well — but even though many PCs/laptops these days have TPM chips on-board it's generally not used in the default setup of generic Linux distributions.

How these technologies currently fit together on generic Linux distributions doesn't really make too much sense to me — and falls short of what they could actually deliver. In this story I'd like to have a closer look at why I think that, and what I propose to do about it.

The Basic Technologies

Let's have a closer look what these technologies actually deliver:

  1. LUKS/dm-crypt/cryptsetup provide disk encryption, and optionally data authentication. Disk encryption means that reading the data in clear-text form is only possible if you possess a secret of some form, usually a password/passphrase. Data authentication means that no one can make changes to the data on disk unless they possess a secret of some form. Most distributions only enable the former though — the latter is a more recent addition to LUKS/cryptsetup, and is not used by default on most distributions (though it probably should be). Closely related to LUKS/dm-crypt is dm-verity (which can authenticate immutable volumes) and dm-integrity (which can authenticate writable volumes, among other things).

  2. UEFI SecureBoot provides mechanisms for authenticating boot loaders and other pre-OS binaries before they are invoked. If those boot loaders then authenticate the next step of booting in a similar fashion there's a chain of trust which can ensure that only code that has some level of trust associated with it will run on the system. Authentication of boot loaders is done via cryptographic signatures: the OS/boot loader vendors cryptographically sign their boot loader binaries. The cryptographic certificates that may be used to validate these signatures are then signed by Microsoft, and since Microsoft's certificates are basically built into all of today's PCs and laptops this will provide some basic trust chain: if you want to modify the boot loader of a system you must have access to the private key used to sign the code (or to the private keys further up the certificate chain).

  3. TPMs do many things. For this text we'll focus one facet: they can be used to protect secrets (for example for use in disk encryption, see above), that are released only if the code that booted the host can be authenticated in some form. This works roughly like this: every component that is used during the boot process (i.e. code, certificates, configuration, …) is hashed with a cryptographic hash function before it is used. The resulting hash is written to some small volatile memory the TPM maintains that is write-only (the so called Platform Configuration Registers, "PCRs"): each step of the boot process will write hashes of the resources needed by the next part of the boot process into these PCRs. The PCRs cannot be written freely: the hashes written are combined with what is already stored in the PCRs — also through hashing and the result of that then replaces the previous value. Effectively this means: only if every component involved in the boot matches expectations the hash values exposed in the TPM PCRs match the expected values too. And if you then use those values to unlock the secrets you want to protect you can guarantee that the key is only released to the OS if the expected OS and configuration is booted. The process of hashing the components of the boot process and writing that to the TPM PCRs is called "measuring". What's also important to mention is that the secrets are not only protected by these PCR values but encrypted with a "seed key" that is generated on the TPM chip itself, and cannot leave the TPM (at least so goes the theory). The idea is that you cannot read out a TPM's seed key, and thus you cannot duplicate the chip: unless you possess the original, physical chip you cannot retrieve the secret it might be able to unlock for you. Finally, TPMs can enforce a limit on unlock attempts per time ("anti-hammering"): this makes it hard to brute force things: if you can only execute a certain number of unlock attempts within some specific time then brute forcing will be prohibitively slow.

How Linux Distributions use these Technologies

As mentioned already, Linux distributions adopted the first two of these technologies widely, the third one not so much.

So typically, here's how the boot process of Linux distributions works these days:

  1. The UEFI firmware invokes a piece of code called "shim" (which is stored in the EFI System Partition — the "ESP" — of your system), that more or less is just a list of certificates compiled into code form. The shim is signed with the aforementioned Microsoft key, that is built into all PCs/laptops. This list of certificates then can be used to validate the next step of the boot process. The shim is measured by the firmware into the TPM. (Well, the shim can do a bit more than what I describe here, but this is outside of the focus of this article.)

  2. The shim then invokes a boot loader (often Grub) that is signed by a private key owned by the distribution vendor. The boot loader is stored in the ESP as well, plus some other places (i.e. possibly a separate boot partition). The corresponding certificate is included in the list of certificates built into the shim. The boot loader components are also measured into the TPM.

  3. The boot loader then invokes the kernel and passes it an initial RAM disk image (initrd), which contains initial userspace code. The kernel itself is signed by the distribution vendor too. It's also validated via the shim. The initrd is not validated, though (!). The kernel is measured into the TPM, the initrd sometimes too.

  4. The kernel unpacks the initrd image, and invokes what is contained in it. Typically, the initrd then asks the user for a password for the encrypted root file system. The initrd then uses that to set up the encrypted volume. No code authentication or TPM measurements take place.

  5. The initrd then transitions into the root file system. No code authentication or TPM measurements take place.

  6. When the OS itself is up the user is prompted for their user name, and their password. If correct, this will unlock the user account: the system is now ready to use. At this point no code authentication, no TPM measurements take place. Moreover, the user's password is not used to unlock any data, it's used only to allow or deny the login attempt — the user's data has already been decrypted a long time ago, by the initrd, as mentioned above.

What you'll notice here of course is that code validation happens for the shim, the boot loader and the kernel, but not for the initrd or the main OS code anymore. TPM measurements might go one step further: the initrd is measured sometimes too, if you are lucky. Moreover, you might notice that the disk encryption password and the user password are inquired by code that is not validated, and is thus not safe from external manipulation. You might also notice that even though TPM measurements of boot loader/OS components are done nothing actually ever makes use of the resulting PCRs in the typical setup.

Attack Scenarios

Of course, before determining whether the setup described above makes sense or not, one should have an idea what one actually intends to protect against.

The most basic attack scenario to focus on is probably that you want to be reasonably sure that if someone steals your laptop that contains all your data then this data remains confidential. The model described above probably delivers that to some degree: the full disk encryption when used with a reasonably strong password should make it hard for the laptop thief to access the data. The data is as secure as the password used is strong. The attacker might attempt to brute force the password, thus if the password is not chosen carefully the attacker might be successful.

Two more interesting attack scenarios go something like this:

  1. Instead of stealing your laptop the attacker takes the harddisk from your laptop while you aren't watching (e.g. while you went for a walk and left it at home or in your hotel room), makes a copy of it, and then puts it back. You'll never notice they did that. The attacker then analyzes the data in their lab, maybe trying to brute force the password. In this scenario you won't even know that your data is at risk, because for you nothing changed — unlike in the basic scenario above. If the attacker manages to break your password they have full access to the data included on it, i.e. everything you so far stored on it, but not necessarily on what you are going to store on it later. This scenario is worse than the basic one mentioned above, for the simple fact that you won't know that you might be attacked. (This scenario could be extended further: maybe the attacker has a chance to watch you type in your password or so, effectively lowering the password strength.)

  2. Instead of stealing your laptop the attacker takes the harddisk from your laptop while you aren't watching, inserts backdoor code on it, and puts it back. In this scenario you won't know your data is at risk, because physically everything is as before. What's really bad though is that the attacker gets access to anything you do on your laptop, both the data already on it, and whatever you will do in the future.

I think in particular this backdoor attack scenario is something we should be concerned about. We know for a fact that attacks like that happen all the time (Pegasus, industry espionage, …), hence we should make them hard.

Are we Safe?

So, does the scheme so far implemented by generic Linux distributions protect us against the latter two scenarios? Unfortunately not at all. Because distributions set up disk encryption the way they do, and only bind it to a user password, an attacker can easily duplicate the disk, and then attempt to brute force your password. What's worse: since code authentication ends at the kernel — and the initrd is not authenticated anymore —, backdooring is trivially easy: an attacker can change the initrd any way they want, without having to fight any kind of protections. And given that FDE unlocking is implemented in the initrd, and it's the initrd that asks for the encryption password things are just too easy: an attacker could trivially easily insert some code that picks up the FDE password as you type it in and send it wherever they want. And not just that: since once they are in they are in, they can do anything they like for the rest of the system's lifecycle, with full privileges — including installing backdoors for versions of the OS or kernel that are installed on the device in the future, so that their backdoor remains open for as long as they like.

That is sad of course. It's particular sad given that the other popular OSes all address this much better. ChromeOS, Android, Windows and MacOS all have way better built-in protections against attacks like this. And it's why one can certainly claim that your data is probably better protected right now if you store it on those OSes then it is on generic Linux distributions.

