dm integrity: change memcmp to strncmp in dm_integrity_ctr

If the string opt_string is small, the function memcmp can access bytes
that are beyond the terminating nul character. In theory, it could cause
segfault, if opt_string were located just below some unmapped memory.

Change from memcmp to strncmp so that we don't read bytes beyond the end
of the string.

Cc: stable@vger.kernel.org # v4.12+
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-integrity.c

@@ -3185,7 +3185,7 @@ static int dm_integrity_ctr(struct dm_target *ti, unsigned argc, char **argv)
 			journal_watermark = val;
 		else if (sscanf(opt_string, "commit_time:%u%c", &val, &dummy) == 1)
 			sync_msec = val;
-		else if (!memcmp(opt_string, "meta_device:", strlen("meta_device:"))) {
+		else if (!strncmp(opt_string, "meta_device:", strlen("meta_device:"))) {
 			if (ic->meta_dev) {
 				dm_put_device(ti, ic->meta_dev);
 				ic->meta_dev = NULL;
@@ -3204,17 +3204,17 @@ static int dm_integrity_ctr(struct dm_target *ti, unsigned argc, char **argv)
 				goto bad;
 			}
 			ic->sectors_per_block = val >> SECTOR_SHIFT;
-		} else if (!memcmp(opt_string, "internal_hash:", strlen("internal_hash:"))) {
+		} else if (!strncmp(opt_string, "internal_hash:", strlen("internal_hash:"))) {
 			r = get_alg_and_key(opt_string, &ic->internal_hash_alg, &ti->error,
 					    "Invalid internal_hash argument");
 			if (r)
 				goto bad;
-		} else if (!memcmp(opt_string, "journal_crypt:", strlen("journal_crypt:"))) {
+		} else if (!strncmp(opt_string, "journal_crypt:", strlen("journal_crypt:"))) {
 			r = get_alg_and_key(opt_string, &ic->journal_crypt_alg, &ti->error,
 					    "Invalid journal_crypt argument");
 			if (r)
 				goto bad;
-		} else if (!memcmp(opt_string, "journal_mac:", strlen("journal_mac:"))) {
+		} else if (!strncmp(opt_string, "journal_mac:", strlen("journal_mac:"))) {
 			r = get_alg_and_key(opt_string, &ic->journal_mac_alg,  &ti->error,
 					    "Invalid journal_mac argument");
 			if (r)