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

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Date:   Wed, 8 Feb 2017 18:36:07 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     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 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?

The second issue that I noticed is that tw_refcnt is set to 4 _after_
we schedule the timer. The timer will probably won't fire before we
set tw_refcnt, but if it somehow does it will corrupt the ref count. I
don't think that it's what I am seeing, though. More likely it's the
first issues (if it's real).

Does it make any sense?