fsverity: skip PKCS#7 parser when keyring is empty

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
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/r/20230820173237.2579-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/verity/signature.c b/fs/verity/signature.c
index b95acae..90c0757 100644 (file)
--- a/fs/verity/signature.c
+++ b/fs/verity/signature.c
@@ -62,6 +62,22 @@ 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, rejecting signed file!");
+               return -ENOKEY;
+       }
+
        d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
        if (!d)
                return -ENOMEM;