[PATCH v7 2/3] LoadPin: Enable loading from trusted dm-verity devices

[Mike Snitzer]

On Mon, Jun 27 2022 at 11:35P -0400,
Matthias Kaehlcke <mka at chromium.org> wrote:

> Extend LoadPin to allow loading of kernel files from trusted dm-verity [1]
> devices.
> 
> This change adds the concept of trusted verity devices to LoadPin. LoadPin
> maintains a list of root digests of verity devices it considers trusted.
> Userspace can populate this list through an ioctl on the new LoadPin
> securityfs entry 'dm-verity'. The ioctl receives a file descriptor of
> a file with verity digests as parameter. Verity reads the digests from
> this file after confirming that the file is located on the pinned root.
> The digest file must contain one digest per line. The list of trusted
> digests can only be set up once, which is typically done at boot time.
> 
> When a kernel file is read LoadPin first checks (as usual) whether the file
> is located on the pinned root, if so the file can be loaded. Otherwise, if
> the verity extension is enabled, LoadPin determines whether the file is
> located on a verity backed device and whether the root digest of that
> device is in the list of trusted digests. The file can be loaded if the
> verity device has a trusted root digest.
> 
> Background:
> 
> As of now LoadPin restricts loading of kernel files to a single pinned
> filesystem, typically the rootfs. This works for many systems, however it
> can result in a bloated rootfs (and OTA updates) on platforms where
> multiple boards with different hardware configurations use the same rootfs
> image. Especially when 'optional' files are large it may be preferable to
> download/install them only when they are actually needed by a given board.
> Chrome OS uses Downloadable Content (DLC) [2] to deploy certain 'packages'
> at runtime. As an example a DLC package could contain firmware for a
> peripheral that is not present on all boards. DLCs use dm-verity to verify
> the integrity of the DLC content.
> 
> [1] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html
> [2] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
> 
> Signed-off-by: Matthias Kaehlcke <mka at chromium.org>
> Acked-by: Kees Cook <keescook at chromium.org>

Acked-by: Mike Snitzer <snitzer at kernel.org>