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Date: Tue, 4 Apr 2017 18:14:54 -0300
From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
To: Xin Long <lucien.xin@gmail.com>
Cc: Andrey Konovalov <andreyknvl@google.com>,
        Vlad Yasevich <vyasevich@gmail.com>,
        Neil Horman <nhorman@tuxdriver.com>,
        "David S. Miller" <davem@davemloft.net>, linux-sctp@vger.kernel.org,
        netdev <netdev@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>, Eric Dumazet <edumazet@google.com>,
        Kostya Serebryany <kcc@google.com>,
        syzkaller <syzkaller@googlegroups.com>
Subject: Re: net/sctp: list double add warning in sctp_endpoint_add_asoc
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On Wed, Apr 05, 2017 at 01:29:19AM +0800, Xin Long wrote:
> On Tue, Apr 4, 2017 at 9:28 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> > Hi,
> >
> > I've got the following error report while fuzzing the kernel with syzkaller.
> >
> > On commit a71c9a1c779f2499fb2afc0553e543f18aff6edf (4.11-rc5).
> >
> > A reproducer and .config are attached.
> The script is pretty hard to reproduce the issue in my env.

I didn't try running it but I also found the reproducer very complicated
to follow. Do you have any plans on having some PoC optimizer, so we can
have a more readable code?
strace is handy for filtering the noise, yes, but sometimes it doesn't
cut it.

> But there seems a case to cause a use-after-free when out of snd_buf.
> 
> the case is like:
> -----------
> one thread:                       another thread:
>                                   sctp_rcv hold asoc (hold transport)
>                                   enqueue the chunk to backlog queue
>                                   [refcnt=2]
> 
> sctp_close free assoc
> [refcnt=1]
> 
> sctp_sendmsg find asoc
> but not hold it
> 
> out of snd_buf
> hold asoc, schedule out
> [refcnt = 2]
> 
>                                   process backlog and put asoc/transport
>                                   [refcnt=1]
> 
> schedule in, put asoc
> [refcnt=0] <--- destroyed
> 
> sctp_sendmsg continue

It shouldn't be continuing here because sctp_wait_for_sndbuf and
sctp_wait_for_connect functions are checking if the asoc is dead
already when it schedules in, even though sctp_wait_for_connect return
value is ignored and sctp_sendmsg() simply returns after that.
Or the checks for dead asocs in there aren't enough somehow.

> using asoc, panic