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

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Date:   Tue, 14 Feb 2017 20:38:30 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        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>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: Re: net: use-after-free in tw_timer_handler

On Wed, Feb 8, 2017 at 8:32 PM, 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?
>>> >
>>> > Yes, atomic_read() is not a proper sync point.
>>>
>>> Do you mean that it does not include read barrier?
>>> I more mean that we can call inet_twsk_deschedule_put twice for the same socket.
>>
>> I meant that this code assumed RTNL being held.
>>
>> This might not be the case now, after some old change.
>
>
> cleanup_net releases rtnl lock right before calling these callbacks.


+Andrey, do you know somebody on your side interested in stability of
network namespace?
This use-after-free seems to be related to net namespace. For context,
full thread is here:
https://groups.google.com/forum/#!msg/syzkaller/p1tn-_Kc6l4/smuL_FMAAgAJ