linux-kernel - Re: [BUG] fbdev: i740fb: Divide error when ‘var->pixclock’ is zero

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Message-ID: <CAMuHMdU=r+TEJmgdnuM+0Mxa0jQ4HjtKT1jzkw_2R+1v9K_9RQ@mail.gmail.com>
Date:   Tue, 5 Apr 2022 08:52:33 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Helge Deller <deller@....de>
Cc:     Zheyu Ma <zheyuma97@...il.com>,
        Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] fbdev: i740fb: Divide error when ‘var->pixclock’ is zero

Hi Helge,

On Tue, Apr 5, 2022 at 8:34 AM Helge Deller <deller@....de> wrote:
> On 4/4/22 13:46, Geert Uytterhoeven wrote:
> > On Sun, Apr 3, 2022 at 5:41 PM Helge Deller <deller@....de> wrote:
> >> On 4/3/22 13:26, Zheyu Ma wrote:
> >>> I found a bug in the function i740fb_set_par().
> >>
> >> Nice catch!
> >>
> >>> When the user calls the ioctl system call without setting the value to
> >>> 'var->pixclock', the driver will throw a divide error.
> >>>
> >>> This bug occurs because the driver uses the value of 'var->pixclock'
> >>> without checking it, as the following code snippet show:
> >>>
> >>> if ((1000000 / var->pixclock) > DACSPEED8) {
> >>>      dev_err(info->device, "requested pixclock %i MHz out of range
> >>> (max. %i MHz at 8bpp)\n",
> >>>          1000000 / var->pixclock, DACSPEED8);
> >>>     return -EINVAL;x
> >>> }
> >>>
> >>> We can fix this by checking the value of 'var->pixclock' in the
> >>> function i740fb_check_var() similar to commit
> >>> b36b242d4b8ea178f7fd038965e3cac7f30c3f09, or we should set the lowest
> >>> supported value when this field is zero.
> >>> I have no idea about which solution is better.
> >>
> >> Me neither.
> >> I think a solution like commit b36b242d4b8ea178f7fd038965e3cac7f30c3f09
> >> is sufficient.
> >>
> >> Note that i740fb_set_par() is called in i740fb_resume() as well.
> >> Since this doesn't comes form userspace I think adding a check for
> >> the return value there isn't necessary.
> >>
> >> Would you mind sending a patch like b36b242d4b8ea178f7fd038965e3cac7f30c3f09 ?
> >
> > When passed an invalid value, .check_var() is supposed to
> > round up the invalid to a valid value, if possible.
>
> I don't disagree.
> The main problem probably is: what is the next valid value?
> This needs to be analyzed on a per-driver base and ideally tested.
> Right now a division-by-zero is tiggered which is probably more worse.
>
> That said, currently I'd prefer to apply the zero-checks patches over
> any untested patches. It's easy to revert such checks if a better solution
> becomes available.
>
> Thoughts?

Fair enough. And you're the maintainer ;-)

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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