From: Jon Maxwell jmaxwell37@gmail.com
[ Upstream commit cf12e6f9124629b18a6182deefc0315f0a73a199 ]
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: fdfc5c8594c2 ("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 Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 5c8d0fb498256..9f53d25e047e3 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -955,7 +955,7 @@ static int tcp_send_mss(struct sock *sk, int *size_goal, int flags) */ static 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);