tcp: don't free a FIN sk_buff in tcp_remove_empty_skb()

v1: Implement a more general statement as recommended by Eric Dumazet. The
sequence number will be advanced, so this check will fix the FIN case and
other cases.

A customer reported sockets stuck in the CLOSING state. A Vmcore revealed that
the write_queue was not empty as determined by tcp_write_queue_empty() but the
sk_buff containing the FIN flag had been freed and the socket was zombied in
that state. Corresponding pcaps show no FIN from the Linux kernel on the wire.

Some instrumentation was added to the kernel and it was found that there is a
timing window where tcp_sendmsg() can run after tcp_send_fin().

tcp_sendmsg() will hit an error, for example:

1269 ▹       if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))↩
1270 ▹       ▹       goto do_error;↩

tcp_remove_empty_skb() will then free the FIN sk_buff as "skb->len == 0". The
TCP socket is now wedged in the FIN-WAIT-1 state because the FIN is never sent.

If the other side sends a FIN packet the socket will transition to CLOSING and
remain that way until the system is rebooted.

Fix this by checking for the FIN flag in the sk_buff and don't free it if that
is the case. Testing confirmed that fixed the issue.

Fixes: fdfc5c85 ("tcp: remove empty skb from write queue in error cases")
Signed-off-by: Jon Maxwell <jmaxwell37@gmail.com>
Reported-by: Monir Zouaoui <Monir.Zouaoui@mail.schwarz>
Reported-by: Simon Stier <simon.stier@mail.schwarz>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -938,7 +938,7 @@ int tcp_send_mss(struct sock *sk, int *size_goal, int flags)
  */
 void tcp_remove_empty_skb(struct sock *sk, struct sk_buff *skb)
 {
-	if (skb && !skb->len) {
+	if (skb && TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq) {
 		tcp_unlink_write_queue(skb, sk);
 		if (tcp_write_queue_empty(sk))
 			tcp_chrono_stop(sk, TCP_CHRONO_BUSY);