iscsi-target: chap auth shouldn't match username with trailing garbage

In iSCSI negotiations with initiator CHAP enabled, usernames with
trailing garbage are permitted, because the string comparison only
checks the strlen of the configured username.

e.g. "usernameXXXXX" will be permitted to match "username".

Just check one more byte so the trailing null char is also matched.

Signed-off-by: Eric Seppanen <eric@purestorage.com>
Cc: <stable@vger.kernel.org> #3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_auth.c

@@ -146,6 +146,7 @@ static int chap_server_compute_md5(
 	unsigned char client_digest[MD5_SIGNATURE_SIZE];
 	unsigned char server_digest[MD5_SIGNATURE_SIZE];
 	unsigned char chap_n[MAX_CHAP_N_SIZE], chap_r[MAX_RESPONSE_LENGTH];
+	size_t compare_len;
 	struct iscsi_chap *chap = conn->auth_protocol;
 	struct crypto_hash *tfm;
 	struct hash_desc desc;
@@ -184,7 +185,9 @@ static int chap_server_compute_md5(
 		goto out;
 	}
 
-	if (memcmp(chap_n, auth->userid, strlen(auth->userid)) != 0) {
+	/* Include the terminating NULL in the compare */
+	compare_len = strlen(auth->userid) + 1;
+	if (strncmp(chap_n, auth->userid, compare_len) != 0) {
 		pr_err("CHAP_N values do not match!\n");
 		goto out;
 	}