tcp: Add logic to check for SYN w/ data in tcp_simple_retransmit

There are cases where a fastopen SYN may trigger either a ICMP_TOOBIG
message in the case of IPv6 or a fragmentation request in the case of
IPv4. This results in the socket stalling for a second or more as it does
not respond to the message by retransmitting the SYN frame.

Normally a SYN frame should not be able to trigger a ICMP_TOOBIG or
ICMP_FRAG_NEEDED however in the case of fastopen we can have a frame that
makes use of the entire MSS. In the case of fastopen it does, and an
additional complication is that the retransmit queue doesn't contain the
original frames. As a result when tcp_simple_retransmit is called and
walks the list of frames in the queue it may not mark the frames as lost
because both the SYN and the data packet each individually are smaller than
the MSS size after the adjustment. This results in the socket being stalled
until the retransmit timer kicks in and forces the SYN frame out again
without the data attached.

In order to resolve this we can reduce the MSS the packets are compared
to in tcp_simple_retransmit to -1 for cases where we are still in the
TCP_SYN_SENT state for a fastopen socket. Doing this we will mark all of
the packets related to the fastopen SYN as lost.
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Link: https://lore.kernel.org/r/160780498125.3272.15437756269539236825.stgit@localhost.localdomainSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/tcp_input.c

@@ -2689,7 +2689,22 @@ void tcp_simple_retransmit(struct sock *sk)
 	const struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct sk_buff *skb;
-	unsigned int mss = tcp_current_mss(sk);
+	int mss;
+
+	/* A fastopen SYN request is stored as two separate packets within
+	 * the retransmit queue, this is done by tcp_send_syn_data().
+	 * As a result simply checking the MSS of the frames in the queue
+	 * will not work for the SYN packet.
+	 *
+	 * Us being here is an indication of a path MTU issue so we can
+	 * assume that the fastopen SYN was lost and just mark all the
+	 * frames in the retransmit queue as lost. We will use an MSS of
+	 * -1 to mark all frames as lost, otherwise compute the current MSS.
+	 */
+	if (tp->syn_data && sk->sk_state == TCP_SYN_SENT)
+		mss = -1;
+	else
+		mss = tcp_current_mss(sk);
 
 	skb_rbtree_walk(skb, &sk->tcp_rtx_queue) {
 		if (tcp_skb_seglen(skb) > mss)