ostree/docs/composefs.md

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# Using composefs with OSTree
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## composefs

The [composefs](https://github.com/containers/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](https://github.com/ostreedev/ostree/issues/2867) 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`.

### Kernel argument ot-composefs

The `ostree-prepare-root` binary will look for a kernel argument called `ot-composefs`.

The default value is `maybe` (this will likely become a build and initramfs-configurable option)
in the future too.

The possible values are:

- `off`: Never use composefs
- `maybe`: Use composefs if supported and there is a composefs image in the deployment directory
- `on`: Require composefs
- `digest=<sha256>`: Require the mounted composefs image to have a particular digest
- `signed=<path>`: Require that the commit is signed as validated by the ed25519 public key specified
   by `path` (the path is resolved in the initrd).

### 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 a ed25519 private key (see `ostree
--sign`), and `signed=/path/to/public.key` is specified on the
commandline, 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 transien 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 kernel commandline has `ot-signed=/path/to/key`
 * After commiting, 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.

## Comparison with other approaches

There is also support for using [IMA](ima.md) 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

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