On Sun Aug 20, 2023 at 8:32 PM 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 Reviewed-by: Jarkko Sakkinen jarkko@kernel.org Signed-off-by: Eric Biggers ebiggers@google.com

---

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@kernel.org

BR, Jarkko