(Yeah, I know that there are some niche distros which do this better, and some hackers hack their own. But I care about general purpose distros here, i.e. the big ones, that most people base their work on.)

Note that there are more problems with the current setup. For example, it's really weird that during boot the user is queried for an FDE password which actually protects their data, and then once the system is up they are queried again – now asking for a username, and another password. And the weird thing is that this second authentication that appears to be user-focused doesn't really protect the user's data anymore — at that moment the data is already unlocked and accessible. The username/password query is supposed to be useful in multi-user scenarios of course, but how does that make any sense, given that these multiple users would all have to know a disk encryption password that unlocks the whole thing during the FDE step, and thus they have access to every user's data anyway if they make an offline copy of the harddisk?

Can we do better?

Of course we can, and that is what this story is actually supposed to be about.

Let's first figure out what the minimal issues we should fix are (at least in my humble opinion):

  1. The initrd must be authenticated before being booted into. (And measured unconditionally.)

  2. The OS binary resources (i.e. /usr/) must be authenticated before being booted into. (But don't need to be encrypted, since everyone has the same anyway, there's nothing to hide here.)

  3. The OS configuration and state (i.e. /etc/ and /var/) must be encrypted, and authenticated before they are used. The encryption key should be bound to the TPM device; i.e system data should be locked to a security concept belonging to the system, not the user.

  4. The user's home directory (i.e. /home/lennart/ and similar) must be encrypted and authenticated. The unlocking key should be bound to a user password or user security token (FIDO2 or PKCS#11 token); i.e. user data should be locked to a security concept belonging to the user, not the system.

Or to summarize this differently:

  1. Every single component of the boot process and OS needs to be authenticated, i.e. all of shim (done), boot loader (done), kernel (done), initrd (missing so far), OS binary resources (missing so far), OS configuration and state (missing so far), the user's home (missing so far).

  2. Encryption is necessary for the OS configuration and state (bound to TPM), and for the user's home directory (bound to a user password or user security token).

In Detail

Let's see how we can achieve the above in more detail.

How to Authenticate the initrd

At the moment initrds are generated on the installed host via scripts (dracut and similar) that try to figure out a minimal set of binaries and configuration data to build an initrd that contains just enough to be able to find and set up the root file system. What is included in the initrd hence depends highly on the individual installation and its configuration. Pretty likely no two initrds generated that way will be fully identical due to this. This model clearly has benefits: the initrds generated this way are very small and minimal, and support exactly what is necessary for the system to boot, and not less or more. It comes with serious drawbacks too though: the generation process is fragile and sometimes more akin to black magic than following clear rules: the generator script natively has to understand a myriad of storage stacks to determine what needs to be included and what not. It also means that authenticating the image is hard: given that each individual host gets a different specialized initrd, it means we cannot just sign the initrd with the vendor key like we sign the kernel. If we want to keep this design we'd have to figure out some other mechanism (e.g. a per-host signature key – that is generated locally; or by authenticating it with a message authentication code bound to the TPM). While these approaches are certainly thinkable, I am not convinced they actually are a good idea though: locally and dynamically generated per-host initrds is something we probably should move away from.

If we move away from locally generated initrds, things become a lot simpler. If the distribution vendor generates the initrds on their build systems then it can be attached to the kernel image itself, and thus be signed and measured along with the kernel image, without any further work. This simplicity is simply lovely. Besides robustness and reproducibility this gives us an easy route to authenticated initrds.

But of course, nothing is really that simple: working with vendor-generated initrds means that we can't adjust them anymore to the specifics of the individual host: if we pre-build the initrds and include them in the kernel image in immutable fashion then it becomes harder to support complex, more exotic storage or to parameterize it with local network server information, credentials, passwords, and so on. Now, for my simple laptop use-case these things don't matter, there's no need to extend/parameterize things, laptops and their setups are not that wildly different. But what to do about the cases where we want both: extensibility to cover for less common storage subsystems (iscsi, LVM, multipath, drivers for exotic hardware…) and parameterization?

Here's a proposal how to achieve that: let's build a basic initrd into the kernel as suggested, but then do two things to make this scheme both extensible and parameterizable, without compromising security.

  1. Let's define a way how the basic initrd can be extended with additional files, which are stored in separate "extension images". The basic initrd should be able to discover these extension images, authenticate them and then activate them, thus extending the initrd with additional resources on-the-fly.

  2. Let's define a way how we can safely pass additional parameters to the kernel/initrd (and actually the rest of the OS, too) in an authenticated (and possibly encrypted) fashion. Parameters in this context can be anything specific to the local installation, i.e. server information, security credentials, certificates, SSH server keys, or even just the root password that shall be able to unlock the root account in the initrd …

In such a scheme we should be able to deliver everything we are looking for:

  1. We'll have a full trust chain for the code: the boot loader will authenticate and measure the kernel and basic initrd. The initrd extension images will then be authenticated by the basic initrd image.

  2. We'll have authentication for all the parameters passed to the initrd.

This so far sounds very unspecific? Let's make it more specific by looking closer at the components I'd suggest to be used for this logic:

  1. The systemd suite since a few months contains a subsystem implementing system extensions (v248). System extensions are ultimately just disk images (for example a squashfs file system in a GPT envelope) that can extend an underlying OS tree. Extending in this regard means they simply add additional files and directories into the OS tree, i.e. below /usr/. For a longer explanation see systemd-sysext(8). When a system extension is activated it is simply mounted and then merged into the main /usr/ tree via a read-only overlayfs mount. Now what's particularly nice about them in this context we are talking about here is that the extension images may carry dm-verity authentication data, and PKCS#7 signatures (once this is merged, that is, i.e. v250).

  2. The systemd suite also contains a concept called service "credentials". These are small pieces of information passed to services in a secure way. One key feature of these credentials is that they can be encrypted and authenticated in a very simple way with a key bound to the TPM (v250). See LoadCredentialEncrypted= and systemd-creds(1) for details. They are great for safely storing SSL private keys and similar on your system, but they also come handy for parameterizing initrds: an encrypted credential is just a file that can only be decoded if the right TPM is around with the right PCR values set.

  3. The systemd suite contains a component called systemd-stub(7). It's an EFI stub, i.e. a small piece of code that is attached to a kernel image, and turns the kernel image into a regular EFI binary that can be directly executed by the firmware (or a boot loader). This stub has a number of nice features (for example, it can show a boot splash before invoking the Linux kernel itself and such). Once this work is merged (v250) the stub will support one more feature: it will automatically search for system extension image files and credential files next to the kernel image file, measure them and pass them on to the main initrd of the host.

Putting this together we have nice way to provide fully authenticated kernel images, initrd images and initrd extension images; as well as encrypted and authenticated parameters via the credentials logic.

How would a distribution actually make us of this? A distribution vendor would pre-build the basic initrd, and glue it into the kernel image, and sign that as a whole. Then, for each supposed extension of the basic initrd (e.g. one for iscsi support, one for LVM, one for multipath, …), the vendor would use a tool such as mkosi to build an extension image, i.e. a GPT disk image containing the files in squashfs format, a Verity partition that authenticates it, plus a PKCS#7 signature partition that validates the root hash for the dm-verity partition, and that can be checked against a key provided by the boot loader or main initrd. Then, any parameters for the initrd will be encrypted using systemd-creds encrypt -T. The resulting encrypted credentials and the initrd extension images are then simply placed next to the kernel image in the ESP (or boot partition). Done.

This checks all boxes: everything is authenticated and measured, the credentials also encrypted. Things remain extensible and modular, can be pre-built by the vendor, and installation is as simple as dropping in one file for each extension and/or credential.

How to Authenticate the Binary OS Resources

Let's now have a look how to authenticate the Binary OS resources, i.e. the stuff you find in /usr/, i.e. the stuff traditionally shipped to the user's system via RPMs or DEBs.

