[PATCH 3/5] fbcon: Prevent that screen size is smaller than font size

[Geert Uytterhoeven]

Hi Helge,

On Thu, Jun 30, 2022 at 9:09 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller at gmx.de> wrote:
> > We need to prevent that users configure a screen size which is smaller than the
> > currently selected font size. Otherwise rendering chars on the screen will
> > access memory outside the graphics memory region.
> > This patch adds a new function fbcon_modechange_possible() which
> > implements this check and which later may be extended with other checks
> > if necessary.  The new function is called from the FBIOPUT_VSCREENINFO
> > ioctl handler in fbmem.c, which will return -EINVAL if userspace asked
> > for a too small screen size.
> >
> > 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/fbcon.c
> > +++ b/drivers/video/fbdev/core/fbcon.c
> > @@ -2736,6 +2736,34 @@ void fbcon_update_vcs(struct fb_info *info, bool all)
> >  }
> >  EXPORT_SYMBOL(fbcon_update_vcs);
> >
> > +/* let fbcon check if it supports a new screen resolution */
> > +int fbcon_modechange_possible(struct fb_info *info, struct fb_var_screeninfo *var)
> > +{
> > +       struct fbcon_ops *ops = info->fbcon_par;
> > +       struct vc_data *vc;
> > +       int i;
> > +
> > +       WARN_CONSOLE_UNLOCKED();
> > +
> > +       if (!ops || ops->currcon < 0)
> > +               return -EINVAL;
>
> So if the virtual console is _not_ used as a text console, we refuse
> mode changes?
>
> > +
> > +       /* prevent setting a screen size which is smaller than font size */
> > +       for (i = first_fb_vc; i <= last_fb_vc; i++) {
> > +               vc = vc_cons[i].d;
> > +               if (!vc || vc->vc_mode != KD_TEXT ||
> > +                          registered_fb[con2fb_map[i]] != info)

Looks like registered_fb[] is wrong since commit cad564ca557f8d3b
("fbcon: Fix boundary checks for fbcon=vc:n1-n2 parameters")?

> > +                       continue;
> > +
> > +               if (vc->vc_font.width  > FBCON_SWAP(var->rotate, var->xres, var->yres) ||
> > +                   vc->vc_font.height > FBCON_SWAP(var->rotate, var->yres, var->xres))
> > +                       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