Messages in this thread | | | Subject | Re: net: use-after-free in tw_timer_handler | From | Eric Dumazet <> | Date | Wed, 08 Feb 2017 09:58:36 -0800 |
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On Wed, 2017-02-08 at 18:36 +0100, Dmitry Vyukov wrote: > On Tue, Jan 24, 2017 at 4:52 PM, Eric Dumazet <edumazet@google.com> wrote: > > On Tue, Jan 24, 2017 at 7:06 AM, Dmitry Vyukov <dvyukov@google.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.
> 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.
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