I think there are three relevant ways how to authenticate this:

  1. Make /usr/ a dm-verity volume. dm-verity is a concept implemented in the Linux kernel that provides authenticity to read-only block devices: every read access is cryptographically verified against a top-level hash value. This top-level hash is typically a 256bit value that you can either encode in the kernel image you are using, or cryptographically sign (which is particularly nice once this is merged). I think this is actually the best approach since it makes the /usr/ tree entirely immutable in a very simple way. However, this also means that the whole of /usr/ needs to be updated as once, i.e. the traditional rpm/apt based update logic cannot work in this mode.

  2. Make /usr/ a dm-integrity volume. dm-integrity is a concept provided by the Linux kernel that offers integrity guarantees to writable block devices, i.e. in some ways it can be considered to be a bit like dm-verity while permitting write access. It can be used in three ways, one of which I think is particularly relevant here. The first way is with a simple hash function in "stand-alone" mode: this is not too interesting here, it just provides greater data safety for file systems that don't hash check their files' data on their own. The second way is in combination with dm-crypt, i.e. with disk encryption. In this case it adds authenticity to confidentiality: only if you know the right secret you can read and make changes to the data, and any attempt to make changes without knowing this secret key will be detected as IO error on next read by those in possession of the secret (more about this below). The third way is the one I think is most interesting here: in "stand-alone" mode, but with a keyed hash function (e.g. HMAC). What's this good for? This provides authenticity without encryption: if you make changes to the disk without knowing the secret this will be noticed on the next read attempt of the data and result in IO errors. This mode provides what we want (authenticity) and doesn't do what we don't need (encryption). Of course, the secret key for the HMAC must be provided somehow, I think ideally by the TPM.

  3. Make /usr/ a dm-crypt (LUKS) + dm-integrity volume. This provides both authenticity and encryption. The latter isn't typically needed for /usr/ given that it generally contains no secret data: anyone can download the binaries off the Internet anyway, and the sources too. By encrypting this you'll waste CPU cycles, but beyond that it doesn't hurt much. (Admittedly, some people might want to hide the precise set of packages they have installed, since it of course does reveal a bit of information about you: i.e. what you are working on, maybe what your job is – think: if you are a hacker you have hacking tools installed – and similar). Going this way might simplify things in some cases, as it means you don't have to distinguish "OS binary resources" (i.e /usr/) and "OS configuration and state" (i.e. /etc/ + /var/, see below), and just make it the same volume. Here too, the secret key must be provided somehow, I think ideally by the TPM.

All three approach are valid. The first approach has my primary sympathies, but for distributions not willing to abandon client-side updates via RPM/dpkg this is not an option, in which case I would propose the other two approaches for these cases.

The LUKS encryption key (and in case of dm-integrity standalone mode the key for the keyed hash function) should be bound to the TPM. Why the TPM for this? You could also use a user password, a FIDO2 or PKCS#11 security token — but I think TPM is the right choice: why that? To reduce the requirement for repeated authentication, i.e. that you first have to provide the disk encryption password, and then you have to login, providing another password. It should be possible that the system boots up unattended and then only one authentication prompt is needed to unlock the user's data properly. The TPM provides a way to do this in a reasonably safe and fully unattended way. Also, when we stop considering just the laptop use-case for a moment: on servers interactive disk encryption prompts don't make much sense — the fact that TPMs can provide secrets without this requiring user interaction and thus the ability to work in entirely unattended environments is quite desirable. Note that crypttab(5) as implemented by systemd (v248) provides native support for authentication via password, via TPM2, via PKCS#11 or via FIDO2, so the choice is ultimately all yours.

How to Encrypt/Authenticate OS Configuration and State

Let's now look at the OS configuration and state, i.e. the stuff in /etc/ and /var/. It probably makes sense to not consider these two hierarchies independently but instead just consider this to be the root file system. If the OS binary resources are in a separate file system it is then mounted onto the /usr/ sub-directory of the root file system.

The OS configuration and state (or: root file system) should be both encrypted and authenticated: it might contain secret keys, user passwords, privileged logs and similar. This data matters and contains plenty data that should remain confidential.

The encryption of choice here is dm-crypt (LUKS) + dm-integrity similar as discussed above, again with the key bound to the TPM.

If the OS binary resources are protected the same way it is safe to merge these two volumes and have a single partition for both (see above)

How to Encrypt/Authenticate the User's Home Directory

The data in the user's home directory should be encrypted, and bound to the user's preferred token of authentication (i.e. a password or FIDO2/PKCS#11 security token). As mentioned, in the traditional mode of operation the user's home directory is not individually encrypted, but only encrypted because FDE is in use. The encryption key for that is a system wide key though, not a per-user key. And I think that's problem, as mentioned (and probably not even generally understood by our users). We should correct that and ensure that the user's password is what unlocks the user's data.

In the systemd suite we provide a service systemd-homed(8) (v245) that implements this in a safe way: each user gets its own LUKS volume stored in a loopback file in /home/, and this is enough to synthesize a user account. The encryption password for this volume is the user's account password, thus it's really the password provided at login time that unlocks the user's data. systemd-homed also supports other mechanisms of authentication, in particular PKCS#11/FIDO2 security tokens. It also provides support for other storage back-ends (such as fscrypt), but I'd always suggest to use the LUKS back-end since it's the only one providing the comprehensive confidentiality guarantees one wants for a UNIX-style home directory.

Note that there's one special caveat here: if the user's home directory (e.g. /home/lennart/) is encrypted and authenticated, what about the file system this data is stored on, i.e. /home/ itself? If that dir is part of the the root file system this would result in double encryption: first the data is encrypted with the TPM root file system key, and then again with the per-user key. Such double encryption is a waste of resources, and unnecessary. I'd thus suggest to make /home/ its own dm-integrity volume with a HMAC, keyed by the TPM. This means the data stored directly in /home/ will be authenticated but not encrypted. That's good not only for performance, but also has practical benefits: it allows extracting the encrypted volume of the various users in case the TPM key is lost, as a way to recover from dead laptops or similar.

Why authenticate /home/, if it only contains per-user home directories that are authenticated on their own anyway? That's a valid question: it's because the kernel file system maintainers made clear that Linux file system code is not considered safe against rogue disk images, and is not tested for that; this means before you mount anything you need to establish trust in some way because otherwise there's a risk that the act of mounting might exploit your kernel.

Summary of Resources and their Protections

So, let's now put this all together. Here's a table showing the various resources we deal with, and how I think they should be protected (in my idealized world).

Resource Needs Authentication Needs Encryption Suggested Technology Validation/Encryption Keys/Certificates acquired via Stored where
Shim yes no SecureBoot signature verification firmware certificate database ESP
Boot loader yes no ditto firmware certificate database/shim ESP/boot partition
Kernel yes no ditto ditto ditto
initrd yes no ditto ditto ditto
initrd parameters yes yes systemd TPM encrypted credentials TPM ditto
initrd extensions yes no systemd-sysext with Verity+PKCS#7 signatures firmware/initrd certificate database ditto
OS binary resources yes no dm-verity root hash linked into kernel image, or firmware/initrd certificate database top-level partition
OS configuration and state yes yes dm-crypt (LUKS) + dm-integrity TPM top-level partition
/home/ itself yes no dm-integrity with HMAC TPM top-level partition
User home directories yes yes dm-crypt (LUKS) + dm-integrity in loopback files User password/FIDO2/PKCS#11 security token loopback file inside /home partition

This should provide all the desired guarantees: everything is authenticated, and the individualized per-host or per-user data is also encrypted. No double encryption takes place. The encryption keys/verification certificates are stored/bound to the most appropriate infrastructure.

Does this address the three attack scenarios mentioned earlier? I think so, yes. The basic attack scenario I described is addressed by the fact that /var/, /etc/ and /home/*/ are encrypted. Brute forcing the former two is harder than in the status quo ante model, since a high entropy key is used instead of one derived from a user provided password. Moreover, the "anti-hammering" logic of the TPM will make brute forcing prohibitively slow. The home directories are protected by the user's password or ideally a personal FIDO2/PKCS#11 security token in this model. Of course, a password isn't better security-wise then the status quo ante. But given the FIDO2/PKCS#11 support built into systemd-homed it should be easier to lock down the home directories securely.

Binding encryption of /var/ and /etc/ to the TPM also addresses the first of the two more advanced attack scenarios: a copy of the harddisk is useless without the physical TPM chip, since the seed key is sealed into that. (And even if the attacker had the chance to watch you type in your password, it won't help unless they possess access to to the TPM chip.) For the home directory this attack is not addressed as long as a plain password is used. However, since binding home directories to FIDO2/PKCS#11 tokens is built into systemd-homed things should be safe here too — provided the user actually possesses and uses such a device.

The backdoor attack scenario is addressed by the fact that every resource in play now is authenticated: it's hard to backdoor the OS if there's no component that isn't verified by signature keys or TPM secrets the attacker hopefully doesn't know.

For general purpose distributions that focus on updating the OS per RPM/dpkg the idealized model above won't work out, since (as mentioned) this implies an immutable /usr/, and thus requires updating /usr/ via an atomic update operation. For such distros a setup like the following is probably more realistic, but see above.

Resource Needs Authentication Needs Encryption Suggested Technology Validation/Encryption Keys/Certificates acquired via Stored where
Shim yes no SecureBoot signature verification firmware certificate database ESP
Boot loader yes no ditto firmware certificate database/shim ESP/boot partition
Kernel yes no ditto ditto ditto
initrd yes no ditto ditto ditto
initrd parameters yes yes systemd TPM encrypted credentials TPM ditto
initrd extensions yes no systemd-sysext with Verity+PKCS#7 signatures firmware/initrd certificate database ditto
OS binary resources, configuration and state yes yes dm-crypt (LUKS) + dm-integrity TPM top-level partition
/home/ itself yes no dm-integrity with HMAC TPM top-level partition
User home directories yes yes dm-crypt (LUKS) + dm-integrity in loopback files User password/FIDO2/PKCS#11 security token loopback file inside /home partition

This means there's only one root file system that contains all of /etc/, /var/ and /usr/.

Recovery Keys

When binding encryption to TPMs one problem that arises is what strategy to adopt if the TPM is lost, due to hardware failure: if I need the TPM to unlock my encrypted volume, what do I do if I need the data but lost the TPM?

The answer here is supporting recovery keys (this is similar to how other OSes approach this). Recovery keys are pretty much the same concept as passwords. The main difference being that they are computer generated rather than user-chosen. Because of that they typically have much higher entropy (which makes them more annoying to type in, i.e you want to use them only when you must, not day-to-day). By having higher entropy they are useful in combination with TPM, FIDO2 or PKCS#11 based unlocking: unlike a combination with passwords they do not compromise the higher strength of protection that TPM/FIDO2/PKCS#11 based unlocking is supposed to provide.

Current versions of systemd-cryptenroll(1) implement a recovery key concept in an attempt to address this problem. You may enroll any combination of TPM chips, PKCS#11 tokens, FIDO2 tokens, recovery keys and passwords on the same LUKS volume. When enrolling a recovery key it is generated and shown on screen both in text form and as QR code you can scan off screen if you like. The idea is write down/store this recovery key at a safe place so that you can use it when you need it. Note that such recovery keys can be entered wherever a LUKS password is requested, i.e. after generation they behave pretty much the same as a regular password.

TPM PCR Brittleness

Locking devices to TPMs and enforcing a PCR policy with this (i.e. configuring the TPM key to be unlockable only if certain PCRs match certain values, and thus requiring the OS to be in a certain state) brings a problem with it: TPM PCR brittleness. If the key you want to unlock with the TPM requires the OS to be in a specific state (i.e. that all OS components' hashes match certain expectations or similar) then doing OS updates might have the affect of making your key inaccessible: the OS updates will cause the code to change, and thus the hashes of the code, and thus certain PCRs. (Thankfully, you unrolled a recovery key, as described above, so this doesn't mean you lost your data, right?).

To address this I'd suggest three strategies:

  1. Most importantly: don't actually use the TPM PCRs that contain code hashes. There are actually multiple PCRs defined, each containing measurements of different aspects of the boot process. My recommendation is to bind keys to PCR 7 only, a PCR that contains measurements of the UEFI SecureBoot certificate databases. Thus, the keys will remain accessible as long as these databases remain the same, and updates to code will not affect it (updates to the certificate databases will, and they do happen too, though hopefully much less frequent then code updates). Does this reduce security? Not much, no, because the code that's run is after all not just measured but also validated via code signatures, and those signatures are validated with the aforementioned certificate databases. Thus binding an encrypted TPM key to PCR 7 should enforce a similar level of trust in the boot/OS code as binding it to a PCR with hashes of specific versions of that code. i.e. using PCR 7 means you say "every code signed by these vendors is allowed to unlock my key" while using a PCR that contains code hashes means "only this exact version of my code may access my key".

  2. Use LUKS key management to enroll multiple versions of the TPM keys in relevant volumes, to support multiple versions of the OS code (or multiple versions of the certificate database, as discussed above). Specifically: whenever an update is done that might result changing the relevant PCRs, pre-calculate the new PCRs, and enroll them in an additional LUKS slot on the relevant volumes. This means that the unlocking keys tied to the TPM remain accessible in both states of the system. Eventually, once rebooted after the update, remove the old slots.

  3. If these two strategies didn't work out (maybe because the OS/firmware was updated outside of OS control, or the update mechanism was aborted at the wrong time) and the TPM PCRs changed unexpectedly, and the user now needs to use their recovery key to get access to the OS back, let's handle this gracefully and automatically reenroll the current TPM PCRs at boot, after the recovery key checked out, so that for future boots everything is in order again.

Other approaches can work too: for example, some OSes simply remove TPM PCR policy protection of disk encryption keys altogether immediately before OS or firmware updates, and then reenable it right after. Of course, this opens a time window where the key bound to the TPM is much less protected than people might assume. I'd try to avoid such a scheme if possible.

Anything Else?

So, given that we are talking about idealized systems: I personally actually think the ideal OS would be much simpler, and thus more secure than this:

I'd try to ditch the Shim, and instead focus on enrolling the distribution vendor keys directly in the UEFI firmware certificate list. This is actually supported by all firmwares too. This has various benefits: it's no longer necessary to bind everything to Microsoft's root key, you can just enroll your own stuff and thus make sure only what you want to trust is trusted and nothing else. To make an approach like this easier, we have been working on doing automatic enrollment of these keys from the systemd-boot boot loader, see this work in progress for details. This way the Firmware will authenticate the boot loader/kernel/initrd without any further component for this in place.

I'd also not bother with a separate boot partition, and just use the ESP for everything. The ESP is required anyway by the firmware, and is good enough for storing the few files we need.

FAQ

Can I implement all of this in my distribution today?

Probably not. While the big issues have mostly been addressed there's a lot of integration work still missing. As you might have seen I linked some PRs that haven't even been merged into our tree yet, and definitely not been released yet or even entered the distributions.

Will this show up in Fedora/Debian/Ubuntu soon?

I don't know. I am making a proposal how these things might work, and am working on getting various building blocks for this into shape. What the distributions do is up to them. But even if they don't follow the recommendations I make 100%, or don't want to use the building blocks I propose I think it's important they start thinking about this, and yes, I think they should be thinking about defaulting to setups like this.

Work for measuring/signing initrds on Fedora has been started, here's a slide deck with some information about it.

But isn't a TPM evil?

Some corners of the community tried (unfortunately successfully to some degree) to paint TPMs/Trusted Computing/SecureBoot as generally evil technologies that stop us from using our systems the way we want. That idea is rubbish though, I think. We should focus on what it can deliver for us (and that's a lot I think, see above), and appreciate the fact we can actually use it to kick out perceived evil empires from our devices instead of being subjected to them. Yes, the way SecureBoot/TPMs are defined puts you in the driver seat if you want — and you may enroll your own certificates to keep out everything you don't like.

What if my system doesn't have a TPM?

TPMs are becoming quite ubiquitous, in particular as the upcoming Windows versions will require them. In general I think we should focus on modern, fully equipped systems when designing all this, and then find fall-backs for more limited systems. Frankly it feels as if so far the design approach for all this was the other way round: try to make the new stuff work like the old rather than the old like the new (I mean, to me it appears this thinking is the main raison d'être for the Grub boot loader).

