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

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Date:   Wed, 8 Feb 2017 19:55:57 +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>,
        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 6:58 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-02-08 at 18:36 +0100, Dmitry Vyukov 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?
>
> 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.


>> 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).
>>
>
> Timer is pinned, it cannot fire under us on this cpu.

Ack