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

[Helge Deller]

On 6/26/22 10:50, Daniel Vetter wrote:
> 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>

Thanks, but sadly pushing commit 409d6c95f9c6 isn't easily possible.
I've just sent a new series v5 which fixes it up..

Helge

>
>> ---
>>  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
>>
>