More specifically, on the systems where we have no TPM we ultimately cannot provide the same security guarantees as for those which have. So depending on the resource to protect we should fall back to different TPM-less mechanisms. For example, if we have no TPM then the root file system should probably be encrypted with a user provided password, typed in at boot as before. And for the encrypted boot credentials we probably should simply not encrypt them, and place them in the ESP unencrypted.

Effectively this means: without TPM you'll still get protection regarding the basic attack scenario, as before, but not the other two.

What if my system doesn't have UEFI?

Many of the mechanisms explained above taken individually do not require UEFI. But of course the chain of trust suggested above requires something like UEFI SecureBoot. If your system lacks UEFI it's probably best to find work-alikes to the technologies suggested above, but I doubt I'll be able to help you there.

rpm/dpkg already cryptographically validates all packages at installation time (gpg), why would I need more than that?

This type of package validation happens once: at the moment of installation (or update) of the package, but not anymore when the data installed is actually used. Thus when an attacker manages to modify the package data after installation and before use they can make any change they like without this ever being noticed. Such package download validation does address certain attack scenarios (i.e. man-in-the-middle attacks on network downloads), but it doesn't protect you from attackers with physical access, as described in the attack scenarios above.

Systems such as ostree aren't better than rpm/dpkg regarding this BTW, their data is not validated on use either, but only during download or when processing tree checkouts.

Key really here is that the scheme explained here provides offline protection for the data "at rest" — even someone with physical access to your device cannot easily make changes that aren't noticed on next use. rpm/dpkg/ostree provide online protection only: as long as the system remains up, and all OS changes are done through the intended program code-paths, and no one has physical access everything should be good. In today's world I am sure this is not good enough though. As mentioned most modern OSes provide offline protection for the data at rest in one way or another. Generic Linux distributions are terribly behind on this.

This is all so desktop/laptop focused, what about servers?

I am pretty sure servers should provide similar security guarantees as outlined above. In a way servers are a much simpler case: there are no users and no interactivity. Thus the discussion of /home/ and what it contains and of user passwords doesn't matter. However, the authenticated initrd and the unattended TPM-based encryption I think are very important for servers too, in a trusted data center environment. It provides security guarantees so far not given by Linux server OSes.

I'd like to help with this, or discuss/comment on this

Submit patches or reviews through GitHub. General discussion about this is best done on the systemd mailing list.


The Wondrous World of Discoverable GPT Disk Images

TL;DR: Tag your GPT partitions with the right, descriptive partition types, and the world will become a better place.

A number of years ago we started the Discoverable Partitions Specification which defines GPT partition type UUIDs and partition flags for the various partitions Linux systems typically deal with. Before the specification all Linux partitions usually just used the same type, basically saying "Hey, I am a Linux partition" and not much else. With this specification the GPT partition type, flags and label system becomes a lot more expressive, as it can tell you:

  1. What kind of data a partition contains (i.e. is this swap data, a file system or Verity data?)
  2. What the purpose/mount point of a partition is (i.e. is this a /home/ partition or a root file system?)
  3. What CPU architecture a partition is intended for (i.e. is this a root partition for x86-64 or for aarch64?)
  4. Shall this partition be mounted automatically? (i.e. without specifically be configured via /etc/fstab)
  5. And if so, shall it be mounted read-only?
  6. And if so, shall the file system be grown to its enclosing partition size, if smaller?
  7. Which partition contains the newer version of the same data (i.e. multiple root file systems, with different versions)

By embedding all of this information inside the GPT partition table disk images become self-descriptive: without requiring any other source of information (such as /etc/fstab) if you look at a compliant GPT disk image it is clear how an image is put together and how it should be used and mounted. This self-descriptiveness in particular breaks one philosophical weirdness of traditional Linux installations: the original source of information which file system the root file system is, typically is embedded in the root file system itself, in /etc/fstab. Thus, in a way, in order to know what the root file system is you need to know what the root file system is. 🤯 🤯 🤯

(Of course, the way this recursion is traditionally broken up is by then copying the root file system information from /etc/fstab into the boot loader configuration, resulting in a situation where the primary source of information for this — i.e. /etc/fstab — is actually mostly irrelevant, and the secondary source — i.e. the copy in the boot loader — becomes the configuration that actually matters.)

Today, the GPT partition type UUIDs defined by the specification have been adopted quite widely, by distributions and their installers, as well as a variety of partitioning tools and other tools.

In this article I want to highlight how the various tools the systemd project provides make use of the concepts the specification introduces.

But before we start with that, let's underline why tagging partitions with these descriptive partition type UUIDs (and the associated partition flags) is a good thing, besides the philosophical points made above.

  1. Simplicity: in particular OS installers become simpler — adjusting /etc/fstab as part of the installation is not necessary anymore, as the partitioning step already put all information into place for assembling the system properly at boot. i.e. installing doesn't mean that you always have to get fdisk and /etc/fstab into place, the former suffices entirely.

  2. Robustness: since partition tables mostly remain static after installation the chance of corruption is much lower than if the data is stored in file systems (e.g. in /etc/fstab). Moreover by associating the metadata directly with the objects it describes the chance of things getting out of sync is reduced. (i.e. if you lose /etc/fstab, or forget to rerun your initrd builder you still know what a partition is supposed to be just by looking at it.)

  3. Programmability: if partitions are self-descriptive it's much easier to automatically process them with various tools. In fact, this blog story is mostly about that: various systemd tools can naturally process disk images prepared like this.

  4. Alternative entry points: on traditional disk images, the boot loader needs to be told which kernel command line option root= to use, which then provides access to the root file system, where /etc/fstab is then found which describes the rest of the file systems. Where precisely root= is configured for the boot loader highly depends on the boot loader and distribution used, and is typically encoded in a Turing complete programming language (Grub…). This makes it very hard to automatically determine the right root file system to use, to implement alternative entry points to the system. By alternative entry points I mean other ways to boot the disk image, specifically for running it as a systemd-nspawn container — but this extends to other mechanisms where the boot loader may be bypassed to boot up the system, for example qemu when configured without a boot loader.

  5. User friendliness: it's simply a lot nicer for the user looking at a partition table if the partition table explains what is what, instead of just saying "Hey, this is a Linux partition!" and nothing else.

Uses for the concept

Now that we cleared up the Why?, lets have a closer look how this is currently used and exposed in systemd's various components.

Use #1: Running a disk image in a container

If a disk image follows the Discoverable Partition Specification then systemd-nspawn has all it needs to just boot it up. Specifically, if you have a GPT disk image in a file foobar.raw and you want to boot it up in a container, just run systemd-nspawn -i foobar.raw -b, and that's it (you can specify a block device like /dev/sdb too if you like). It becomes easy and natural to prepare disk images that can be booted either on a physical machine, inside a virtual machine manager or inside such a container manager: the necessary meta-information is included in the image, easily accessible before actually looking into its file systems.

Use #2: Booting an OS image on bare-metal without /etc/fstab or kernel command line root=

If a disk image follows the specification in many cases you can remove /etc/fstab (or never even install it) — as the basic information needed is already included in the partition table. The systemd-gpt-auto-generator logic implements automatic discovery of the root file system as well as all auxiliary file systems. (Note that the former requires an initrd that uses systemd, some more conservative distributions do not support that yet, unfortunately). Effectively this means you can boot up a kernel/initrd with an entirely empty kernel command line, and the initrd will automatically find the root file system (by looking for a suitably marked partition on the same drive the EFI System Partition was found on).

(Note, if /etc/fstab or root= exist and contain relevant information they always takes precedence over the automatic logic. This is in particular useful to tweaks thing by specifying additional mount options and such.)

Use #3: Mounting a complex disk image for introspection or manipulation

The systemd-dissect tool may be used to introspect and manipulate OS disk images that implement the specification. If you pass the path to a disk image (or block device) it will extract various bits of useful information from the image (e.g. what OS is this? what partitions to mount?) and display it.

