[PATCH v2 0/3] LoadPin: Enable loading from trusted dm-verity devices

[Matthias Kaehlcke]

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) [1] 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 [2] to verify the
integrity of the DLC content.

This series extends LoadPin to allow loading of kernel files
from trusted dm-verity devices. It adds the concept of
trusted verity devices to LoadPin. Userspace can use the new
systl file 'loadpin/trusted_verity_root_digests_path' to
provide LoadPin with the path of a file with a list of root
digests from dm-verity devices that LoadPin should consider
as trusted. This file must be located on the pinned root.

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.

The path of the file with the trusted root digests can
only be written once, which is typically done at boot time.

[1] https://chromium.googlesource.com/chromiumos/platform2/+/HEAD/dlcservice/docs/developer.md
[2] https://www.kernel.org/doc/html/latest/admin-guide/device-mapper/verity.html

(no changes since v1)

Matthias Kaehlcke (3):
  dm: Add verity helpers for LoadPin
  LoadPin: Enable loading from trusted dm-verity devices
  dm: verity-loadpin: Use CONFIG_SECURITY_LOADPIN_VERITY for conditional
    compilation

 drivers/md/Makefile               |   1 +
 drivers/md/dm-verity-loadpin.c    |  80 ++++++++++++
 drivers/md/dm-verity-target.c     |  33 +++++
 drivers/md/dm-verity.h            |   4 +
 include/linux/dm-verity-loadpin.h |  27 ++++
 security/loadpin/Kconfig          |  16 +++
 security/loadpin/loadpin.c        | 200 +++++++++++++++++++++++++++++-
 7 files changed, 360 insertions(+), 1 deletion(-)
 create mode 100644 drivers/md/dm-verity-loadpin.c
 create mode 100644 include/linux/dm-verity-loadpin.h

-- 
2.36.0.rc2.479.g8af0fa9b8e-goog