tcp: Fix SYN option room calculation for TCP-AO.

When building SYN packet in tcp_syn_options(), MSS, TS, WS, and
SACKPERM are used without checking the remaining bytes in the
options area.

To keep that logic as is, we limit the TCP-AO MAC length in
tcp_ao_parse_crypto().  Currently, the limit is calculated as below.

  MAX_TCP_OPTION_SPACE - TCPOLEN_TSTAMP_ALIGNED
                       - TCPOLEN_WSCALE_ALIGNED
                       - TCPOLEN_SACKPERM_ALIGNED

This looks confusing as (1) we pack SACKPERM into the leading
2-bytes of the aligned 12-bytes of TS and (2) TCPOLEN_MSS_ALIGNED
is not used.  Fortunately, the calculated limit is not wrong as
TCPOLEN_SACKPERM_ALIGNED and TCPOLEN_MSS_ALIGNED are the same value.

However, we should use the proper constant in the formula.

  MAX_TCP_OPTION_SPACE - TCPOLEN_MSS_ALIGNED
                       - TCPOLEN_TSTAMP_ALIGNED
                       - TCPOLEN_WSCALE_ALIGNED

Fixes: 4954f17 ("net/tcp: Introduce TCP_AO setsockopt()s")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: q2ven

net/ipv4/tcp_ao.c

@@ -1315,7 +1315,8 @@ static int tcp_ao_parse_crypto(struct tcp_ao_add *cmd, struct tcp_ao_key *key)
 	key->maclen = cmd->maclen ?: 12; /* 12 is the default in RFC5925 */
 
 	/* Check: maclen + tcp-ao header <= (MAX_TCP_OPTION_SPACE - mss
-	 *					- tstamp - wscale - sackperm),
+	 *					- tstamp (including sackperm)
+	 *					- wscale),
 	 * see tcp_syn_options(), tcp_synack_options(), commit 33ad798c924b.
 	 *
 	 * In order to allow D-SACK with TCP-AO, the header size should be:
@@ -1342,9 +1343,9 @@ static int tcp_ao_parse_crypto(struct tcp_ao_add *cmd, struct tcp_ao_key *key)
 	 * large to leave sufficient option space.
 	 */
 	syn_tcp_option_space = MAX_TCP_OPTION_SPACE;
+	syn_tcp_option_space -= TCPOLEN_MSS_ALIGNED;
 	syn_tcp_option_space -= TCPOLEN_TSTAMP_ALIGNED;
 	syn_tcp_option_space -= TCPOLEN_WSCALE_ALIGNED;
-	syn_tcp_option_space -= TCPOLEN_SACKPERM_ALIGNED;
 	if (tcp_ao_len(key) > syn_tcp_option_space) {
 		err = -EMSGSIZE;
 		goto err_kfree;