linux-kernel - Re: KASAN: slab-out-of-bounds Read in fbcon_get

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Date:   Thu, 5 Dec 2019 11:31:27 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     syzbot <syzbot+4455ca3b3291de891abc@...kaller.appspotmail.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        DRI <dri-devel@...ts.freedesktop.org>, ghalat@...hat.com,
        Gleb Natapov <gleb@...nel.org>, gwshan@...ux.vnet.ibm.com,
        "H. Peter Anvin" <hpa@...or.com>, James Morris <jmorris@...ei.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        KVM list <kvm@...r.kernel.org>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Russell Currey <ruscur@...sell.cc>,
        Sam Ravnborg <sam@...nborg.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, stewart@...ux.vnet.ibm.com,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        Thomas Gleixner <tglx@...utronix.de>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: KASAN: slab-out-of-bounds Read in fbcon_get_font

On Thu, Dec 5, 2019 at 11:22 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 05/12/19 11:16, Dmitry Vyukov wrote:
> > On Thu, Dec 5, 2019 at 11:13 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
> >>
> >> On 04/12/19 22:41, syzbot wrote:
> >>> syzbot has bisected this bug to:
> >>>
> >>> commit 2de50e9674fc4ca3c6174b04477f69eb26b4ee31
> >>> Author: Russell Currey <ruscur@...sell.cc>
> >>> Date:   Mon Feb 8 04:08:20 2016 +0000
> >>>
> >>>     powerpc/powernv: Remove support for p5ioc2
> >>>
> >>> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=127a042ae00000
> >>> start commit:   76bb8b05 Merge tag 'kbuild-v5.5' of
> >>> git://git.kernel.org/p..
> >>> git tree:       upstream
> >>> final crash:    https://syzkaller.appspot.com/x/report.txt?x=117a042ae00000
> >>> console output: https://syzkaller.appspot.com/x/log.txt?x=167a042ae00000
> >>> kernel config:  https://syzkaller.appspot.com/x/.config?x=dd226651cb0f364b
> >>> dashboard link:
> >>> https://syzkaller.appspot.com/bug?extid=4455ca3b3291de891abc
> >>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11181edae00000
> >>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=105cbb7ae00000
> >>>
> >>> Reported-by: syzbot+4455ca3b3291de891abc@...kaller.appspotmail.com
> >>> Fixes: 2de50e9674fc ("powerpc/powernv: Remove support for p5ioc2")
> >>>
> >>> For information about bisection process see:
> >>> https://goo.gl/tpsmEJ#bisection
> >>>
> >>
> >> Why is everybody being CC'd, even if the bug has nothing to do with the
> >> person's subsystem?
> >
> > The To list should be intersection of 2 groups of emails: result of
> > get_maintainers.pl on the file identified as culprit in the crash
> > message + emails extracted from the bisected to commit.
>
> Ah, and because the machine is a KVM guest, kvm_wait appears in a lot of
> backtrace and I get to share syzkaller's joy every time. :)

I don't see any mention of "kvm" in the crash report. And it's only 1
file, not all of them, in this case I would expect it to be
drivers/video/fbdev/core/fbcon.c. So it should be something different.

> This bisect result is bogus, though Tetsuo found the bug anyway.
> Perhaps you can exclude commits that only touch architectures other than
> x86?

We do this. It work sometimes. But sometimes it hits non-deterministic
kernel build bugs:
https://github.com/google/syzkaller/issues/1271#issuecomment-559093018
And in this case it hit some git bisect weirdness which I can't explain yet:
https://github.com/google/syzkaller/issues/1527