This is a note to let you know that I've just added the patch titled fsverity: skip PKCS#7 parser when keyring is empty to the 6.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: fsverity-skip-pkcs-7-parser-when-keyring-is-empty.patch and it can be found in the queue-6.4 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 919dc320956ea353a7fb2d84265195ad5ef525ac Mon Sep 17 00:00:00 2001 From: Eric Biggers <ebiggers@xxxxxxxxxx> Date: Tue, 1 Aug 2023 21:03:53 -0700 Subject: fsverity: skip PKCS#7 parser when keyring is empty From: Eric Biggers <ebiggers@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx Reviewed-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx> Link: https://lore.kernel.org/r/20230820173237.2579-1-ebiggers@xxxxxxxxxx Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/verity/signature.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) --- a/fs/verity/signature.c +++ b/fs/verity/signature.c @@ -54,6 +54,22 @@ int fsverity_verify_signature(const stru 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; Patches currently in stable-queue which might be from ebiggers@xxxxxxxxxx are queue-6.4/fsverity-skip-pkcs-7-parser-when-keyring-is-empty.patch