Re: [net-next v1] tcp: don't free a FIN sk_buff in tcp_remove_empty_skb()

[Eric Dumazet]

On Sun, Oct 24, 2021 at 4:59 PM Jon Maxwell <jmaxwell37@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxx>
> Reported-by: Monir Zouaoui <Monir.Zouaoui@mail.schwarz>
> Reported-by: Simon Stier <simon.stier@mail.schwarz>

Thanks for this fix !
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>