[PATCH v2 2/4] fbcon: Add fbcon_modechange_possible() check

[Helge Deller]

On 6/25/22 14:55, Daniel Vetter wrote:
> On Sat, Jun 25, 2022 at 02:25:00PM +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 may be extended with other checks later if necessary.  The
>> new function will be called from the FBIOPUT_VSCREENINFO ioctl handler in
>> fbmem.c, which will then return -EINVAL to the user if the new screen size is
>> too small.
>>
>> Signed-off-by: Helge Deller <deller at gmx.de>
>> Cc: stable at vger.kernel.org # v5.4+
>> ---
>>  drivers/video/fbdev/core/fbcon.c | 26 ++++++++++++++++++++++++++
>>  include/linux/fbcon.h            |  4 ++++
>>  2 files changed, 30 insertions(+)
>>
>> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
>> index e162d5e753e5..e4cc4841ed7f 100644
>> --- a/drivers/video/fbdev/core/fbcon.c
>> +++ b/drivers/video/fbdev/core/fbcon.c
>> @@ -2736,6 +2736,32 @@ 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();
> here please.

Yes, good idea.

>> +
>> +	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++) {
>
> Maybe a follow up patch to make this an interator? Kinda like what I've
> done for fbcon_for_each_registered_fb, maybe call it fbcon_for_each_fb_vc
> or so.

Yes, that would be possible later on.
Right now I'd like to limit changes to minimum to make backporting easy.


>> +		vc = vc_cons[i].d;
>> +		if (!vc || vc->vc_mode != KD_TEXT ||
>
> I don't think it's good to filter for !KD_TEXT here, because then we'd
> need to recheck fonts when Xorg would try to switch back to text mode, and
> if that then fails we kinda broke the system.
>
> I can't think of a use-case where you'd want to upload a font which breaks
> your console that Xorg is using right now, so best to just drop this
> check.

Yes, probably right.
Will drop that.

>> +		    fbcon_info_from_console(i) != info)
>
> So I think, but not 100% sure, that with my recent rework for
> fbcon_info_from_console this should be impossible, since the races are
> gone. I guess it doesn't hurt to cargo-cult this, but a follow up patch to
> roll out fbcon_for_each_fb_vc and then delete this check from all of the
> loop iterations would be really good to make this clear.
>
> If you're not sure this is safe we could add this consistency check in a
> if (WARN_ON(fbcon_info_from_console(i))!= info) continue; into the loop
> iterator itself.

Since we now added the WARN_CONSOLE_UNLOCKED() as suggested by you above
I don't think more additional checks are needed.

>
>> +			continue;
>> +
>> +		if (FBCON_SWAP(var->rotate, var->xres, var->yres) < vc->vc_font.width ||
>> +		    FBCON_SWAP(var->rotate, var->yres, var->xres) < vc->vc_font.height)
>
> Bit a bikeshed, but please do the check the same way around as in the
> other place.

Fixed in upcoming series now.

Helge

> Maybe even extract a helper that you pass the vc and the var
> struct too and it checks that it fits, and then use it here and in the
> previous patch.
>
> Cheers, Daniel
>
>> +			return -EINVAL;
>> +	}
>> +
>> +	return 0;
>> +}
>> +EXPORT_SYMBOL(fbcon_modechange_possible);
>> +
>>  int fbcon_mode_deleted(struct fb_info *info,
>>  		       struct fb_videomode *mode)
>>  {
>> 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
>>
>