tcp: fix tcp header size miscalculation when window scale is unused

The size of the TCP header is miscalculated when the window scale ends
up being 0. Additionally, this can be induced by sending a SYN to a
passive open port with a window scale option with value 0.
Signed-off-by: NPhilip Love <love_phil@emc.com>
Signed-off-by: NAdam Langley <agl@imperialviolet.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/tcp_output.c

@@ -468,7 +468,8 @@ static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
 	}
 	if (likely(sysctl_tcp_window_scaling)) {
 		opts->ws = tp->rx_opt.rcv_wscale;
-		size += TCPOLEN_WSCALE_ALIGNED;
+		if(likely(opts->ws))
+			size += TCPOLEN_WSCALE_ALIGNED;
 	}
 	if (likely(sysctl_tcp_sack)) {
 		opts->options |= OPTION_SACK_ADVERTISE;
@@ -509,7 +510,8 @@ static unsigned tcp_synack_options(struct sock *sk,
 
 	if (likely(ireq->wscale_ok)) {
 		opts->ws = ireq->rcv_wscale;
-		size += TCPOLEN_WSCALE_ALIGNED;
+		if(likely(opts->ws))
+			size += TCPOLEN_WSCALE_ALIGNED;
 	}
 	if (likely(doing_ts)) {
 		opts->options |= OPTION_TS;