Re: [PATCH v4 00/14] security: digest_cache LSM

[Paul Moore <paul@paul-moore.com>]

On Thu, Jun 20, 2024 at 12:31 PM Roberto Sassu
<roberto.sassu@huaweicloud.com> wrote:
> On Thu, 2024-06-20 at 12:08 -0400, Paul Moore wrote:
> > On Thu, Jun 20, 2024 at 11:14 AM Roberto Sassu
> > <roberto.sassu@huaweicloud.com> wrote:
> > > On Thu, 2024-06-20 at 10:48 -0400, Paul Moore wrote:
> > > > On Thu, Jun 20, 2024 at 5:12 AM Roberto Sassu
> > > > <roberto.sassu@huaweicloud.com> wrote:
> > > > > On Wed, 2024-06-19 at 14:43 -0400, Paul Moore wrote:
> > > > > > On Wed, Jun 19, 2024 at 12:38 PM Roberto Sassu
> > > > > > <roberto.sassu@huaweicloud.com> wrote:
> > > > > > >
> > > > > > > Making it a kernel subsystem would likely mean replicating what the LSM
> > > > > > > infrastructure is doing, inode (security) blob and being notified about
> > > > > > > file/directory changes.
> > > > > >
> > > > > > Just because the LSM framework can be used for something, perhaps it
> > > > > > even makes the implementation easier, it doesn't mean the framework
> > > > > > should be used for everything.
> > > > >
> > > > > It is supporting 3 LSMs: IMA, IPE and BPF LSM.
> > > > >
> > > > > That makes it a clear target for the security subsystem, and as you
> > > > > suggested to start for IMA, if other kernel subsystems require them, we
> > > > > can make it as an independent subsystem.
> > > >
> > > > Have you discussed the file digest cache functionality with either the
> > > > IPE or BPF LSM maintainers?  While digest_cache may support these
> > >
> > > Well, yes. I was in a discussion since long time ago with Deven and
> > > Fan. The digest_cache LSM is listed in the Use Case section of the IPE
> > > cover letter:
> > >
> > > https://lore.kernel.org/linux-integrity/1716583609-21790-1-git-send-email-wufan@linux.microsoft.com/
> >
> > I would hope to see more than one sentence casually mentioning that
> > there might be some integration in the future.
>
> Sure, I can work more with Fan to do a proper integration.

That seems like a good pre-requisite for turning digest_cache into a
general purpose subsystem.

> > > I also developed an IPE module back in the DIGLIM days:
> > >
> > > https://lore.kernel.org/linux-integrity/a16a628b9e21433198c490500a987121@huawei.com/
> >
> > That looks like more of an fs-verity integration to me.  Yes, of
> > course there would be IPE changes to accept a signature/digest from a
> > digest cache, but that should be minor.
>
> True, but IPE will also benefit from not needing to specify every
> digest in the policy.

Sure, but that isn't really that important from a code integration
perspective, that's an admin policy issue.  I expect there would be
much more integration work with fs-verity than with IPE, and I think
the fs-verity related work might be a challenge.

> Also, the design choice of attaching the digest cache to the inode
> helps LSMs like IPE that don't have a per inode cache on their own.
> Sure, IPE would have to do a digest lookup every time, but at least on
> an already populated hash table.

Just because you need to attach some state to an inode does not mean a
file digest cache must be a LSM.  It could be integrated into the VFS
or it could be a separate subsystem; either way it could provide an
API (either through well defined data structures or functions) that
could be used by various LSMs and filesystems that provide integrity
protection.

-- 
paul-moore.com