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

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Date:   Wed, 08 Feb 2017 11:17:42 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.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, 2017-02-08 at 19:55 +0100, Dmitry Vyukov wrote:
> 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.

I meant that this code assumed RTNL being held.

This might not be the case now, after some old change.