Re: [PATCH] fbcon: Check font dimension limits

[Greg KH]

On Thu, Jan 26, 2023 at 01:49:12AM +0100, Samuel Thibault wrote:
> blit_x and blit_y are uint32_t, so fbcon currently cannot support fonts
> larger than 32x32.

"u32" you mean, right?

> The 32x32 case also needs shifting an unsigned int, to properly set bit
> 31, otherwise we get "UBSAN: shift-out-of-bounds in fbcon_set_font",
> as reported on
> 
> http://lore.kernel.org/all/IA1PR07MB98308653E259A6F2CE94A4AFABCE9@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Odd blank line?


> Kernel Branch: 6.2.0-rc5-next-20230124
> Kernel config: https://drive.google.com/file/d/1F-LszDAizEEH0ZX0HcSR06v5q8FPl2Uv/view?usp=sharing
> Reproducer: https://drive.google.com/file/d/1mP1jcLBY7vWCNM60OMf-ogw-urQRjNrm/view?usp=sharing

What are all of these lines for?

> 
> Reported-by: Sanan Hasanov <sanan.hasanov@xxxxxxxxxxxxxxx>
> Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>

What commit id does this fix?  Should it go to stable kernels?

> 
> Index: linux-6.0/drivers/video/fbdev/core/fbcon.c
> ===================================================================
> --- linux-6.0.orig/drivers/video/fbdev/core/fbcon.c
> +++ linux-6.0/drivers/video/fbdev/core/fbcon.c
> @@ -2489,9 +2489,12 @@ static int fbcon_set_font(struct vc_data
>  	    h > FBCON_SWAP(info->var.rotate, info->var.yres, info->var.xres))
>  		return -EINVAL;
>  
> +	if (font->width > 32 || font->height > 32)
> +		return -EINVAL;
> +
>  	/* Make sure drawing engine can handle the font */
> -	if (!(info->pixmap.blit_x & (1 << (font->width - 1))) ||
> -	    !(info->pixmap.blit_y & (1 << (font->height - 1))))
> +	if (!(info->pixmap.blit_x & (1U << (font->width - 1))) ||
> +	    !(info->pixmap.blit_y & (1U << (font->height - 1))))

Are you sure this is still needed with the above check added?  If so,
why?  What is the difference in the compiled code?

thanks,

greg k-h