linux-kernel - Re: general protection fault in put

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Date:   Thu, 20 Dec 2018 16:36:53 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Manfred Spraul <manfred@...orfullife.com>
Cc:     syzbot+1145ec2e23165570c3ac@...kaller.appspotmail.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        ktsanaktsidis@...desk.com, LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Matthew Wilcox <willy@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: general protection fault in put_pid

On Wed, Dec 19, 2018 at 10:04 AM Manfred Spraul
<manfred@...orfullife.com> wrote:
>
> Hello Dmitry,
>
> On 12/12/18 11:55 AM, Dmitry Vyukov wrote:
> > On Tue, Dec 11, 2018 at 9:23 PM syzbot
> > <syzbot+1145ec2e23165570c3ac@...kaller.appspotmail.com> wrote:
> >> Hello,
> >>
> >> syzbot found the following crash on:
> >>
> >> HEAD commit:    f5d582777bcb Merge branch 'for-linus' of git://git.kernel...
> >> git tree:       upstream
> >> console output: https://syzkaller.appspot.com/x/log.txt?x=135bc547400000
> >> kernel config:  https://syzkaller.appspot.com/x/.config?x=c8970c89a0efbb23
> >> dashboard link: https://syzkaller.appspot.com/bug?extid=1145ec2e23165570c3ac
> >> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> >> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16803afb400000
> > +Manfred, this looks similar to the other few crashes related to
> > semget$private(0x0, 0x4000, 0x3f) that you looked at.
>
> I found one unexpected (incorrect?) locking, see the attached patch.
>
> But I doubt that this is the root cause of the crashes.


But why? These one-off sporadic crashes reported by syzbot looks
exactly like a subtle race and your patch touches sem_exit_ns involved
in all reports.
So if you don't spot anything else, I would say close these 3 reports
with this patch (I see you already included Reported-by tags which is
great!) and then wait for syzbot reaction. Since we got 3 of them, if
it's still not fixed I would expect that syzbot will be able to
retrigger this later again.


> Any remarks on the patch?
>
> I would continue to search, and then send a series with all findings.
>
> --
>
>      Manfred
>