[PATCH] fsverity: don't check builtin signatures when require_signatures=0

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

An issue that arises when migrating from builtin signatures to userspace
signatures is that existing files that have builtin signatures cannot be
opened unless either CONFIG_FS_VERITY_BUILTIN_SIGNATURES is disabled or
the signing certificate is left in the .fs-verity keyring.

Since builtin signatures provide no security benefit when
fs.verity.require_signatures=0 anyway, let's just skip the signature
verification in this case.

Fixes: 432434c9f8e1 ("fs-verity: support builtin file signatures")
Cc: <stable@vger.kernel.org> # v5.4+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/verity/signature.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/fs/verity/signature.c b/fs/verity/signature.c
index 143a530a80088..dc6935701abda 100644
--- a/fs/verity/signature.c
+++ b/fs/verity/signature.c
@@ -13,8 +13,8 @@
 #include <linux/verification.h>
 
 /*
- * /proc/sys/fs/verity/require_signatures
- * If 1, all verity files must have a valid builtin signature.
+ * /proc/sys/fs/verity/require_signatures.  If 1, then builtin signatures are
+ * verified and all verity files must have a valid builtin signature.
  */
 static int fsverity_require_signatures;
 
@@ -54,6 +54,20 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
 		return 0;
 	}
 
+	/*
+	 * If require_signatures=0, don't verify builtin signatures.
+	 * Originally, builtin signatures were verified opportunistically in
+	 * this case.  However, no security property is possible when
+	 * require_signatures=0 anyway.  Skipping the builtin signature
+	 * verification makes it easier to migrate existing files from builtin
+	 * signature verification to userspace signature verification.
+	 */
+	if (!fsverity_require_signatures) {
+		fsverity_warn(inode,
+			      "Not checking builtin signature due to require_signatures=0");
+		return 0;
+	}
+
 	d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
 	if (!d)
 		return -ENOMEM;

base-commit: 479174d402bcf60789106eedc4def3957c060bad
-- 
2.38.1