tcp: Do not call IPv4 specific func in tcp_check_req

Calling IPv4 specific inet_csk_route_req in tcp_check_req
is a bad idea and crashes machine on IPv6 connections, as reported
by Valdis Kletnieks

Also, all we are really interested in is the timestamp
option in the header, so calling tcp_parse_options()
with the "estab" set to false flag is an overkill as
it tries to parse half a dozen other TCP options.

We know whether timestamp should be enabled or not
using data from request_sock.
Signed-off-by: Gilad Ben-Yossef <gilad@codefidence.com>
Tested-by: Valdis.Kletnieks@vt.edu
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_minisocks.c

@@ -500,11 +500,10 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	int paws_reject = 0;
 	struct tcp_options_received tmp_opt;
 	struct sock *child;
-	struct dst_entry *dst = inet_csk_route_req(sk, req);
 
-	tmp_opt.saw_tstamp = 0;
-	if (th->doff > (sizeof(struct tcphdr)>>2)) {
-		tcp_parse_options(skb, &tmp_opt, 0, dst);
+	if ((th->doff > (sizeof(struct tcphdr)>>2)) && (req->ts_recent)) {
+		tmp_opt.tstamp_ok = 1;
+		tcp_parse_options(skb, &tmp_opt, 1, NULL);
 
 		if (tmp_opt.saw_tstamp) {
 			tmp_opt.ts_recent = req->ts_recent;
@@ -517,8 +516,6 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 		}
 	}
 
-	dst_release(dst);
-
 	/* Check for pure retransmitted SYN. */
 	if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn &&
 	    flg == TCP_FLAG_SYN &&