ANDROID: fs-verity: Export function to check signatures

Allows a file system to provide its own fs-verity implementation
but still to hook into the signature check and control file from
fs-verity

Bug: 160634504
Bug: 170978993
Test: incfs_test running on this + subsequent changes
Signed-off-by: Paul Lawrence <paullawrence@google.com>
Change-Id: I02020af460d62fa5eb459a083419208e175005e8

fs/verity/signature.c

@@ -40,11 +40,38 @@ static struct key *fsverity_keyring;
 int fsverity_verify_signature(const struct fsverity_info *vi,
 			      const u8 *signature, size_t sig_size)
 {
-	const struct inode *inode = vi->inode;
-	const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
+	unsigned int digest_algorithm =
+		vi->tree_params.hash_alg - fsverity_hash_algs;
+
+	return __fsverity_verify_signature(vi->inode, signature, sig_size,
+					   vi->file_digest, digest_algorithm);
+}
+
+/**
+ * __fsverity_verify_signature() - check a verity file's signature
+ * @inode: the file's inode
+ * @signature: the file's signature
+ * @sig_size: size of @signature. Can be 0 if there is no signature
+ * @file_digest: the file's digest
+ * @digest_algorithm: the digest algorithm used
+ *
+ * Takes the file's digest and optional signature and verifies the signature
+ * against the digest and the fs-verity keyring if appropriate
+ *
+ * Return: 0 on success (signature valid or not required); -errno on failure
+ */
+int __fsverity_verify_signature(const struct inode *inode, const u8 *signature,
+				size_t sig_size, const u8 *file_digest,
+				unsigned int digest_algorithm)
+{
 	struct fsverity_formatted_digest *d;
+	struct fsverity_hash_alg *hash_alg = fsverity_get_hash_alg(inode,
+							digest_algorithm);
 	int err;
 
+	if (IS_ERR(hash_alg))
+		return PTR_ERR(hash_alg);
+
 	if (sig_size == 0) {
 		if (fsverity_require_signatures) {
 			fsverity_err(inode,
@@ -60,7 +87,7 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
 	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);
+	memcpy(d->digest, file_digest, hash_alg->digest_size);
 
 	err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
 				     signature, sig_size, fsverity_keyring,
@@ -83,9 +110,10 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
 	}
 
 	pr_debug("Valid signature for file digest %s:%*phN\n",
-		 hash_alg->name, hash_alg->digest_size, vi->file_digest);
+		 hash_alg->name, hash_alg->digest_size, file_digest);
 	return 0;
 }
+EXPORT_SYMBOL_GPL(__fsverity_verify_signature);
 
 #ifdef CONFIG_SYSCTL
 static struct ctl_table_header *fsverity_sysctl_header;

include/linux/fsverity.h

@@ -233,4 +233,18 @@ static inline bool fsverity_active(const struct inode *inode)
 	return fsverity_get_info(inode) != NULL;
 }
 
+#ifdef CONFIG_FS_VERITY_BUILTIN_SIGNATURES
+int __fsverity_verify_signature(const struct inode *inode, const u8 *signature,
+				size_t sig_size, const u8 *file_digest,
+				unsigned int digest_algorithm);
+#else /* !CONFIG_FS_VERITY_BUILTIN_SIGNATURES */
+static inline int __fsverity_verify_signature(const struct inode *inode,
+				const u8 *signature, size_t sig_size,
+				const u8 *file_digest,
+				unsigned int digest_algorithm)
+{
+	return 0;
+}
+#endif /* !CONFIG_FS_VERITY_BUILTIN_SIGNATURES */
+
 #endif	/* _LINUX_FSVERITY_H */