linux-kernel - Re: BUG: unable to handle kernel NULL pointer dereference in fbcon

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Message-ID: <YAVU/vIwsqAzC7Qm@phenom.ffwll.local>
Date:   Mon, 18 Jan 2021 10:29:34 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     syzbot <syzbot+b67aaae8d3a927f68d20@...kaller.appspotmail.com>
Cc:     b.zolnierkie@...sung.com, daniel.vetter@...ll.ch,
        daniel.vetter@...el.com, dri-devel@...ts.freedesktop.org,
        george.kennedy@...cle.com, gregkh@...uxfoundation.org,
        jirislaby@...nel.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, melissa.srw@...il.com,
        natechancellor@...il.com, sam@...nborg.org,
        syzkaller-bugs@...glegroups.com, tzimmermann@...e.de,
        yepeilin.cs@...il.com
Subject: Re: BUG: unable to handle kernel NULL pointer dereference in
 fbcon_cursor

On Sun, Jan 17, 2021 at 03:29:05AM -0800, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit ea40d7857d5250e5400f38c69ef9e17321e9c4a2
> Author: Daniel Vetter <daniel.vetter@...ll.ch>
> Date:   Fri Oct 9 23:21:56 2020 +0000
> 
>     drm/vkms: fbdev emulation support

Not sure you want to annotate this, but this just makes the bug
reproducible on vkms. It's a preexisting issue (probably a few decades
old) of the fbcon code afaict. It might also be that you can only repro
this when you have multiple fbcon drivers (vkms plus whatever your virtual
machine has I guess).
-Daniel

> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=148e2748d00000
> start commit:   b3a3cbde Add linux-next specific files for 20210115
> git tree:       linux-next
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=168e2748d00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=128e2748d00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=6ea08dae6aab586f
> dashboard link: https://syzkaller.appspot.com/bug?extid=b67aaae8d3a927f68d20
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15cd8fe0d00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17af5258d00000
> 
> Reported-by: syzbot+b67aaae8d3a927f68d20@...kaller.appspotmail.com
> Fixes: ea40d7857d52 ("drm/vkms: fbdev emulation support")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch