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Using composefs with OSTree

  1. composefs
    1. Enabling composefs (unsigned)
    2. composefs configuration
    3. Integrity of backing OSTree objects
    4. Injecting composefs digests
    5. Signatures
  2. Requirements
  3. Status
  4. Compatiblity
  5. Comparison with other approaches
  6. Further references

composefs

The composefs project is a new hybrid Linux stacking filesystem that provides many benefits when used for bootable host systems, such as a strong story for integrity.

At the current time, integration of composefs and ostree is experimental. This issue tracks the latest status.

Enabling composefs (unsigned)

When building a disk image or to transition an existing system, run:

ostree config --repo=/ostree/repo set ex-integrity.composefs true

This will ensure that any future deployments (e.g. created by ostree admin upgrade) have a .ostree.cfs file in the deployment directory which is a mountable composefs metadata file, with a “backing store” directory that is shared with the current /ostree/repo/objects.

composefs configuration

The ostree-prepare-root binary will look for ostree/prepare-root.conf in /etc and /usr/lib in the initramfs. Using that configuration file you can enable composefs, and specify an Ed25519 public key to validate the booted commit.

See the manpage for ostree-prepare-root for details of how to configure it.

Integrity of backing OSTree objects

In ostree/prepare-root.conf, if composefs.enabled is set to signed or verity, before the content of a file in the mounted composefs is read, the integrity of its backing OSTree object in /ostree/repo/objects is validated by the digest stored in .ostree.cfs. This can ensure the integrity of the “backing store”.

Injecting composefs digests

When generating an OSTree commit, there is a CLI switch --generate-composefs-metadata and a corresponding C API ostree_repo_commit_add_composefs_metadata. This will inject the composefs digest as metadata into the ostree commit under a metadata key ostree.composefs.v0. Because an OSTree commit can be signed, this allows covering the composefs fsverity digest with a signature.

Signatures

If a commit is signed with an Ed25519 private key (see ostree sign), and composefs.keyfile is specified in prepare-root.conf, then the initrd will find the commit being booted in the system repo and validate its signature against the public key. It will then ensure that the composefs digest being booted has an fs-verity digest matching the one in the commit. This allows a fully trusted read-only /usr.

The exact usage of the signature is up to the user, but a common way to use it with transient keys. This is done like this:

  • Generate a new keypair before each build
  • Embed the public key in the initrd that is part of the commit.
  • Ensure the initrd has a prepare-root.conf with [composefs] enabled=signed, and either use keypath or inject /etc/ostree/initramfs-root-binding.key; for more see man ostree-prepare-root
  • After committing, run ostree sign with the private key.
  • Throw away the private key.

When a transient key is used this way, that ties the initrd with the userspace part from the commit. This means each initrd can only boot the very same userspace it was made for. For example, if an older version of the OS has a security flaw, you can’t boot a new fixed (signed) initrd and have it boot the older userspace with the flaw.

Requirements

The current default composefs integration in ostree does not have any requirements from the underlying kernel and filesystem other than having the following kernel options set:

  • CONFIG_OVERLAY_FS
  • CONFIG_BLK_DEV_LOOP
  • CONFIG_EROFS_FS

At the current time, there are no additional userspace runtime requirements.

Status

IMPORTANT The integration with composefs is experimental and subject to change. Please try it and report issues but do not deploy to production systems yet.

Compatiblity

One issue that ostree users transitioning to composefs may hit is that it is no longer possible to add new toplevel directories via the chattr -i / && mkdir /somedir && chattr -i trick. A bit more on this in the following issues:

However, users who were doing things like this probably want to enable the root.transient option; see man ostree-prepare-root which will allow this (but also change other behaviors too).

Comparison with other approaches

There is also support for using IMA with ostree. In short, composefs provides much stronger and more efficient integrity:

  • composefs validates an entire filesystem tree, not just individual files
  • composefs makes files actually read-only, whereas IMA does not by default
  • composefs uses fs-verity which does on-demand verification (IMA by default does a full readahead of every file accessed, though IMA can also use fs-verity as a backend)

Further references

  • https://github.com/containers/composefs
  • https://www.kernel.org/doc/html/next/filesystems/fsverity.html