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

Message

Matthias Kaehlcke April 18, 2022, 9:15 p.m. UTC

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' to
provide LoadPin with a list of root digests from dm-verity
devices that LoadPin should consider as trusted. 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 list of trusted root digests can only be written once
(typically at boot time), to limit the possiblity of
attackers setting up rogue verity devices and marking them
as trusted.

[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


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     |  34 ++++++
 drivers/md/dm-verity.h            |   4 +
 include/linux/dm-verity-loadpin.h |  27 +++++
 security/loadpin/Kconfig          |  11 ++
 security/loadpin/loadpin.c        | 168 +++++++++++++++++++++++++++++-
 7 files changed, 324 insertions(+), 1 deletion(-)
 create mode 100644 drivers/md/dm-verity-loadpin.c
 create mode 100644 include/linux/dm-verity-loadpin.h