linux-kernel - Re: net: use-after-free in tw_timer

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Date:   Fri, 17 Feb 2017 21:36:57 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        James Morris <jmorris@...ei.org>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Patrick McHardy <kaber@...sh.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net: use-after-free in tw_timer_handler

On Fri, Feb 17, 2017 at 7:51 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>>>>>
>>>>> This code was changed a long time ago :
>>>>>
>>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ed2e923945892a8372ab70d2f61d364b0b6d9054
>>>>>
>>>>> So I suspect a recent patch broke the logic.
>>>>>
>>>>> You might start a bisection :
>>>>>
>>>>> I would check if 4.7 and 4.8 trigger the issue you noticed.
>>>>
>>>>
>>>> It happens with too low rate for bisecting (few times per day). I
>>>> could add some additional checks into code, but I don't know what
>>>> checks could be useful.
>>>
>>> If you can not tell if 4.7 and/or 4.8 have the problem, I am not sure
>>> we are able to help.
>>
>>
>> There are also chances that the problem is older.
>>
>> Looking at the code, this part of inet_twsk_purge looks fishy:
>>
>> 285                         if (unlikely((tw->tw_family != family) ||
>> 286                                      atomic_read(&twsk_net(tw)->count))) {
>>
>> It uses net->count == 0 check to find the right sockets. But what if
>> there are several nets with count == 0 in flight, can't there be
>> several inet_twsk_purge calls running concurrently freeing each other
>> sockets? If so it looks like inet_twsk_purge can call
>> inet_twsk_deschedule_put twice for a socket. Namely, two calls for
>> different nets discover the socket, check that net->count==0 and both
>> call inet_twsk_deschedule_put. Shouldn't we just give inet_twsk_purge
>> net that it needs to purge?
>
> I don't think this could happen, because cleanup_net() is called in a
> work struct, and this work can't be scheduled twice, so there should
> not be any parallel cleanup_net().
>
> Also, inet_twsk_deschedule_put() already waits for the flying timer,
> net->count==0 at this point and all sockets in this netns are already
> gone, so I don't know how a timer could be still fired after this.


One thing that I noticed is that use-after-free always happens in
tw_timer_handler, but never in timer code. This suggests that:
1. Whoever frees the struct waits for the timer mutex unlock.
2. Free happens almost at the same time as timer fires (otherwise the
timer would probably be cancelled).

That could happen if there is a race in timer code during cancellation
or rearming. I understand that it's heavily stressed code, but who
knows...

I still wonder if twsk_net(tw)->count==0 is the right condition. This
net may not yet have been scheduled for destruction, but another task
could pick it up and start destroying...

Cong, do you have any ideas as to what debugging checks I could add to
help track this down?