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

["Jarkko Sakkinen" <jarkko@kernel.org>]

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