linux-kernel - Re: [PATCH v2] fbdev: Detect integer underflow at "struct fbcon_ops"->clear

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Message-ID: <9e6eac10-c5c3-f518-36cc-9ea32fb5d7fe@i-love.sakura.ne.jp>
Date:   Wed, 15 Jul 2020 20:17:44 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        George Kennedy <george.kennedy@...cle.com>,
        dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        syzbot <syzbot+e5fd3e65515b48c02a30@...kaller.appspotmail.com>
Subject: Re: [PATCH v2] fbdev: Detect integer underflow at "struct
 fbcon_ops"->clear_margins.

On 2020/07/15 18:48, Dan Carpenter wrote:
>> @@ -216,7 +216,7 @@ static void bit_clear_margins(struct vc_data *vc, struct fb_info *info,
>>  	region.color = color;
>>  	region.rop = ROP_COPY;
>>  
>> -	if (rw && !bottom_only) {
>> +	if ((int) rw > 0 && !bottom_only) {
>>  		region.dx = info->var.xoffset + rs;
>                             ^^^^^^^^^^^^^^^^^^^^^^
> 
> If you choose a very high positive "rw" then this addition can overflow.
> info->var.xoffset comes from the user and I don't think it's checked...

Well, I think it would be checked by "struct fb_ops"->check_var hook.
For example, vmw_fb_check_var() has

	if ((var->xoffset + var->xres) > par->max_width ||
	    (var->yoffset + var->yres) > par->max_height) {
		DRM_ERROR("Requested geom can not fit in framebuffer\n");
		return -EINVAL;
	}

check. Of course, there might be integer overflow in that check...
Having sanity check at caller of "struct fb_ops"->check_var might be nice.