[PATCH v4 2/3] fbcon: Prevent that screen size is smaller than font size

[Daniel Vetter]

On Sun, Jun 26, 2022 at 01:27:02AM +0200, Helge Deller 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.
> 
> This patch depends on commit 409d6c95f9c6 ("fbcon: Introduce wrapper for console->fb_info lookup"),
> so it needs to be backported as well.
> 
> Signed-off-by: Helge Deller <deller at gmx.de>
> Cc: stable at vger.kernel.org # v5.4+

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> ---
>  drivers/video/fbdev/core/fbcon.c | 27 +++++++++++++++++++++++++++
>  drivers/video/fbdev/core/fbmem.c |  4 +++-
>  include/linux/fbcon.h            |  4 ++++
>  3 files changed, 34 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index e162d5e753e5..2ab7515ac842 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2736,6 +2736,33 @@ 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;
> +
> +	/* 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 || fbcon_info_from_console(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;
> +	}
> +
> +	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 afa2863670f3..160389365a36 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1106,7 +1106,9 @@ 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);
> +		if (!ret)
> +			ret = fb_set_var(info, &var);
>  		if (!ret)
>  			fbcon_update_vcs(info, var.activate & FB_ACTIVATE_ALL);
>  		unlock_fb_info(info);
> diff --git a/include/linux/fbcon.h b/include/linux/fbcon.h
> index ff5596dd30f8..2382dec6d6ab 100644
> --- a/include/linux/fbcon.h
> +++ b/include/linux/fbcon.h
> @@ -15,6 +15,8 @@ void fbcon_new_modelist(struct fb_info *info);
>  void fbcon_get_requirement(struct fb_info *info,
>  			   struct fb_blit_caps *caps);
>  void fbcon_fb_blanked(struct fb_info *info, int blank);
> +int  fbcon_modechange_possible(struct fb_info *info,
> +			       struct fb_var_screeninfo *var);
>  void fbcon_update_vcs(struct fb_info *info, bool all);
>  void fbcon_remap_all(struct fb_info *info);
>  int fbcon_set_con2fb_map_ioctl(void __user *argp);
> @@ -33,6 +35,8 @@ static inline void fbcon_new_modelist(struct fb_info *info) {}
>  static inline void fbcon_get_requirement(struct fb_info *info,
>  					 struct fb_blit_caps *caps) {}
>  static inline void fbcon_fb_blanked(struct fb_info *info, int blank) {}
> +static inline int  fbcon_modechange_possible(struct fb_info *info,
> +				struct fb_var_screeninfo *var) { return 0; }
>  static inline void fbcon_update_vcs(struct fb_info *info, bool all) {}
>  static inline void fbcon_remap_all(struct fb_info *info) {}
>  static inline int fbcon_set_con2fb_map_ioctl(void __user *argp) { return 0; }
> --
> 2.35.3
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch