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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Thu, 3 Feb 2022 08:53:53 +0100
Message-ID: <CAMuHMdXxYf2+skr3TN4ZmMx-=Eghe9Prm8KgDvFskovMG0MeOw@mail.gmail.com>
Subject: Re: [PATCH v2 RESEND 3] video: fbdev: cirrusfb: check pixclock to
 avoid divide by zero
To:     George Kennedy <george.kennedy@oracle.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Dan Carpenter <dan.carpenter@oracle.com>
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Hi George,

CC Helge, linux-fbdev

On Wed, Feb 2, 2022 at 7:23 PM George Kennedy <george.kennedy@oracle.com> wrote:
> Do a sanity check on pixclock value to avoid divide by zero.
>
> If the pixclock value is zero, the cirrusfb driver will round up
> pixclock to get the derived frequency as close to maxclock as
> possible.
>
> Syzkaller reported a divide error in cirrusfb_check_pixclock.
>
> divide error: 0000 [#1] SMP KASAN PTI
> CPU: 0 PID: 14938 Comm: cirrusfb_test Not tainted 5.15.0-rc6 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2
> RIP: 0010:cirrusfb_check_var+0x6f1/0x1260
>
> Call Trace:
>   fb_set_var+0x398/0xf90
>   do_fb_ioctl+0x4b8/0x6f0
>   fb_ioctl+0xeb/0x130
>   __x64_sys_ioctl+0x19d/0x220
>   do_syscall_64+0x3a/0x80
>   entry_SYSCALL_64_after_hwframe+0x44/0xae
>
> Cc: <stable@vger.kernel.org> # 5.4.y
> Reported-by: syzkaller <syzkaller@googlegroups.com>
> Signed-off-by: George Kennedy<george.kennedy@oracle.com>
> Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
> ---
> Hi Andrew, can you pick this patch up?  Some of the fbdev drivers have
> maintainers but the cirrusfb.c is basically orphaned.  I sent this patch
> in Oct and again in Dec and no one picked it up.  Geert has reviewed it.
> Obviously the original code just crashed so this shouldn't be
> controversial.
>
> v2: Geert says that instead of just returning an error, this is supposed
> to find a valid pixclock and use that.
>
> RESEND: Added the Reviewed by: and Cc: <stable@vger.kernel.org>

This is already queued in the fbdev for-next branch[1], as announced
by Helge in [2].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/deller/linux-fbdev.git/commit/?h=for-next&id=5c6f402bdcf9e7239c6bc7087eda71ac99b31379
[2] https://lore.kernel.org/all/e1e5b7d1-ea09-6e28-9c39-45b983734a85@gmx.de/

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds