On Sun Aug 20, 2023 at 8:32 PM 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
> Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
>
> v3: improve the error message slightly
> v2: check keyring and return early before allocating formatted digest
>
> fs/verity/signature.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/fs/verity/signature.c b/fs/verity/signature.c
> index b95acae64eac6..90c07573dd77b 100644
> --- a/fs/verity/signature.c
> +++ b/fs/verity/signature.c
> @@ -55,20 +55,36 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
>
> if (sig_size == 0) {
> if (fsverity_require_signatures) {
> fsverity_err(inode,
> "require_signatures=1, rejecting unsigned file!");
> return -EPERM;
> }
> 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;
> memcpy(d->magic, "FSVerity", 8);
> d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
> d->digest_size = cpu_to_le16(hash_alg->digest_size);
> memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
>
> err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
> signature, sig_size, fsverity_keyring,
>
> base-commit: 456ae5fe9b448f44ebe98b391a3bae9c75df465e
> --
> 2.41.0
Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
BR, Jarkko
