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

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Date:   Fri, 17 Feb 2017 10:51:38 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.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 Wed, Feb 8, 2017 at 9:36 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Tue, Jan 24, 2017 at 4:52 PM, Eric Dumazet <edumazet@...gle.com> wrote:
>> On Tue, Jan 24, 2017 at 7:06 AM, Dmitry Vyukov <dvyukov@...gle.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.