panic in tcp_do_segment()

[Peter Holm]

On Mon, Apr 08, 2013 at 02:13:40PM +0200, Andre Oppermann wrote:
> On 05.04.2013 13:09, Matt Miller wrote:
> > Hey Rick,
> >
> > I believe Juan and I have root caused this crash recently.  The t_state =
> > 0x1, TCPS_LISTEN, in the link provided at the time of the assertion.
> >
> > In tcp_input(), if we're in TCPS_LISTEN, SO_ACCEPTCONN should be set on the
> > socket and we should never enter tcp_do_segment() for this state.  I think
> > if you look in your corefile, you'll see the socket *doesn't* have this
> > flag set in your case.
> >
> > 1043         /*
> > 1044          * When the socket is accepting connections (the INPCB is in
> > LISTEN
> > 1045          * state) we look into the SYN cache if this is a new
> > connection
> > 1046          * attempt or the completion of a previous one.  Because listen
> > 1047          * sockets are never in TCPS_ESTABLISHED, the V_tcbinfo lock
> > will be
> > 1048          * held in this case.
> > 1049          */
> > 1050         if (so->so_options & SO_ACCEPTCONN) {
> > 1051                 struct in_conninfo inc;
> > 1052
> > 1053                 KASSERT(tp->t_state == TCPS_LISTEN, ("%s: so accepting
> > but "
> > 1054                     "tp not listening", __func__));
> > ...
> > 1356                 syncache_add(&inc, &to, th, inp, &so, m, NULL, NULL);
> > 1357                 /*
> > 1358                  * Entry added to syncache and mbuf consumed.
> > 1359                  * Everything already unlocked by syncache_add().
> > 1360                  */
> > 1361                 INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
> > 1362                 return;
> > 1363         }
> > ...
> > 1384         /*
> > 1385          * Segment belongs to a connection in SYN_SENT, ESTABLISHED or
> > later
> > 1386          * state.  tcp_do_segment() always consumes the mbuf chain,
> > unlocks
> > 1387          * the inpcb, and unlocks pcbinfo.
> > 1388          */
> > 1389         tcp_do_segment(m, th, so, tp, drop_hdrlen, tlen, iptos,
> > ti_locked);
> >
> > I think this has to do with this patch in soclose() where SO_ACCEPTCONN is
> > being turned off in soclose().  I suspect if you look at the other threads
> > in your corefile, you'll see one at this point in soclose() operating on
> > the same socket as the one in the tcp_do_segment() thread.
> >
> > http://svnweb.freebsd.org/base?view=revision&revision=243627
> >
> >   817                 /*
> >   818                  * Prevent new additions to the accept queues due
> >   819                  * to ACCEPT_LOCK races while we are draining them.
> >   820                  */
> >   821                 so->so_options &= ~SO_ACCEPTCONN;
> >   822                 while ((sp = TAILQ_FIRST(&so->so_incomp)) != NULL) {
> >   823                         TAILQ_REMOVE(&so->so_incomp, sp, so_list);
> >   824                         so->so_incqlen--;
> >   825                         sp->so_qstate &= ~SQ_INCOMP;
> >   826                         sp->so_head = NULL;
> >   827                         ACCEPT_UNLOCK();
> >   828                         soabort(sp);
> >   829                         ACCEPT_LOCK();
> >   830                 }
> >
> > Juan had evaluated this code path and it seemed safe to just drop the
> > packet in this case:
> >
> > +     /*
> > +      * In closing down the socket, the SO_ACCEPTCONN flag is removed to
> > +      * prevent new connections from being established.  This means that
> > +      * any frames in that were in the midst of being processed could
> > +      * make it here.  Need to just drop the frame.
> > +      */
> > +     if (TCPS_LISTEN == tp->t_state) {
> > +         TCPSTAT_INC(tcps_rcvwhileclosing);
> > +         goto drop;
> > +     }
> >        KASSERT(tp->t_state > TCPS_LISTEN, ("%s: TCPS_LISTEN",
> >            __func__));
> >
> > Or, if there's someone more familiar with the locking in these paths, they
> > may be able to come up with a way to restructure the locks and logic to
> > close this window.
> 
> Matt, Juan,
> 
> excellent analysis.  I don't see a better approach to handle this
> under the current ACCEPT_LOCK model.
> 
> Compared to your patch I'd like to handle this race earlier before
> we hit tcp_do_segment().
> 
> Could you please review the attached patch which handles it right
> after the SO_ACCEPTCONN / syncache check?
> 
> -- 
> Andre
> 
> Index: netinet/tcp_input.c
> ===================================================================
> --- netinet/tcp_input.c	(revision 249253)
> +++ netinet/tcp_input.c	(working copy)
> @@ -1351,6 +1351,16 @@
>   		 */
>   		INP_INFO_UNLOCK_ASSERT(&V_tcbinfo);
>   		return;
> +	} else if (tp->t_state == TCPS_LISTEN) {
> +		/*
> +		 * When a listen socket is torn down the SO_ACCEPTCONN
> +		 * flag is removed first while connections are drained
> +		 * from the accept queue in a unlock/lock cycle of the
> +		 * ACCEPT_LOCK, opening a race condition allowing a SYN
> +		 * attempt go through unhandled.
> +		 */
> +		TCPSTAT_INC(tcps_rcvdwhileclosing);
> +		goto drop;
>   	}
> 
>   #ifdef TCP_SIGNATURE

I was able to reproduce the original "panic: tcp_do_segment:
TCPS_LISTEN" with ease; see
http://people.freebsd.org/~pho/stress/log/tcp.txt.

With your patch (minus the TCPSTAT_INC) I got this "panic: Lock (rw)
tcp locked @ netinet/tcp_input.c:1432."

http://people.freebsd.org/~pho/stress/log/tcp2.txt

- Peter