linux-kernel - Re: net/packet: use-after-free in packet_rcv

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Date:   Fri, 10 Feb 2017 10:02:17 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>, jarno@....org,
        Sowmini Varadhan <sowmini.varadhan@...cle.com>,
        Philip Pettersson <philip.pettersson@...il.com>,
        weongyo.linux@...il.com, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: net/packet: use-after-free in packet_rcv_fanout

On Fri, Feb 10, 2017 at 9:59 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2017-02-10 at 09:49 -0800, Cong Wang wrote:
>> On Thu, Feb 9, 2017 at 7:23 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Thu, 2017-02-09 at 19:19 -0800, Eric Dumazet wrote:
>> >
>> >> More likely the bug is in fanout_add(), with a buggy sequence in error
>> >> case, and not correct locking.
>> >>
>> >> kfree(po->rollover);
>> >> po->rollover = NULL;
>> >>
>> >> Two cpus entering fanout_add() (using the same af_packet socket,
>> >> syzkaller courtesy...) might both see po->fanout being NULL.
>> >>
>> >> Then they grab the mutex.  Too late...
>> >
>> > Patch could be :
>> >
>>
>> For me, clearly the data structure that use-after-free'd is struct sock
>> rather than struct packet_rollover.
>
> Fine. But your patch makes absolutely no sense.

I don't have to give a 100% correct patch to prove my explanation
of the crash. At least it makes more sense than yours...