With the --mount switch a disk image (or block device) can be mounted to some location. This is useful for looking what is inside it, or changing its contents. This will dissect the image and then automatically mount all contained file systems matching their GPT partition description to the right places, so that you subsequently could chroot into it. (But why chroot if you can just use systemd-nspawn? 😎)

Use #4: Copying files in and out of a disk image

The systemd-dissect tool also has two switches --copy-from and --copy-to which allow copying files out of or into a compliant disk image, taking all included file systems and the resulting mount hierarchy into account.

Use #5: Running services directly off a disk image

The RootImage= setting in service unit files accepts paths to compliant disk images (or block device nodes), and can mount them automatically, running service binaries directly off them (in chroot() style). In fact, this is the base for the Portable Service concept of systemd.

Use #6: Provisioning disk images

systemd provides various tools that can run operations provisioning disk images in an "offline" mode. Specifically:

systemd-tmpfiles

With the --image= switch systemd-tmpfiles can directly operate on a disk image, and for example create all directories and other inodes defined in its declarative configuration files included in the image. This can be useful for example to set up the /var/ or /etc/ tree according to such configuration before first boot.

systemd-sysusers

Similar, the --image= switch of systemd-sysusers tells the tool to read the declarative system user specifications included in the image and synthesizes system users from it, writing them to the /etc/passwd (and related) files in the image. This is useful for provisioning these users before the first boot, for example to ensure UID/GID numbers are pre-allocated, and such allocations not delayed until first boot.

systemd-machine-id-setup

The --image= switch of systemd-machine-id-setup may be used to provision a fresh machine ID into /etc/machine-id of a disk image, before first boot.

systemd-firstboot

The --image= switch of systemd-firstboot may be used to set various basic system setting (such as root password, locale information, hostname, …) on the specified disk image, before booting it up.

Use #7: Extracting log information

The journalctl switch --image= may be used to show the journal log data included in a disk image (or, as usual, the specified block device). This is very useful for analyzing failed systems offline, as it gives direct access to the logs without any further, manual analysis.

Use #8: Automatic repartitioning/growing of file systems

The systemd-repart tool may be used to repartition a disk or image in an declarative and additive way. One primary use-case for it is to run during boot on physical or VM systems to grow the root file system to the disk size, or to add in, format, encrypt, populate additional partitions at boot.

With its --image= switch it the tool may operate on compliant disk images in offline mode of operation: it will then read the partition definitions that shall be grown or created off the image itself, and then apply them to the image. This is particularly useful in combination with the --size= which allows growing disk images to the specified size.

Specifically, consider the following work-flow: you download a minimized disk image foobar.raw that contains only the minimized root file system (and maybe an ESP, if you want to boot it on bare-metal, too). You then run systemd-repart --image=foo.raw --size=15G to enlarge the image to the 15G, based on the declarative rules defined in the repart.d/ drop-in files included in the image (this means this can grow the root partition, and/or add in more partitions, for example for /srv or so, maybe encrypted with a locally generated key or so). Then, you proceed to boot it up with systemd-nspawn --image=foo.raw -b, making use of the full 15G.

Versioning + Multi-Arch

Disk images implementing this specifications can carry OS executables in one of three ways:

  1. Only a root file system

  2. Only a /usr/ file system (in which case the root file system is automatically picked as tmpfs).

  3. Both a root and a /usr/file system (in which case the two are combined, the /usr/ file system mounted into the root file system, and the former possibly in read-only fashion`)

They may also contain OS executables for different architectures, permitting "multi-arch" disk images that can safely boot up on multiple CPU architectures. As the root and /usr/ partition type UUIDs are specific to architectures this is easily done by including one such partition for x86-64, and another for aarch64. If the image is now used on an x86-64 system automatically the former partition is used, on aarch64 the latter.

Moreover, these OS executables may be contained in different versions, to implement a simple versioning scheme: when tools such as systemd-nspawn or systemd-gpt-auto-generator dissect a disk image, and they find two or more root or /usr/ partitions of the same type UUID, they will automatically pick the one whose GPT partition label (a 36 character free-form string every GPT partition may have) is the newest according to strverscmp() (OK, truth be told, we don't use strverscmp() as-is, but a modified version with some more modern syntax and semantics, but conceptually identical).

This logic allows to implement a very simple and natural A/B update scheme: an updater can drop multiple versions of the OS into separate root or /usr/ partitions, always updating the partition label to the version included there-in once the download is complete. All of the tools described here will then honour this, and always automatically pick the newest version of the OS.

Verity

When building modern OS appliances, security is highly relevant. Specifically, offline security matters: an attacker with physical access should have a difficult time modifying the OS in a way that isn't noticed. i.e. think of a car or a cell network base station: these appliances are usually parked/deployed in environments attackers can get physical access to: it's essential that in this case the OS itself sufficiently protected, so that the attacker cannot just mount the OS file system image, make modifications (inserting a backdoor, spying software or similar) and the system otherwise continues to run without this being immediately detected.

A great way to implement offline security is via Linux' dm-verity subsystem: it allows to securely bind immutable disk IO to a single, short trusted hash value: if an attacker manages to offline modify the disk image the modified disk image won't match the trusted hash anymore, and will not be trusted anymore (depending on policy this then just result in IO errors being generated, or automatic reboot/power-off).

The Discoverable Partitions Specification declares how to include Verity validation data in disk images, and how to relate them to the file systems they protect, thus making if very easy to deploy and work with such protected images. For example systemd-nspawn supports a --root-hash= switch, which accepts the Verity root hash and then will automatically assemble dm-verity with this, automatically matching up the payload and verity partitions. (Alternatively, just place a .roothash file next to the image file).

Future

The above already is a powerful tool set for working with disk images. However, there are some more areas I'd like to extend this logic to:

bootctl

Similar to the other tools mentioned above, bootctl (which is a tool to interface with the boot loader, and install/update systemd's own EFI boot loader sd-boot) should learn a --image= switch, to make installation of the boot loader on disk images easy and natural. It would automatically find the ESP and other relevant partitions in the image, and copy the boot loader binaries into them (or update them).

coredumpctl

Similar to the existing journalctl --image= logic the coredumpctl tool should also gain an --image= switch for extracting coredumps from compliant disk images. The combination of journalctl --image= and coredumpctl --image= would make it exceptionally easy to work with OS disk images of appliances and extracting logging and debugging information from them after failures.

And that's all for now. Please refer to the specification and the man pages for further details. If your distribution's installer does not yet tag the GPT partition it creates with the right GPT type UUIDs, consider asking them to do so.

Thank you for your time.


File Descriptor Limits

TL;DR: don't use select() + bump the RLIMIT_NOFILE soft limit to the hard limit in your modern programs.

The primary way to reference, allocate and pin runtime OS resources on Linux today are file descriptors ("fds"). Originally they were used to reference open files and directories and maybe a bit more, but today they may be used to reference almost any kind of runtime resource in Linux userspace, including open devices, memory (memfd_create(2)), timers (timefd_create(2)) and even processes (with the new pidfd_open(2) system call). In a way, the philosophically skewed UNIX concept of "everything is a file" through the proliferation of fds actually acquires a bit of sensible meaning: "everything has a file descriptor" is certainly a much better motto to adopt.

Because of this proliferation of fds, non-trivial modern programs tend to have to deal with substantially more fds at the same time than they traditionally did. Today, you'll often encounter real-life programs that have a few thousand fds open at the same time.

Like on most runtime resources on Linux limits are enforced on file descriptors: once you hit the resource limit configured via RLIMIT_NOFILE any attempt to allocate more is refused with the EMFILE error — until you close a couple of those you already have open.

Because fds weren't such a universal concept traditionally, the limit of RLIMIT_NOFILE used to be quite low. Specifically, when the Linux kernel first invokes userspace it still sets RLIMIT_NOFILE to a low value of 1024 (soft) and 4096 (hard). (Quick explanation: the soft limit is what matters and causes the EMFILE issues, the hard limit is a secondary limit that processes may bump their soft limit to — if they like — without requiring further privileges to do so. Bumping the limit further would require privileges however.). A limit of 1024 fds made fds a scarce resource: APIs tried to be careful with using fds, since you simply couldn't have that many of them at the same time. This resulted in some questionable coding decisions and concepts at various places: often secondary descriptors that are very similar to fds — but were not actually fds — were introduced (e.g. inotify watch descriptors), simply to avoid for them the low limits enforced on true fds. Or code tried to aggressively close fds when not absolutely needing them (e.g. ftw()/nftw()), losing the nice + stable "pinning" effect of open fds.

