net/ipv4: disable SMC TCP option with SYN Cookies

Currently, the SMC experimental TCP option in a SYN packet is lost on
the server side when SYN Cookies are active. However, the corresponding
SYNACK sent back to the client contains the SMC option. This causes an
inconsistent view of the SMC capabilities on the client and server.

This patch disables the SMC option in the SYNACK when SYN Cookies are
active to avoid this issue.

Fixes: 60e2a7780793b ("tcp: TCP experimental option for SMC")
Signed-off-by: Hans Wippel <[e-mail]>
Signed-off-by: Ursula Braun <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/syncookies.c

···
 	req->ts_recent		= tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0;
 	treq->snt_synack	= 0;
 	treq->tfo_listener	= false;
+	if (IS_ENABLED(CONFIG_SMC))
+		ireq->smc_ok = 0;
 
 	ireq->ir_iif = inet_request_bound_dev_if(sk, skb);
 

net/ipv4/tcp_input.c

···
 	if (want_cookie && !tmp_opt.saw_tstamp)
 		tcp_clear_options(&tmp_opt);
 
+	if (IS_ENABLED(CONFIG_SMC) && want_cookie)
+		tmp_opt.smc_ok = 0;
+
 	tmp_opt.tstamp_ok = tmp_opt.saw_tstamp;
 	tcp_openreq_init(req, &tmp_opt, skb, sk);
 	inet_rsk(req)->no_srccheck = inet_sk(sk)->transparent;

net/ipv6/syncookies.c

···
 	treq->snt_isn = cookie;
 	treq->ts_off = 0;
 	treq->txhash = net_tx_rndhash();
+	if (IS_ENABLED(CONFIG_SMC))
+		ireq->smc_ok = 0;
 
 	/*
 	 * We need to lookup the dst_entry to get the correct window size.