linux-kernel - Re: [PATCH] fbcon: Check font dimension limits

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Message-ID: <3bcd9911-5fdd-2a1a-0a76-55e1b8f7642a@kernel.org>
Date:   Thu, 26 Jan 2023 10:02:55 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Samuel Thibault <samuel.thibault@...-lyon.org>,
        gregkh@...uxfoundation.org, Daniel Vetter <daniel@...ll.ch>,
        Helge Deller <deller@....de>
Cc:     linux-fbdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>
Subject: Re: [PATCH] fbcon: Check font dimension limits

On 26. 01. 23, 1:49, Samuel Thibault wrote:
> blit_x and blit_y are uint32_t, so fbcon currently cannot support fonts
> larger than 32x32.
> 
> 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@IA1PR07MB9830.namprd07.prod.outlook.com
> 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
> 
> Reported-by: Sanan Hasanov <sanan.hasanov@...ghts.ucf.edu>
> Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>
> 
> 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))))

So use BIT() properly then? That should be used in all these shifts 
anyway. Exactly to avoid UB.

thanks,
-- 
js
suse labs