On May 13, 2022 9:32:12 AM PDT, Mike Snitzer [off-list ref] wrote:

On Wed, May 04 2022 at  3:54P -0400,
Matthias Kaehlcke [off-list ref] wrote:

quoted

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 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@chromium.org>
---

Changes in v3:
- added securityfs for LoadPin (currently only populated when
  CONFIG_SECURITY_LOADPIN_VERITY=y)
- added uapi include for LoadPin
- changed the interface for setting up the list of trusted
  digests from sysctl to ioctl on securityfs entry
- added stub for loadpin_is_fs_trusted() to be used
  CONFIG_SECURITY_LOADPIN_VERITY is not select
- depend on CONFIG_SECURITYFS instead of CONFIG_SYSTCL
- updated Kconfig help
- minor changes in read_trusted_verity_root_digests()
- updated commit message

Changes in v2:
- userspace now passes the path of the file with the verity digests
  via systcl, instead of the digests themselves
- renamed sysctl file to 'trusted_verity_root_digests_path'
- have CONFIG_SECURITY_LOADPIN_VERITY depend on CONFIG_SYSCTL
- updated Kconfig doc
- updated commit message

 include/uapi/linux/loadpin.h |  19 ++++
 security/loadpin/Kconfig     |  16 +++
 security/loadpin/loadpin.c   | 184 ++++++++++++++++++++++++++++++++++-
 3 files changed, 218 insertions(+), 1 deletion(-)
 create mode 100644 include/uapi/linux/loadpin.h

I would certainly need some Reviewed-by:s from security and/or loadpin
experts if I were to pick this patch up.

Alternatively, since it's mostly touching loadpin, I can carry it in my tree, as long as you've Acked the dm bits. :)

Did you see the issues the kernel test robot emailed about?

You'd do well to fix those issues up when submitting another revision
of this patchset.

Agreed.


-- 
Kees Cook