tcp: tcp_synack_options() fix

Commit 33ad798c (tcp: options clean up) introduced a problem
if MD5+SACK+timestamps were used in initial SYN message.

Some stacks (old linux for example) try to negotiate MD5+SACK+TSTAMP
sessions, but since 40 bytes of tcp options space are not enough to
store all the bits needed, we chose to disable timestamps in this case.

We send a SYN-ACK _without_ timestamp option, but socket has timestamps
enabled and all further outgoing messages contain a TS block, all with
the initial timestamp of the remote peer.

Fix is to really disable timestamps option for the whole session.
Reported-by: NBijay Singh <Bijay.Singh@guavus.com>
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -668,7 +668,6 @@ static unsigned tcp_synack_options(struct sock *sk,
 	u8 cookie_plus = (xvp != NULL && !xvp->cookie_out_never) ?
 			 xvp->cookie_plus :
 			 0;
-	bool doing_ts = ireq->tstamp_ok;
 
 #ifdef CONFIG_TCP_MD5SIG
 	*md5 = tcp_rsk(req)->af_specific->md5_lookup(sk, req);
@@ -681,7 +680,7 @@ static unsigned tcp_synack_options(struct sock *sk,
 		 * rather than TS in order to fit in better with old,
 		 * buggy kernels, but that was deemed to be unnecessary.
 		 */
-		doing_ts &= !ireq->sack_ok;
+		ireq->tstamp_ok &= !ireq->sack_ok;
 	}
 #else
 	*md5 = NULL;
@@ -696,7 +695,7 @@ static unsigned tcp_synack_options(struct sock *sk,
 		opts->options |= OPTION_WSCALE;
 		remaining -= TCPOLEN_WSCALE_ALIGNED;
 	}
-	if (likely(doing_ts)) {
+	if (likely(ireq->tstamp_ok)) {
 		opts->options |= OPTION_TS;
 		opts->tsval = TCP_SKB_CB(skb)->when;
 		opts->tsecr = req->ts_recent;
@@ -704,7 +703,7 @@ static unsigned tcp_synack_options(struct sock *sk,
 	}
 	if (likely(ireq->sack_ok)) {
 		opts->options |= OPTION_SACK_ADVERTISE;
-		if (unlikely(!doing_ts))
+		if (unlikely(!ireq->tstamp_ok))
 			remaining -= TCPOLEN_SACKPERM_ALIGNED;
 	}
 
@@ -712,7 +711,7 @@ static unsigned tcp_synack_options(struct sock *sk,
 	 * If the <SYN> options fit, the same options should fit now!
 	 */
 	if (*md5 == NULL &&
-	    doing_ts &&
+	    ireq->tstamp_ok &&
 	    cookie_plus > TCPOLEN_COOKIE_BASE) {
 		int need = cookie_plus; /* has TCPOLEN_COOKIE_BASE */