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@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxx
> > Acked-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> > ---
> >
> > 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@xxxxxxxxxx>
>
> 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