Worse though is that certain OS level APIs were designed having only the low limits in mind. The worst offender being the BSD/POSIX select(2) system call: it only works with fds in the numeric range of 0…1023 (aka FD_SETSIZE-1). If you have an fd outside of this range, tough luck: select() won't work, and only if you are lucky you'll detect that and can handle it somehow.

Linux fds are exposed as simple integers, and for most calls it is guaranteed that the lowest unused integer is allocated for new fds. Thus, as long as the RLIMIT_NOFILE soft limit is set to 1024 everything remains compatible with select(): the resulting fds will also be below 1024. Yay. If we'd bump the soft limit above this threshold though and at some point in time an fd higher than the threshold is allocated, this fd would not be compatible with select() anymore.

Because of that, indiscriminately increasing the soft RLIMIT_NOFILE resource limit today for every userspace process is problematic: as long as there's userspace code still using select() doing so will risk triggering hard-to-handle, hard-to-debug errors all over the place.

However, given the nowadays ubiquitous use of fds for all kinds of resources (did you know, an eBPF program is an fd? and a cgroup too? and attaching an eBPF program to cgroup is another fd? …), we'd really like to raise the limit anyway. 🤔

So before we continue thinking about this problem, let's make the problem more complex (…uh, I mean… "more exciting") first. Having just one hard and one soft per-process limit on fds is boring. Let's add more limits on fds to the mix. Specifically on Linux there are two system-wide sysctls: fs.nr_open and fs.file-max. (Don't ask me why one uses a dash and the other an underscore, or why there are two of them...) On today's kernels they kinda lost their relevance. They had some originally, because fds weren't accounted by any other counter. But today, the kernel tracks fds mostly as small pieces of memory allocated on userspace requests — because that's ultimately what they are —, and thus charges them to the memory accounting done anyway.

So now, we have four limits (actually: five if you count the memory accounting) on the same kind of resource, and all of them make a resource artificially scarce that we don't want to be scarce. So what to do?

Back in systemd v240 already (i.e. 2019) we decided to do something about it. Specifically:

  • Automatically at boot we'll now bump the two sysctls to their maximum, making them effectively ineffective. This one was easy. We got rid of two pretty much redundant knobs. Nice!

  • The RLIMIT_NOFILE hard limit is bumped substantially to 512K. Yay, cheap fds! You may have an fd, and you, and you as well, everyone may have an fd!

  • But … we left the soft RLIMIT_NOFILE limit at 1024. We weren't quite ready to break all programs still using select() in 2019 yet. But it's not as bad as it might sound I think: given the hard limit is bumped every program can easily opt-in to a larger number of fds, by setting the soft limit to the hard limit early on — without requiring privileges.

So effectively, with this approach fds should be much less scarce (at least for programs that opt into that), and the limits should be much easier to configure, since there are only two knobs now one really needs to care about:

  • Configure the RLIMIT_NOFILE hard limit to the maximum number of fds you actually want to allow a process.

  • In the program code then either bump the soft to the hard limit, or not. If you do, you basically declare "I understood the problem, I promise to not use select(), drown me fds please!". If you don't then effectively everything remains as it always was.

Apparently this approach worked, since the negative feedback on change was even scarcer than fds traditionally were (ha, fun!). We got reports from pretty much only two projects that were bitten by the change (one being a JVM implementation): they already bumped their soft limit automatically to their hard limit during program initialization, and then allocated an array with one entry per possible fd. With the new high limit this resulted in one massive allocation that traditionally was just a few K, and this caused memory checks to be hit.

Anyway, here's the take away of this blog story:

  • Don't use select() anymore in 2021. Use poll(), epoll, iouring, …, but for heaven's sake don't use select(). It might have been all the rage in the 1990s but it doesn't scale and is simply not designed for today's programs. I wished the man page of select() would make clearer how icky it is and that there are plenty of more preferably APIs.

  • If you hack on a program that potentially uses a lot of fds, add some simple code somewhere to its start-up that bumps the RLIMIT_NOFILE soft limit to the hard limit. But if you do this, you have to make sure your code (and any code that you link to from it) refrains from using select(). (Note: there's at least one glibc NSS plugin using select() internally. Given that NSS modules can end up being loaded into pretty much any process such modules should probably be considered just buggy.)

  • If said program you hack on forks off foreign programs, make sure to reset the RLIMIT_NOFILE soft limit back to 1024 for them. Just because your program might be fine with fds >= 1024 it doesn't mean that those foreign programs might. And unfortunately RLIMIT_NOFILE is inherited down the process tree unless explicitly set.

And that's all I have for today. I hope this was enlightening.


Unlocking LUKS2 volumes with TPM2, FIDO2, PKCS#11 Security Hardware on systemd 248

TL;DR: It's now easy to unlock your LUKS2 volume with a FIDO2 security token (e.g. YubiKey, Nitrokey FIDO2, AuthenTrend ATKey.Pro). And TPM2 unlocking is easy now too.

Blogging is a lot of work, and a lot less fun than hacking. I mostly focus on the latter because of that, but from time to time I guess stuff is just too interesting to not be blogged about. Hence here, finally, another blog story about exciting new features in systemd.

With the upcoming systemd v248 the systemd-cryptsetup component of systemd (which is responsible for assembling encrypted volumes during boot) gained direct support for unlocking encrypted storage with three types of security hardware:

  1. Unlocking with FIDO2 security tokens (well, at least with those which implement the hmac-secret extension; most do). i.e. your YubiKeys (series 5 and above), Nitrokey FIDO2, AuthenTrend ATKey.Pro and such.

  2. Unlocking with TPM2 security chips (pretty ubiquitous on non-budget PCs/laptops/…)

  3. Unlocking with PKCS#11 security tokens, i.e. your smartcards and older YubiKeys (the ones that implement PIV). (Strictly speaking this was supported on older systemd already, but was a lot more "manual".)

For completeness' sake, let's keep in mind that the component also allows unlocking with these more traditional mechanisms:

  1. Unlocking interactively with a user-entered passphrase (i.e. the way most people probably already deploy it, supported since about forever)

  2. Unlocking via key file on disk (optionally on removable media plugged in at boot), supported since forever.

  3. Unlocking via a key acquired through trivial AF_UNIX/SOCK_STREAM socket IPC. (Also new in v248)

  4. Unlocking via recovery keys. These are pretty much the same thing as a regular passphrase (and in fact can be entered wherever a passphrase is requested) — the main difference being that they are always generated by the computer, and thus have guaranteed high entropy, typically higher than user-chosen passphrases. They are generated in a way they are easy to type, in many cases even if the local key map is misconfigured. (Also new in v248)

In this blog story, let's focus on the first three items, i.e. those that talk to specific types of hardware for implementing unlocking.

To make working with security tokens and TPM2 easy, a new, small tool was added to the systemd tool set: systemd-cryptenroll. It's only purpose is to make it easy to enroll your security token/chip of choice into an encrypted volume. It works with any LUKS2 volume, and embeds a tiny bit of meta-information into the LUKS2 header with parameters necessary for the unlock operation.

Unlocking with FIDO2

So, let's see how this fits together in the FIDO2 case. Most likely this is what you want to use if you have one of these fancy FIDO2 tokens (which need to implement the hmac-secret extension, as mentioned). Let's say you already have your LUKS2 volume set up, and previously unlocked it with a simple passphrase. Plug in your token, and run:

# systemd-cryptenroll --fido2-device=auto /dev/sda5

(Replace /dev/sda5 with the underlying block device of your volume).

This will enroll the key as an additional way to unlock the volume, and embeds all necessary information for it in the LUKS2 volume header. Before we can unlock the volume with this at boot, we need to allow FIDO2 unlocking via /etc/crypttab. For that, find the right entry for your volume in that file, and edit it like so:

myvolume /dev/sda5 - fido2-device=auto

Replace myvolume and /dev/sda5 with the right volume name, and underlying device of course. Key here is the fido2-device=auto option you need to add to the fourth column in the file. It tells systemd-cryptsetup to use the FIDO2 metadata now embedded in the LUKS2 header, wait for the FIDO2 token to be plugged in at boot (utilizing systemd-udevd, …) and unlock the volume with it.

