Re: [PATCH v2] fsverity: skip PKCS#7 parser when keyring is empty

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Aug 02, 2023 at 07:27:15AM +0300, Jarkko Sakkinen wrote:
> On Wed Aug 2, 2023 at 7:15 AM EEST, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> >
> > If an fsverity builtin signature is given for a file but the
> > ".fs-verity" keyring is empty, there's no real reason to run the PKCS#7
> > parser.  Skip this to avoid the PKCS#7 attack surface when builtin
> > signature support is configured into the kernel but is not being used.
> >
> > This is a hardening improvement, not a fix per se, but I've added
> > Fixes and Cc stable to get it out to more users.
> >
> > Fixes: 432434c9f8e1 ("fs-verity: support builtin file signatures")
> > Cc: stable@vger.kernel.org
> > Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
> > Signed-off-by: Eric Biggers <ebiggers@google.com>
> > ---
> >
> > v2: check keyring and return early before allocating formatted digest
> >
> >  fs/verity/signature.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> >
> > diff --git a/fs/verity/signature.c b/fs/verity/signature.c
> > index b95acae64eac6..8f474702aa249 100644
> > --- a/fs/verity/signature.c
> > +++ b/fs/verity/signature.c
> > @@ -62,6 +62,21 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
> >  		return 0;
> >  	}
> >  
> > +	if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
> > +		/*
> > +		 * The ".fs-verity" keyring is empty, due to builtin signatures
> > +		 * being supported by the kernel but not actually being used.
> > +		 * In this case, verify_pkcs7_signature() would always return an
> > +		 * error, usually ENOKEY.  It could also be EBADMSG if the
> > +		 * PKCS#7 is malformed, but that isn't very important to
> > +		 * distinguish.  So, just skip to ENOKEY to avoid the attack
> > +		 * surface of the PKCS#7 parser, which would otherwise be
> > +		 * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
> > +		 */
> > +		fsverity_err(inode, "fs-verity keyring is empty");
> > +		return -ENOKEY;
> > +	}
> > +
> >  	d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
> >  	if (!d)
> >  		return -ENOMEM;
> >
> > base-commit: 456ae5fe9b448f44ebe98b391a3bae9c75df465e
> > -- 
> > 2.41.0
> 
> Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
> 
> applied
> 
> BR, Jarkko

Hi Jarkko, thanks for the review!

I actually intended to take this through the fsverity tree.  Is that okay?

BTW, we could actually make this change to verify_pkcs7_signature() itself.
I wasn't sure it would be appropriate for all callers, though.  Any thoughts?

- Eric