dm-integrity: check mac_size against HASH_MAX_DIGESTSIZE in sb_mac()

sb_mac() verifies that the superblock + MAC don't exceed 512 bytes.
Because the superblock is currently 64 bytes, this really verifies
mac_size <= 448.  This confuses smatch into thinking that mac_size may
be as large as 448, which is inconsistent with the later code that
assumes the MAC fits in a buffer of size HASH_MAX_DIGESTSIZE (64).

In fact mac_size <= HASH_MAX_DIGESTSIZE is guaranteed by the crypto API,
as that is the whole point of HASH_MAX_DIGESTSIZE.  But, let's be
defensive and explicitly check for this.  This suppresses the false
positive smatch warning.  It does not fix an actual bug.

Reported-by: kernel test robot <[email protected]>
Reported-by: Dan Carpenter <[email protected]>
Closes: https://lore.kernel.org/r/[email protected]/
Signed-off-by: Eric Biggers <[email protected]>
Signed-off-by: Mikulas Patocka <[email protected]>

Author: ebiggers

drivers/md/dm-integrity.c

@@ -494,7 +494,8 @@ static int sb_mac(struct dm_integrity_c *ic, bool wr)
 	__u8 *sb = (__u8 *)ic->sb;
 	__u8 *mac = sb + (1 << SECTOR_SHIFT) - mac_size;
 
-	if (sizeof(struct superblock) + mac_size > 1 << SECTOR_SHIFT) {
+	if (sizeof(struct superblock) + mac_size > 1 << SECTOR_SHIFT ||
+	    mac_size > HASH_MAX_DIGESTSIZE) {
 		dm_integrity_io_error(ic, "digest is too long", -EINVAL);
 		return -EINVAL;
 	}