linux-kernel - Re: [syzbot] BUG: unable to handle kernel paging request in vga16fb

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Date:   Sun, 29 Aug 2021 19:31:53 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        syzbot <syzbot+04168c8063cfdde1db5e@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, b.zolnierkie@...sung.com,
        colin.king@...onical.com, dri-devel@...ts.freedesktop.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        masahiroy@...nel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] BUG: unable to handle kernel paging request in
 vga16fb_fillrect

On 8/29/21 7:27 PM, Tetsuo Handa wrote:
> On 2021/08/30 9:24, Randy Dunlap wrote:
>> Note that yres_virtual is set to 0x10000000. Is there no practical limit
>> (hence limit check) that can be used here?
>>
>> Also, in vga16fb_check_var(), beginning at line 404:
>>
>>    404        if (yres > vyres)
>>    405            vyres = yres;
>>    406        if (vxres * vyres > maxmem) {
>>    407            vyres = maxmem / vxres;
>>    408            if (vyres < yres)
>>    409                return -ENOMEM;
>>    410        }
>>
>> At line 406, the product of vxres * vyres overflows 32 bits (is 0 in this
>> case/example), so any protection from this block is lost.
> 
> OK. Then, we can check overflow like below.
> 
> diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
> index e2757ff1c23d..e483a3f5fd47 100644
> --- a/drivers/video/fbdev/vga16fb.c
> +++ b/drivers/video/fbdev/vga16fb.c
> @@ -403,7 +403,7 @@ static int vga16fb_check_var(struct fb_var_screeninfo *var,
>   
>   	if (yres > vyres)
>   		vyres = yres;
> -	if (vxres * vyres > maxmem) {
> +	if ((u64) vxres * vyres > (u64) maxmem) {
>   		vyres = maxmem / vxres;
>   		if (vyres < yres)
>   			return -ENOMEM;
> 
> But I think we can check overflow in the common code like below. (Both patch fixed the oops.)

OK, great. Thanks.

> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 1c855145711b..8899679bbc46 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -1008,6 +1008,11 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
>   	if (var->xres < 8 || var->yres < 8)
>   		return -EINVAL;
>   
> +	/* Don't allow u32 * u32 to overflow. */
> +	if ((u64) var->xres * var->yres > (u64) UINT_MAX ||
> +	    (u64) var->xres_virtual * var->yres_virtual > (u64) UINT_MAX)
> +		return -EINVAL;
> +
>   	ret = info->fbops->fb_check_var(var, info);
>   
>   	if (ret)
> 
>>
>> But even if yres_virtual (aka vyres) is "only" 0x01000000, so no
>> multiplication overflow occurs, the resulting value of vyres "seems"
>> to still be too large and can cause an error [I'm not sure about this
>> last part -- I need to use a new gcc so that KASAN will work.]
> 


-- 
~Randy