[PATCH v6 3/4] fbcon: Prevent that screen size is smaller than font size

[Geert Uytterhoeven]

Hi Helge,

On Sun, Jun 26, 2022 at 12:33 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>
> Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: stable at vger.kernel.org # v5.4+

Thanks for your patch, which is now commit f0b6a66d33ca6e7e ("fbcon:
Prevent that screen size is smaller than font size") in fbdev/for-next

> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c

> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1112,7 +1112,9 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
>                         return -EFAULT;
>                 console_lock();
>                 lock_fb_info(info);
> -               ret = fb_set_var(info, &var);
> +               ret = fbcon_modechange_possible(info, &var);

Again, this should be done (if done at all) after the call to
fb_ops.check_var(), as it breaks the FBIOPUT_VSCREENINFO rounding rule.

What if the user just wants to display graphics, not text?
Can't the text console be disabled instead?

> +               if (!ret)
> +                       ret = fb_set_var(info, &var);
>                 if (!ret)
>                         fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
>                 unlock_fb_info(info);

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