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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Wed, 29 May 2019 17:14:17 +0200
Message-ID: <CACT4Y+bWyNawZBQkV3TyyFF0tyHnJ9UPsCW-EzmC7rwwh3yk2g@mail.gmail.com>
Subject: Re: KASAN: use-after-free Read in br_mdb_ip_get
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Nikolay Aleksandrov <nikolay@cumulusnetworks.com>,
        Thomas Graf <tgraf@suug.ch>,
        syzbot <syzbot+bc5ab0af2dbf3b0ae897@syzkaller.appspotmail.com>,
        bridge@lists.linux-foundation.org,
        David Miller <davem@davemloft.net>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Roopa Prabhu <roopa@cumulusnetworks.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
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On Wed, May 29, 2019 at 4:58 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> Hi Dmitry:
>
> On Thu, Feb 21, 2019 at 11:54:42AM +0100, Dmitry Vyukov wrote:
> >
> > Taking into account that this still happened only once, I tend to
> > write it off onto a previous silent memory corruption (we have dozens
> > of known bugs that corrupt memory). So if several people already
> > looked at it and don't see the root cause, it's probably time to stop
> > spending time on this until we have more info.
> >
> > Although, there was also this one:
> > https://groups.google.com/d/msg/syzkaller-bugs/QfCCSxdB1aM/y2cn9IZJCwAJ
> > I have not checked if it can be the root cause of this report, but it
> > points suspiciously close to this stack and when I looked at it, it
> > the report looked legit.
>
> Have you had any more reports of this kind coming from br_multicast?
>
> It looks like
>
> ommit 1515a63fc413f160d20574ab0894e7f1020c7be2
> Author: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
> Date:   Wed Apr 3 23:27:24 2019 +0300
>
>     net: bridge: always clear mcast matching struct on reports and leaves
>
> may have at least fixed the uninitialised value error.


The most up-to-date info is always available here:

>> dashboard link: https://syzkaller.appspot.com/bug?extid=bc5ab0af2dbf3b0ae897

It says no new crashes happened besides the original one.

We now have the following choices:

1. Invalidate with "#syz invalid"
2. Mark as tentatively fixed by that commit (could it fix it?) with
"#syz fix: net: bridge: always clear mcast matching struct on reports
and leaves"
3. Do nothing, then syzbot will auto-close it soon (bugs without
reproducers that did not happen in the past 180 days)