[PATCH V4] fbcon: fix fbcon out-of-bounds write in sys_imageblit

[Daniel Vetter]

On Mon, Aug 30, 2021 at 11:30:23AM +0800, tcs.kernel at gmail.com wrote:
> From: Haimin Zhang <tcs_kernel at tencent.com>
> 
> yres and vyres can be controlled by user mode parameters, and cause
> p->vrows to become a negative value. While this value be passed to real_y
> function, the ypos will be out of screen range.This is an out-of-bounds
> write bug.
> some driver will check xres and yres in fb_check_var callback,but some not
> so we add a common check after that callback.
> 
> Signed-off-by: Haimin Zhang <tcs_kernel at tencent.com>
> Signed-off-by: Tetsuo Handa <penguin-kernel at i-love.sakura.ne.jp>

Does this fix a syzbot crash or how was this discovered?
-Daniel

> ---
>  drivers/video/fbdev/core/fbmem.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 1c85514..5599372 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1013,6 +1013,10 @@ static int fb_check_caps(struct fb_info *info, struct fb_var_screeninfo *var,
>  	if (ret)
>  		return ret;
>  
> +	/* virtual resolution cannot be smaller than visible resolution. */
> +	if (var->yres_virtual < var->yres || var->xres_virtual < var->xres)
> +		return -EINVAL;
> +
>  	if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
>  		return 0;
>  
> -- 
> 1.8.3.1
> 

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