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fsverity: skip PKCS#7 parser when keyring is empty

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commit 919dc320956ea353a7fb2d84265195ad5ef525ac upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Eric Biggers 2023-08-01 21:03:53 -07:00 committed by Greg Kroah-Hartman
parent 712491c9ab
commit b6c9d04019
1 changed files with 16 additions and 0 deletions
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16
fs/verity/signature.c
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@ -58,6 +58,22 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
return -EBADMSG;
}
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;