linux-kernel - Re: [RFC PATCH v13 17/20] ipe: enable support for fs-verity as a trust provider

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Message-ID: <20240229194237.GA1454@sol.localdomain>
Date: Thu, 29 Feb 2024 11:42:37 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: Fan Wu <wufan@...ux.microsoft.com>
Cc: corbet@....net, zohar@...ux.ibm.com, jmorris@...ei.org,
	serge@...lyn.com, tytso@....edu, axboe@...nel.dk, agk@...hat.com,
	snitzer@...nel.org, eparis@...hat.com, paul@...l-moore.com,
	linux-doc@...r.kernel.org, linux-integrity@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	linux-fscrypt@...r.kernel.org, linux-block@...r.kernel.org,
	dm-devel@...ts.linux.dev, audit@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Deven Bowers <deven.desai@...ux.microsoft.com>
Subject: Re: [RFC PATCH v13 17/20] ipe: enable support for fs-verity as a
 trust provider

On Thu, Feb 29, 2024 at 10:59:21AM -0800, Fan Wu wrote:
> > 
> > So IPE is interested in whether a file has an fsverity builtin signature, but it
> > doesn't care what the signature is or whether it has been checked.  What is the
> > point?
> > 
> > - Eric
> 
> It does make sure the signature is checked. This hook call can only be
> triggered after fsverity_verify_signature() succeed. Therefore, for files
> that are marked with the security blob inode_sec->fs_verity_sign as true,
> they must successfully pass the fsverity_verify_signature() check.
> 
> Regarding the other question, the current version does not support defining
> policies to trust files based on the inner content of their signatures
> because the current patch set is already too large.
> 
> We plan to introduce new policy grammars to enable the policy to define
> which certificate of the signature can be trusted after this version is
> accepted.

Ah, I see, you're relying on the fact that fsverity_verify_signature() verifies
the signature (if present) even if fs.verity.require_signatures hasn't been set.
That does happen to be its behavior, but this isn't clearly documented since
there previously wasn't really a use case for the builtin signatures without
setting fs.verity.require_signatures.  Can you please make sure this behavior is
documented properly in Documentation/filesystems/fsverity.rst and in function
comments?  Otherwise I worry that it could get changed and break your code.

- Eric