fs-verity: add SHA-512 support

Add SHA-512 support to fs-verity.  This is primarily a demonstration of
the trivial changes needed to support a new hash algorithm in fs-verity;
most users will still use SHA-256, due to the smaller space required to
store the hashes.  But some users may prefer SHA-512.

Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/verity/fsverity_private.h

@@ -29,7 +29,7 @@ struct ahash_request;
  * Largest digest size among all hash algorithms supported by fs-verity.
  * Currently assumed to be <= size of fsverity_descriptor::root_hash.
  */
-#define FS_VERITY_MAX_DIGEST_SIZE	SHA256_DIGEST_SIZE
+#define FS_VERITY_MAX_DIGEST_SIZE	SHA512_DIGEST_SIZE
 
 /* A hash algorithm supported by fs-verity */
 struct fsverity_hash_alg {

fs/verity/hash_algs.c

@@ -17,6 +17,11 @@ struct fsverity_hash_alg fsverity_hash_algs[] = {
 		.digest_size = SHA256_DIGEST_SIZE,
 		.block_size = SHA256_BLOCK_SIZE,
 	},
+	[FS_VERITY_HASH_ALG_SHA512] = {
+		.name = "sha512",
+		.digest_size = SHA512_DIGEST_SIZE,
+		.block_size = SHA512_BLOCK_SIZE,
+	},
 };
 
 /**

include/uapi/linux/fsverity.h

@@ -14,6 +14,7 @@
 #include <linux/types.h>
 
 #define FS_VERITY_HASH_ALG_SHA256	1
+#define FS_VERITY_HASH_ALG_SHA512	2
 
 struct fsverity_enable_arg {
 	__u32 version;