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

[Geert Uytterhoeven]

Hi Helge,

On Sun, Apr 3, 2022 at 5:41 PM Helge Deller <deller@xxxxxx> 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.

Commit b36b242d4b8ea178 ("video: fbdev: asiliantfb: Error out if
'pixclock' equals zero") does not do that.

Gr{oetje,eeting}s,

                        Geert

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

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