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Date:	Wed, 09 Jul 2008 14:45:16 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	hemao77@...il.com, bugme-daemon@...zilla.kernel.org,
	netdev@...r.kernel.org,
	Netfilter Development Mailinglist 
	<netfilter-devel@...r.kernel.org>,
	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
Subject: Re: [Bugme-new] [Bug 11058] New: DEADLOOP in kernel network module

Andrew Morton wrote:
> (switched to email.  Please respond via emailed reply-to-all, not via the
> bugzilla web interface).
> 
> On Tue,  8 Jul 2008 20:13:20 -0700 (PDT) bugme-daemon@...zilla.kernel.org wrote:
> 
>> http://bugzilla.kernel.org/show_bug.cgi?id=11058
>>
>>            Summary: DEADLOOP in kernel network module
>> ...
>> Problem Description:
>>
>> We have seen a deadloop in softirq and we got the cause by creating and
>> analysising the core dump of the kernel.I dont know where to submit a patch ,
>> so I report it here,wish you can apply it to furthur version of linux kernel.
>>
>> The Cause:
>>
>> Both  ctnetlink_del_conntrack() and the tcp_packet() run
>> the code below:
>>
>>     if (del_timer(&ct->timeout))                /*deactive the timer*/
>>        ct->timeout.function((unsigned long) ct);/*remove conntrack from
>> conntrack table*/
>>
>> the ctnetlink_del_conntrack() context is:
>> ................
>>         if (cda[CTA_ID-1]) {
>>                 u_int32_t id = ntohl(*(u_int32_t *)NFA_DATA(cda[CTA_ID-1]));
>>                 if (ct->id != id) {
>>                         ip_conntrack_put(ct);
>>                         return -ENOENT;
>>                 }
>>         }       
>> *       if (del_timer(&ct->timeout))
>> *               ct->timeout.function((unsigned long)ct);
>>
>>         ip_conntrack_put(ct);
>> ...........
>>
>> the tcp_packet() context is:
>> ...........
>>         case TCP_CONNTRACK_SYN_SENT:
>>                 if (old_state < TCP_CONNTRACK_TIME_WAIT)
>>                         break;
>>                 if ((conntrack->proto.tcp.seen[dir].flags &
>>                          IP_CT_TCP_FLAG_CLOSE_INIT)
>>                     || after(ntohl(th->seq),
>>                              conntrack->proto.tcp.seen[dir].td_end)) {  
>>                         /* Attempt to reopen a closed connection.
>>                         * Delete this connection and look up again. */
>>                         write_unlock_bh(&tcp_lock);
>> *                       if (del_timer(&conntrack->timeout))
>> *                               conntrack->timeout.function((unsigned long)
>> *                                                           conntrack);
>>                         return -NF_REPEAT;
>> ......
>>
>> How the DEADLOOP happened?
>>
>>     (1)in ctnetlink_del_conntrack()(runs in system call context): the del_timer
>> is called and then goes to timeout.function.
>>     (2)before timeout.function finish excution(means the conntrack not
>> removed),an interrupt happens and a SYN packet of  the same conntrack
>> comes.CPU goes to irq handle and enventually runs tcp_packet().
>>     (3)in tcp_packet() ,del_timer() will fail because the timer was
>> already deleted. the timeout.function in tcp_packet will not run,
>> -NF_REPEAT is returned, the SYN packet will be passed back again.
>>     (4)Neither side has the chance to run timeout.function,the
>> conntrack remains there,deadloop happen,the SYN packet will be passed back
>> again and again.
>>
>> The fix maybe,add lock the softirq when doing conntrack removing:
>> +++    local_bh_disable();
>>       if (del_timer(&ct->timeout))                /*deactive the timer*/
>>          ct->timeout.function((unsigned long) ct);/*remove conntrack from
>> conntrack table*/
>> +++    local_bh_enable();
>>
>> Thanks, may this be helpful.
>> My email: hemao77@...il.com
>>
>> It is hard to reproduce , but it really happen on our linux box.
>>
> 
> Thanks.
> 
> Please submit patches via email as described in
> Documentation/SubmittingPatches.  The file ./MAINTAINERS can be used to
> determine which individuals and mailing lists the patch should be sent to.
> 
> But that's for next time - this patch is small enough for the netfilter
> developers to be able to type in again ;)

Good catch, thanks. Basically all del_timer()/timeout.function calls
in conntrack can happen in process context, so we'd have to disable
BHs every time we do this. I think this fix should also work. The
only spot where we return NF_REPEAT is in TCP conntrack, so we can
simply make sure we only do this if we actually managed to kill the
connection.

Jozsef, what do you think?

[patch for review against net-next, I'll backport it later unless
there are objections]


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