Re: net: use-after-free in tw_timer_handler

From: Cong Wang
Date: Fri Feb 17 2017 - 13:52:15 EST

On Wed, Feb 8, 2017 at 9:36 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Tue, Jan 24, 2017 at 4:52 PM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>> On Tue, Jan 24, 2017 at 7:06 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>>> This code was changed a long time ago :
>>>> 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.