linux-kernel - Re: KASAN: use-after-free Read in tty

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Date:   Wed, 4 Dec 2019 07:53:54 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     syzbot <syzbot+9af6d43c1beabec8fd05@...kaller.appspotmail.com>
Cc:     Gleb Natapov <gleb@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        gwshan@...ux.vnet.ibm.com, "H. Peter Anvin" <hpa@...or.com>,
        Jiri Slaby <jslaby@...e.com>, KVM list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Russell Currey <ruscur@...sell.cc>, stewart@...ux.vnet.ibm.com,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: KASAN: use-after-free Read in tty_open

On Wed, Dec 4, 2019 at 3:45 AM syzbot
<syzbot+9af6d43c1beabec8fd05@...kaller.appspotmail.com> 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=15e5fc32e00000
> 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=17e5fc32e00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=13e5fc32e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=dd226651cb0f364b
> dashboard link: https://syzkaller.appspot.com/bug?extid=9af6d43c1beabec8fd05
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16d15061e00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14b69aeae00000
>
> Reported-by: syzbot+9af6d43c1beabec8fd05@...kaller.appspotmail.com
> Fixes: 2de50e9674fc ("powerpc/powernv: Remove support for p5ioc2")
>
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

This should have been detected as "does not affect binary", but there
is something I don't understand/missing:
This is bisected to 2de50e9674fc4ca3c6174b04477f69eb26b4ee31
and it has this parent:
$ git log -n 1 --format="%P" 2de50e9674fc4ca3c6174b04477f69eb26b4ee31
388f7b1d6e8ca06762e2454d28d6c3c55ad0fe95
But the parent was never tested during bisection... how is this possible?
Mentioned this here:
https://github.com/google/syzkaller/issues/1271#issuecomment-561504032