Messages in this thread | | | From | Eric Dumazet <> | Date | Wed, 20 Oct 2021 18:10:54 -0700 | Subject | Re: [net-next] tcp: don't free a FIN sk_buff in tcp_remove_empty_skb() |
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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 !
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)
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