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From:   Zheyu Ma <zheyuma97@gmail.com>
Date:   Wed, 3 Aug 2022 17:26:51 +0800
Message-ID: <CAMhUBjk-nounZeqN3xq1Yp7+YG=iG+L2_3e1JOnWTJasiups-w@mail.gmail.com>
Subject: Re: [BUG] video: fbdev: arkfb: Found a divide-by-zero bug which may
 cause DoS
To:     Helge Deller <deller@gmx.de>
Cc:     Geert Uytterhoeven <geert@linux-m68k.org>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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Hi,

On Mon, Aug 1, 2022 at 12:35 PM Helge Deller <deller@gmx.de> wrote:
>
> * Zheyu Ma <zheyuma97@gmail.com>:
> > I found a bug in the arkfb driver in the latest kernel, which may cause DoS.
> >
> > The reason for this bug is that the user controls some input to ioctl,
> > making 'mode' 0x7 on line 704, which causes hdiv = 1, hmul = 2, and if
> > the pixclock is controlled to be 1, it will cause a division error in
> > the function ark_set_pixclock().
>
> You are right.
> I see in:
>   drivers/video/fbdev/arkfb.c:784: ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul);
> with hdiv=1, pixclock=1 and hmul=2 you end up with (1*1)/2 = (int) 0.
> and then in
>   drivers/video/fbdev/arkfb.c:504: rv = dac_set_freq(par->dac, 0, 1000000000 / pixclock);
> you'll get a division-by-zero.
>
> > The easiest patch is to check the value of the argument 'pixclock' in
> > the ark_set_pixclock function, but this is perhaps too late, should we
> > do this check earlier? I'm not sure, so I'll report this bug to you.
>
> Yes, I think it should be done earlier.
>
> Geert always mentioned that an invalid pixclock from userspace should be
> rounded up to the next valid pixclock.
> But since I don't have that hardware, I'm not sure how this can be done
> best for this driver.
>
> Do you have the hardware to test?
> If so, could you check the patch below?

Thanks for your patch, it works for me :)

> It should at least prevent the division-by-zero.
> If it works, I'm happy if you could send a final patch...

I've sent a patch to the mailing list, thanks again for your reminder.

regards,

Zheyu Ma