tcp: Defer ts_recent changes until req is owned

Recently a bug was discovered where the server had entered TCP_ESTABLISHED
state, but the upper layers were not notified.

The same 5-tuple packet may be processed by different CPUSs, so two
CPUs may receive different ack packets at the same time when the
state is TCP_NEW_SYN_RECV.

In that case, req->ts_recent in tcp_check_req may be changed concurrently,
which will probably cause the newsk's ts_recent to be incorrectly large.
So that tcp_validate_incoming will fail. At this point, newsk will not be
able to enter the TCP_ESTABLISHED.

cpu1                                    cpu2
tcp_check_req
                                        tcp_check_req
 req->ts_recent = rcv_tsval = t1
                                         req->ts_recent = rcv_tsval = t2

 syn_recv_sock
  tcp_sk(child)->rx_opt.ts_recent = req->ts_recent = t2 // t1 < t2
tcp_child_process
 tcp_rcv_state_process
  tcp_validate_incoming
   tcp_paws_check
    if ((s32)(rx_opt->ts_recent - rx_opt->rcv_tsval) <= paws_win)
        // t2 - t1 > paws_win, failed
                                        tcp_v4_do_rcv
                                         tcp_rcv_state_process
                                         // TCP_ESTABLISHED

The cpu2's skb or a newly received skb will call tcp_v4_do_rcv to get
the newsk into the TCP_ESTABLISHED state, but at this point it is no
longer possible to notify the upper layer application. A notification
mechanism could be added here, but the fix is more complex, so the
current fix is used.

In tcp_check_req, req->ts_recent is used to assign a value to
tcp_sk(child)->rx_opt.ts_recent, so removing the change in req->ts_recent
and changing tcp_sk(child)->rx_opt.ts_recent directly after owning the
req fixes this bug.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_minisocks.c

@@ -815,12 +815,6 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 
 	/* In sequence, PAWS is OK. */
 
-	/* TODO: We probably should defer ts_recent change once
-	 * we take ownership of @req.
-	 */
-	if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
-		WRITE_ONCE(req->ts_recent, tmp_opt.rcv_tsval);
-
 	if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
 		/* Truncate SYN, it is out of window starting
 		   at tcp_rsk(req)->rcv_isn + 1. */
@@ -869,6 +863,10 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
 	if (!child)
 		goto listen_overflow;
 
+	if (own_req && tmp_opt.saw_tstamp &&
+	    !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
+		tcp_sk(child)->rx_opt.ts_recent = tmp_opt.rcv_tsval;
+
 	if (own_req && rsk_drop_req(req)) {
 		reqsk_queue_removed(&inet_csk(req->rsk_listener)->icsk_accept_queue, req);
 		inet_csk_reqsk_queue_drop_and_put(req->rsk_listener, req);