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

[Jonathan Maxwell <jmaxwell37@gmail.com>]

On Thu, Oct 21, 2021 at 12:11 PM Eric Dumazet <edumazet@google.com> wrote:
>
> On Wed, Oct 20, 2021 at 4:25 PM Jon Maxwell <jmaxwell37@gmail.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@gmail.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 !
>

Thanks Eric.

> 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)

Good call as the end_seq will be +1 for a FIN. So that's better.

Let me give the customer a kernel with your idea:

--- net/ipv4/tcp.c 2021-10-20 22:50:35.836001950 +0530
+++ net/ipv4/tcp.c.patch 2021-10-21 01:42:08.493569483 +0530
@@ -955,7 +955,7 @@
  */
 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);

I'll ask the customer to confirm that the v1 patch as above also resolves
the issue. Although I expect it will.

Then I'll resubmit a v1 patch with your suggestion probably early next week.

Regards

Jon