netdev - Re: kernel BUG at net/core/skbuff.c:LINE! (3)

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Date:   Tue, 3 Dec 2019 06:56:16 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        syzbot <syzbot+b2bf2652983d23734c5c@...kaller.appspotmail.com>,
        David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        LKML <linux-kernel@...r.kernel.org>, linux-sctp@...r.kernel.org,
        Xin Long <lucien.xin@...il.com>, mvohra@...are.com,
        netdev <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        William Tu <u9012063@...il.com>,
        Vladislav Yasevich <vyasevich@...il.com>,
        websitedesignservices4u@...il.com,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>
Subject: Re: kernel BUG at net/core/skbuff.c:LINE! (3)

On Tue, Dec 03, 2019 at 09:42:14AM +0100, Dmitry Vyukov wrote:
> On Mon, Dec 2, 2019 at 7:39 PM Marcelo Ricardo Leitner
> <marcelo.leitner@...il.com> wrote:
> >
> > On Sat, Nov 30, 2019 at 04:37:56PM +0100, Dmitry Vyukov wrote:
> > > On Sat, Nov 30, 2019 at 3:50 PM syzbot
> > > <syzbot+b2bf2652983d23734c5c@...kaller.appspotmail.com> wrote:
> > > >
> > > > syzbot has bisected this bug to:
> > > >
> > > > commit 84e54fe0a5eaed696dee4019c396f8396f5a908b
> > > > Author: William Tu <u9012063@...il.com>
> > > > Date:   Tue Aug 22 16:40:28 2017 +0000
> > > >
> > > >      gre: introduce native tunnel support for ERSPAN
> > > >
> > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=158a2f86e00000
> > > > start commit:   f9f1e414 Merge tag 'for-linus-4.16-rc1-tag' of git://git.k..
> > > > git tree:       upstream
> > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=178a2f86e00000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=138a2f86e00000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=34a80ee1ac29767b
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=b2bf2652983d23734c5c
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=147bfebd800000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13d8d543800000
> > > >
> > > > Reported-by: syzbot+b2bf2652983d23734c5c@...kaller.appspotmail.com
> > > > Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN")
> > > >
> > > > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> > >
> > > Humm... the repro contains syz_emit_ethernet, wonder if it's
> > > remote-triggerable...
> >
> > The call trace is still from the tx path. Packet never left the system
> > in this case.
> 
> My understanding is that this does not necessarily mean that the
> remote side is not involved. There is enough state on the host for L4
> protocols, so that the remote side can mess things and then the bad
> thing will happen with local trigger. But that local trigger can be
> just anything trivial that everybody does.
> 
But thats not really helpful.  Unless you see an explicit path from the receive
side to ip6_append_data, Theres no real way for a received packet to reach this
code, so we can't really call it remotely triggerable.

My guess is, since this is coming from the rawv6_sendmsg path, that the raw
protocol is somehow not marshaling its data in a way that ip6_append_data
expects.

Neil