Commit b6c9d040 authored Aug 01, 2023 by Eric Biggers Committed by Greg Kroah-Hartman Sep 23, 2023

fsverity: skip PKCS#7 parser when keyring is empty



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: 432434c9 ("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>

parent 712491c9

fs/verity/signature.c

+16 −0

Original line number	Diff line number	Diff line
		@@ -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;