Re: [dm-devel] [PATCH v4 1/3] dm: Add verity helpers for LoadPin

[Mike Snitzer <snitzer@kernel.org>]

On Wed, May 18 2022 at 11:13P -0400,
Matthias Kaehlcke <mka@chromium.org> wrote:

> Hi Milan,
> 
> On Wed, May 18, 2022 at 09:57:43AM +0200, Milan Broz wrote:
> > On 18/05/2022 01:34, Matthias Kaehlcke wrote:
> > > 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>
> > 
> > ...
> > 
> > > +
> > > +	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.
> 
> > > +
> > > +	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.
> 
> > >   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).

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