Re: general protection fault in mm_update_next_owner

Re: general protection fault in mm_update_next_owner

[Hillf Danton]

On Tue, 11 Jun 2019 09:00:09 +0200 Dmitry Vyukov wrote:
>On Mon, Jun 10, 2019 at 11:27 PM Eric W. Biederman wrote:
>>
>> syzbot <syzbot+f625baafb9a1c4bfc3f6@syzkaller.appspotmail.com> writes:
>>
>> > syzbot has bisected this bug to:
>> >
>> > commit e9db4ef6bf4ca9894bb324c76e01b8f1a16b2650
>> > Author: John Fastabend <john.fastabend@gmail.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@syzkaller.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.

Sorry for reading this message at lore today and late reply because it
did not land in my inbox in Jun 2019.

A couple of days ago, I saw an offline linux-4.18 page fault Oops report
that could trigger the check for X86_PF_USER and X86_PF_INSTR added in

  03c81ea33316 ("x86/fault: Improve kernel-executing-user-memory handling") 

and given the reported CPU is Intel Atom, any light on how to reproduce
it is highly appreciated.

Hillf