[PATCH 4/5] fbmem: Prevent invalid virtual screen sizes in fb_set_var()

[Geert Uytterhoeven]

Hi Helge,

On Thu, Jun 30, 2022 at 9:38 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> On Thu, Jun 30, 2022 at 9:17 PM Helge Deller <deller at gmx.de> wrote:
> > On 6/30/22 21:11, Geert Uytterhoeven wrote:
> > > On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller at 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.
> > >>
> > >> 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
> > >> @@ -1006,6 +1006,12 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
> > >>         if (var->xres < 8 || var->yres < 8)
> > >>                 return -EINVAL;
> > >>
> > >> +       /* make sure virtual resolution >= physical resolution */
> > >> +       if (WARN_ON(var->xres_virtual < var->xres))
> > >> +               var->xres_virtual = var->xres;
> > >> +       if (WARN_ON(var->yres_virtual < var->yres))
> > >> +               var->yres_virtual = var->yres;
> > >
> > > This should be moved below the call to info->fbops->fb_check_var(),
> > > so the WARN_ON() catches buggy fbdev drivers, not userspace fuzzers.
> >
> > Yes, makes sense.
>
> And print the name of the frame buffer device driver, so people know
> who to blame.

Or better, do not continue, but return with a failure:

    if (WARN(var->xres_virtual < var->xres || var->yres_virtual < var->yres,
        "%ps for %s is broken\n", info->fbops->fb_check_var, info->fix.id)
            return -EINVAL;

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