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

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Message-ID: <edcf26d4-64a0-04ea-435c-17d9efde7dd9@kernel.org>
Date:   Thu, 26 Jan 2023 10:08:07 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Samuel Thibault <samuel.thibault@...-lyon.org>
Cc:     Daniel Vetter <daniel@...ll.ch>, Helge Deller <deller@....de>,
        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, 8:43, Greg KH wrote:
>> --- 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?

For font->{width,height} == 32, definitely. IMO, 1 << 31 is undefined as 
1 << 31 cannot be represented by an (signed) int.

-- 
js
suse labs