tcp: call tcp_try_undo_recovery when an RTOd TFO SYNACK is ACKed

For passive TCP Fast Open sockets that had SYN/ACK timeout and did not
send more data in SYN_RECV, upon receiving the final ACK in 3WHS, the
congestion state may awkwardly stay in CA_Loss mode unless the CA state
was undone due to TCP timestamp checks. However, if
tcp_rcv_synrecv_state_fastopen() decides not to undo, then we should
enter CA_Open, because at that point we have received an ACK covering
the retransmitted SYNACKs. Currently, the icsk_ca_state is only set to
CA_Open after we receive an ACK for a data-packet. This is because
tcp_ack does not call tcp_fastretrans_alert (and tcp_process_loss) if
!prior_packets

Note that tcp_process_loss() calls tcp_try_undo_recovery(), so having
tcp_rcv_synrecv_state_fastopen() decide that if we're in CA_Loss we
should call tcp_try_undo_recovery() is consistent with that, and
low risk.

Fixes: dad8cea ("tcp: fix TFO SYNACK undo to avoid double-timestamp-undo")
Signed-off-by: Aananth V <aananthv@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: AananthV

net/ipv4/tcp_input.c

@@ -6444,22 +6444,23 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 
 static void tcp_rcv_synrecv_state_fastopen(struct sock *sk)
 {
+	struct tcp_sock *tp = tcp_sk(sk);
 	struct request_sock *req;
 
 	/* If we are still handling the SYNACK RTO, see if timestamp ECR allows
 	 * undo. If peer SACKs triggered fast recovery, we can't undo here.
 	 */
-	if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss)
-		tcp_try_undo_loss(sk, false);
+	if (inet_csk(sk)->icsk_ca_state == TCP_CA_Loss && !tp->packets_out)
+		tcp_try_undo_recovery(sk);
 
 	/* Reset rtx states to prevent spurious retransmits_timed_out() */
-	tcp_sk(sk)->retrans_stamp = 0;
+	tp->retrans_stamp = 0;
 	inet_csk(sk)->icsk_retransmits = 0;
 
 	/* Once we leave TCP_SYN_RECV or TCP_FIN_WAIT_1,
 	 * we no longer need req so release it.
 	 */
-	req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk,
+	req = rcu_dereference_protected(tp->fastopen_rsk,
 					lockdep_sock_is_held(sk));
 	reqsk_fastopen_remove(sk, req, false);