On Wed, May 18 2022 at 11:13P -0400,
Matthias Kaehlcke [off-list ref] wrote:

Hi Milan,

On Wed, May 18, 2022 at 09:57:43AM +0200, Milan Broz wrote:

quoted

On 18/05/2022 01:34, Matthias Kaehlcke wrote:

quoted

LoadPin limits loading of kernel modules, firmware and certain
other files to a 'pinned' file system (typically a read-only
rootfs). To provide more flexibility LoadPin is being extended
to also allow loading these files from trusted dm-verity
devices. For that purpose LoadPin can be provided with a list
of verity root digests that it should consider as trusted.

Add a bunch of helpers to allow LoadPin to check whether a DM
device is a trusted verity device. The new functions broadly
fall in two categories: those that need access to verity
internals (like the root digest), and the 'glue' between
LoadPin and verity. The new file dm-verity-loadpin.c contains
the glue functions.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>

...

quoted

+
+	if (dm_verity_get_root_digest(ti, &root_digest, &digest_size))
+		return false;

Almost unrelated note, but as there are more and more situations
that checks verity root digest, shouldn't we export this as read-only
sysfs attribute for DM verity devices?

Attacker can always calculate (but not change) Merkle tree, so this
is not something that need to be hidden.

It would allow userspace to easily enumerate trusted DM devices without
calling kernel ioctls...

I guess that's an option if there are scenarios where it is useful. It
should probably be a separate patch, since it isn't directly related with
extending LoadPin support to trusted verity devices.

quoted

quoted

+
+	table = dm_get_live_table(md, &srcu_idx);
+
+	if (dm_table_get_num_targets(table) != 1)
+		goto out;
+
+	ti = dm_table_get_target(table, 0);
+
+	if (is_trusted_verity_target(ti))
+		trusted = true;

What happens is someone reloads verity table later with
a different content (or even different target type)?
Does LoadPin even care here?

LoadPin cares, but only when new kernel files are loaded. It will then check
against the new verity table, and only allow loading of the file if it comes
from a verity target with a trusted digest.

quoted

quoted

  static struct target_type verity_target = {
  	.name		= "verity",
  	.version	= {1, 8, 0},

Please increase the minor version, it is very useful to detect (in logs)
that the target driver has compatible extensions.

I can do that, but would like to confirm that this is really needed/desired.
This patch adds kernel-internal APIs which aren't accessible to userspace,
that don't impact verity directly, so I'm not sure an increased minor version
would be useful.

Bumping to 1.8.1 is useful to indicate new changes that offer expanded
use of the verity target (even if by LoadPin).