[PATCH 2/5] fbcon: Fix up user-provided virtual screen size

[Geert Uytterhoeven]

Hi Helge,

On Thu, Jun 30, 2022 at 9:31 PM Helge Deller <deller at gmx.de> wrote:
> On 6/30/22 21:00, Geert Uytterhoeven wrote:
> > On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller at gmx.de> wrote:
> >> The virtual screen size can't be smaller than the physical screen size.
> >> Based on the general rule that we round up user-provided input if
> >> neccessary, adjust the virtual screen size as well if needed.
> >>
> >> Signed-off-by: Helge Deller <deller at gmx.de>
> >> Cc: stable at vger.kernel.org # v5.4+
> >
> > Thanks for your patch!
> >
> >> --- a/drivers/video/fbdev/core/fbmem.c
> >> +++ b/drivers/video/fbdev/core/fbmem.c
> >> @@ -1106,6 +1106,11 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
> >>                         return -EFAULT;
> >>                 console_lock();
> >>                 lock_fb_info(info);
> >> +               /* adjust virtual screen size if user missed it */
> >> +               if (var.xres_virtual < var.xres)
> >> +                       var.xres_virtual = var.xres;
> >> +               if (var.yres_virtual < var.yres)
> >> +                       var.yres_virtual = var.yres;
> >>                 ret = fb_set_var(info, &var);
> >>                 if (!ret)
> >>                         fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
> >
> > Given "[PATCH 4/5] fbmem: Prevent invalid virtual screen sizes in
> > fb_set_var", I don't think we need this patch.
>
> We do.

Why? It will be caught by [PATCH 4/5].

> > Moreover, this
> > patch will prevent triggering the WARN_ON()s in [PATCH 4/5]
>
> Right.
>
> > in the most common buggy case of drivers plainly ignoring var.[xy]res_virtual.
>
> In summary:
> This patch #2 is fixing up user-space invalid input and is not
> allowed to trigger any WARN_ON().

It's the responsibility of the driver to at least look at its parameters.
What other invalid values might it let pass, that we cannot catch
at the generic level?

> We could drop patch #4, but then we wouldn't catch bad drivers.

I do want to keep patch #4.

Gr{oetje,eeting}s,

                        Geert

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