And that's it already. Easy-peasy, no?

Note that all of this doesn't modify the FIDO2 token itself in any way. Moreover you can enroll the same token in as many volumes as you like. Since all enrollment information is stored in the LUKS2 header (and not on the token) there are no bounds on any of this. (OK, well, admittedly, there's a cap on LUKS2 key slots per volume, i.e. you can't enroll more than a bunch of keys per volume.)

Unlocking with PKCS#11

Let's now have a closer look how the same works with a PKCS#11 compatible security token or smartcard. For this to work, you need a device that can store an RSA key pair. I figure most security tokens/smartcards that implement PIV qualify. How you actually get the keys onto the device might differ though. Here's how you do this for any YubiKey that implements the PIV feature:

# ykman piv reset
# ykman piv generate-key -a RSA2048 9d pubkey.pem
# ykman piv generate-certificate --subject "Knobelei" 9d pubkey.pem
# rm pubkey.pem

(This chain of commands erases what was stored in PIV feature of your token before, be careful!)

For tokens/smartcards from other vendors a different series of commands might work. Once you have a key pair on it, you can enroll it with a LUKS2 volume like so:

# systemd-cryptenroll --pkcs11-token-uri=auto /dev/sda5

Just like the same command's invocation in the FIDO2 case this enrolls the security token as an additional way to unlock the volume, any passphrases you already have enrolled remain enrolled.

For the PKCS#11 case you need to edit your /etc/crypttab entry like this:

myvolume /dev/sda5 - pkcs11-uri=auto

If you have a security token that implements both PKCS#11 PIV and FIDO2 I'd probably enroll it as FIDO2 device, given it's the more contemporary, future-proof standard. Moreover, it requires no special preparation in order to get an RSA key onto the device: FIDO2 keys typically just work.

Unlocking with TPM2

Most modern (non-budget) PC hardware (and other kind of hardware too) nowadays comes with a TPM2 security chip. In many ways a TPM2 chip is a smartcard that is soldered onto the mainboard of your system. Unlike your usual USB-connected security tokens you thus cannot remove them from your PC, which means they address quite a different security scenario: they aren't immediately comparable to a physical key you can take with you that unlocks some door, but they are a key you leave at the door, but that refuses to be turned by anyone but you.

Even though this sounds a lot weaker than the FIDO2/PKCS#11 model TPM2 still bring benefits for securing your systems: because the cryptographic key material stored in TPM2 devices cannot be extracted (at least that's the theory), if you bind your hard disk encryption to it, it means attackers cannot just copy your disk and analyze it offline — they always need access to the TPM2 chip too to have a chance to acquire the necessary cryptographic keys. Thus, they can still steal your whole PC and analyze it, but they cannot just copy the disk without you noticing and analyze the copy.

Moreover, you can bind the ability to unlock the harddisk to specific software versions: for example you could say that only your trusted Fedora Linux can unlock the device, but not any arbitrary OS some hacker might boot from a USB stick they plugged in. Thus, if you trust your OS vendor, you can entrust storage unlocking to the vendor's OS together with your TPM2 device, and thus can be reasonably sure intruders cannot decrypt your data unless they both hack your OS vendor and steal/break your TPM2 chip.

Here's how you enroll your LUKS2 volume with your TPM2 chip:

# systemd-cryptenroll --tpm2-device=auto --tpm2-pcrs=7 /dev/sda5

This looks almost as straightforward as the two earlier sytemd-cryptenroll command lines — if it wasn't for the --tpm2-pcrs= part. With that option you can specify to which TPM2 PCRs you want to bind the enrollment. TPM2 PCRs are a set of (typically 24) hash values that every TPM2 equipped system at boot calculates from all the software that is invoked during the boot sequence, in a secure, unfakable way (this is called "measurement"). If you bind unlocking to a specific value of a specific PCR you thus require the system has to follow the same sequence of software at boot to re-acquire the disk encryption key. Sounds complex? Well, that's because it is.

For now, let's see how we have to modify your /etc/crypttab to unlock via TPM2:

myvolume /dev/sda5 - tpm2-device=auto

This part is easy again: the tpm2-device= option is what tells systemd-cryptsetup to use the TPM2 metadata from the LUKS2 header and to wait for the TPM2 device to show up.

Bonus: Recovery Key Enrollment

FIDO2, PKCS#11 and TPM2 security tokens and chips pair well with recovery keys: since you don't need to type in your password everyday anymore it makes sense to get rid of it, and instead enroll a high-entropy recovery key you then print out or scan off screen and store a safe, physical location. i.e. forget about good ol' passphrase-based unlocking, go for FIDO2 plus recovery key instead! Here's how you do it:

# systemd-cryptenroll --recovery-key /dev/sda5

This will generate a key, enroll it in the LUKS2 volume, show it to you on screen and generate a QR code you may scan off screen if you like. The key has highest entropy, and can be entered wherever you can enter a passphrase. Because of that you don't have to modify /etc/crypttab to make the recovery key work.

Future

There's still plenty room for further improvement in all of this. In particular for the TPM2 case: what the text above doesn't really mention is that binding your encrypted volume unlocking to specific software versions (i.e. kernel + initrd + OS versions) actually sucks hard: if you naively update your system to newer versions you might lose access to your TPM2 enrolled keys (which isn't terrible, after all you did enroll a recovery key — right? — which you then can use to regain access). To solve this some more integration with distributions would be necessary: whenever they upgrade the system they'd have to make sure to enroll the TPM2 again — with the PCR hashes matching the new version. And whenever they remove an old version of the system they need to remove the old TPM2 enrollment. Alternatively TPM2 also knows a concept of signed PCR hash values. In this mode the distro could just ship a set of PCR signatures which would unlock the TPM2 keys. (But quite frankly I don't really see the point: whether you drop in a signature file on each system update, or enroll a new set of PCR hashes in the LUKS2 header doesn't make much of a difference). Either way, to make TPM2 enrollment smooth some more integration work with your distribution's system update mechanisms need to happen. And yes, because of this OS updating complexity the example above — where I referenced your trusty Fedora Linux — doesn't actually work IRL (yet? hopefully…). Nothing updates the enrollment automatically after you initially enrolled it, hence after the first kernel/initrd update you have to manually re-enroll things again, and again, and again … after every update.

The TPM2 could also be used for other kinds of key policies, we might look into adding later too. For example, Windows uses TPM2 stuff to allow short (4 digits or so) "PINs" for unlocking the harddisk, i.e. kind of a low-entropy password you type in. The reason this is reasonably safe is that in this case the PIN is passed to the TPM2 which enforces that not more than some limited amount of unlock attempts may be made within some time frame, and that after too many attempts the PIN is invalidated altogether. Thus making dictionary attacks harder (which would normally be easier given the short length of the PINs).

Postscript

(BTW: Yubico sent me two YubiKeys for testing, Nitrokey a Nitrokey FIDO2, and AuthenTrend three ATKey.Pro tokens, thank you! — That's why you see all those references to YubiKey/Nitrokey/AuthenTrend devices in the text above: it's the hardware I had to test this with. That said, I also tested the FIDO2 stuff with a SoloKey I bought, where it also worked fine. And yes, you!, other vendors!, who might be reading this, please send me your security tokens for free, too, and I might test things with them as well. No promises though. And I am not going to give them back, if you do, sorry. ;-))


ASG! 2019 CfP Re-Opened!

The All Systems Go! 2019 Call for Participation Re-Opened for ONE DAY!

Due to popular request we have re-opened the Call for Participation (CFP) for All Systems Go! 2019 for one day. It will close again TODAY, on 15 of July 2019, midnight Central European Summit Time! If you missed the deadline so far, we’d like to invite you to submit your proposals for consideration to the CFP submission site quickly! (And yes, this is the last extension, there's not going to be any more extensions.)

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All Systems Go! is everybody's favourite low-level Userspace Linux conference, taking place in Berlin, Germany in September 20-22, 2019.

For more information please visit our conference website!

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