Re: [PATCH v2 3/4] fbmem: Prevent invalid virtual screen sizes in fb_set_var()

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Helge,

On Fri, Jul 1, 2022 at 10:23 PM Helge Deller <deller@gmx.de> wrote:
> Prevent that drivers configure a virtual screen resolution smaller than
> the physical screen resolution.  This is important, because otherwise we
> may access memory outside of the graphics memory area.
>
> Give a kernel WARNing and show the driver name to help locating the buggy
> driver.
>
> Signed-off-by: Helge Deller <deller@gmx.de>
> Cc: stable@vger.kernel.org # v5.4+

Thanks for your patch!

> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1016,6 +1016,18 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
>         if (ret)
>                 return ret;
>
> +       /* make sure virtual resolution >= physical resolution */
> +       if (WARN_ON(var->xres_virtual < var->xres)) {

WARN_ON_ONCE()?
This does mean we would miss two or more buggy drivers in a single system.

> +               pr_warn("fbcon: Fix up invalid xres %d for %s\n",

xres_virtual?

> +                       var->xres_virtual, info->fix.id);
> +               var->xres_virtual = var->xres;

I think it's better to not fix this up, but return -EINVAL instead.
After all if we get here, we have a buggy driver that needs to be fixed.

> +       }
> +       if (WARN_ON(var->yres_virtual < var->yres)) {
> +               pr_warn("fbcon: Fix up invalid yres %d for %s\n",
> +                       var->yres_virtual, info->fix.id);
> +               var->yres_virtual = var->yres;
> +       }

Same for yres.

> +
>         if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
>                 return 0;
>

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