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

@@ -27,26 +27,30 @@ static int fsverity_require_signatures;
 static struct key *fsverity_keyring;
 
 /**
- * fsverity_verify_signature() - check a verity file's signature
- * @vi: the file's fsverity_info
- * @desc: the file's fsverity_descriptor
- * @desc_size: size of @desc
+ * __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
  *
- * If the file's fs-verity descriptor includes a signature of the file digest,
- * verify it against the certificates in the fs-verity keyring.
+ * 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 fsverity_info *vi,
-			      const struct fsverity_descriptor *desc,
-			      size_t desc_size)
+int __fsverity_verify_signature(const struct inode *inode, const u8 *signature,
+				u32 sig_size, const u8 *file_digest,
+				unsigned int digest_algorithm)
 {
-	const struct inode *inode = vi->inode;
-	const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
-	const u32 sig_size = le32_to_cpu(desc->sig_size);
 	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,
@@ -56,21 +60,16 @@ int fsverity_verify_signature(const struct fsverity_info *vi,
 		return 0;
 	}
 
-	if (sig_size > desc_size - sizeof(*desc)) {
-		fsverity_err(inode, "Signature overflows verity descriptor");
-		return -EBADMSG;
-	}
-
 	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);
+	memcpy(d->digest, file_digest, hash_alg->digest_size);
 
 	err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
-				     desc->signature, sig_size,
+				     signature, sig_size,
 				     fsverity_keyring,
 				     VERIFYING_UNSPECIFIED_SIGNATURE,
 				     NULL, NULL);
@@ -91,9 +90,38 @@ 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);
+
+/**
+ * fsverity_verify_signature() - check a verity file's signature
+ * @vi: the file's fsverity_info
+ * @desc: the file's fsverity_descriptor
+ * @desc_size: size of @desc
+ *
+ * If the file's fs-verity descriptor includes a signature of the file digest,
+ * verify it against the certificates in the fs-verity keyring.
+ *
+ * Return: 0 on success (signature valid or not required); -errno on failure
+ */
+int fsverity_verify_signature(const struct fsverity_info *vi,
+			      const struct fsverity_descriptor *desc,
+			      size_t desc_size)
+{
+	const struct inode *inode = vi->inode;
+	const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
+	const u32 sig_size = le32_to_cpu(desc->sig_size);
+
+	if (sig_size > desc_size - sizeof(*desc)) {
+		fsverity_err(inode, "Signature overflows verity descriptor");
+		return -EBADMSG;
+	}
+
+	return __fsverity_verify_signature(inode, desc->signature, sig_size,
+				vi->file_digest, hash_alg - fsverity_hash_algs);
+}
 
 #ifdef CONFIG_SYSCTL
 static struct ctl_table_header *fsverity_sysctl_header;

include/linux/fsverity.h

@@ -216,4 +216,18 @@ 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,
+				u32 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, u32 sig_size,
+				const u8 *file_digest,
+				unsigned int digest_algorithm)
+{
+	return 0;
+}
+#endif /* !CONFIG_FS_VERITY_BUILTIN_SIGNATURES */
+
 #endif	/* _LINUX_FSVERITY_H */