linux-kernel - Re: general protection fault in mm_update_next

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Message-ID: <CACT4Y+btAivG8iYQFM=Qy_qMoE0SFNhx-ngjN=1hgf7UGrNViw@mail.gmail.com>
Date:   Tue, 11 Jun 2019 09:00:09 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     syzbot <syzbot+f625baafb9a1c4bfc3f6@...kaller.appspotmail.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Alexei Starovoitov <ast@...nel.org>, avagin@...il.com,
        Daniel Borkmann <daniel@...earbox.net>, dbueso@...e.de,
        John Fastabend <john.fastabend@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>, Oleg Nesterov <oleg@...hat.com>,
        prsood@...eaurora.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        bpf <bpf@...r.kernel.org>
Subject: Re: general protection fault in mm_update_next_owner

On Mon, Jun 10, 2019 at 11:27 PM Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> syzbot <syzbot+f625baafb9a1c4bfc3f6@...kaller.appspotmail.com> writes:
>
> > syzbot has bisected this bug to:
> >
> > commit e9db4ef6bf4ca9894bb324c76e01b8f1a16b2650
> > Author: John Fastabend <john.fastabend@...il.com>
> > Date:   Sat Jun 30 13:17:47 2018 +0000
> >
> >     bpf: sockhash fix omitted bucket lock in sock_close
> >
> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15e978e1a00000
> > start commit:   38e406f6 Merge git://git.kernel.org/pub/scm/linux/kernel/g..
> > git tree:       net
> > final crash:    https://syzkaller.appspot.com/x/report.txt?x=17e978e1a00000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=13e978e1a00000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=60564cb52ab29d5b
> > dashboard link: https://syzkaller.appspot.com/bug?extid=f625baafb9a1c4bfc3f6
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1193d81ea00000
> >
> > Reported-by: syzbot+f625baafb9a1c4bfc3f6@...kaller.appspotmail.com
> > Fixes: e9db4ef6bf4c ("bpf: sockhash fix omitted bucket lock in sock_close")
> >
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
>
> How is mm_update_next_owner connected to bpf?


There seems to be a nasty bug in bpf that causes assorted crashes
throughout the kernel for some time. I've seen a bunch of reproducers
that do something with bpf and then cause a random crash. The more
unpleasant ones are the bugs without reproducers, because for these we
don't have a way to link them back to the bpf bug but they are still
hanging there without good explanation, e.g. maybe a part of one-off
crashes in moderation:
https://syzkaller.appspot.com/upstream#moderation2

Such bugs are nice to fix asap to not produce more and more random
crash reports.

Hillf, did you understand the mechanics of this bug and memory
corruption? A good question is why this was unnoticed by KASAN. If we
could make it catch it at the point of occurrence, then it would be a
single bug report clearly attributed to bpf rather then dozens of
assorted crashes.