netdev - Re: [net-next] tcp: don't free a FIN sk_buff in tcp_remove_empty

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Date:   Wed, 20 Oct 2021 18:10:54 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jon Maxwell <jmaxwell37@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [net-next] tcp: don't free a FIN sk_buff in tcp_remove_empty_skb()

On Wed, Oct 20, 2021 at 4:25 PM Jon Maxwell <jmaxwell37@...il.com> wrote:
>
> 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@...il.com>
> ---
>  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 c2d9830136d2..d2b06d8f0c37 100644
> --- a/net/ipv4/tcp.c
> +++ b/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 && !skb->len && !TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) {
>                 tcp_unlink_write_queue(skb, sk);
>                 if (tcp_write_queue_empty(sk))
>                         tcp_chrono_stop(sk, TCP_CHRONO_BUSY);
>

Very nice catch !

The FIN flag is a really special case here.

What we need is to make sure the skb is 'empty' .

What about using a single condition ?

if (skb && TCP_SKB_CB(skb)->seq == TCP_SKB_CB(skb)->end_seq)