Hi Helge,
On Wed, Jun 29, 2022 at 10:00 PM Helge Deller <deller@xxxxxx> 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@xxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 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)
> + 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;
> + }
IMHO this looks way too fragile, and we should fix the rendering code
to handle cols == 0 || rows == 0 instead...
> +
> + return 0;
> +}
> +EXPORT_SYMBOL(fbcon_modechange_possible);
> +
> int fbcon_mode_deleted(struct fb_info *info,
> struct fb_videomode *mode)
> {
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index e645b96d2700..324f726739c4 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1111,7 +1111,9 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
> var.xres_virtual = var.xres;
> if (var.yres_virtual < var.yres)
> var.yres_virtual = var.yres;
> - ret = fb_set_var(info, &var);
> + ret = fbcon_modechange_possible(info, &var);
> + 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@xxxxxxxxxxxxxx
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
