Re: [syzbot] KASAN: vmalloc-out-of-bounds Write in imageblit (2)

From: Dan Carpenter <dan.carpenter@oracle.com>
To: Khalid Masum <khalid.masum.92@gmail.com>
Cc: linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
	syzbot+14b0e8f3fd1612e35350@syzkaller.appspotmail.com,
	khalid.masum@aol.com
Subject: Re: [syzbot] KASAN: vmalloc-out-of-bounds Write in imageblit (2)
Date: Mon, 1 Aug 2022 13:43:43 +0300	[thread overview]
Message-ID: <20220801104343.GA3438@kadam> (raw)
In-Reply-To: <20220730081246.4567-1-khalid.masum.92@gmail.com>

On Sat, Jul 30, 2022 at 02:12:46PM +0600, Khalid Masum wrote:
> Currently the if block's condition has an unhandled case, where the
> result of ret might get greater than vc->vc_scr_end, and therefore
> the corresponding handler in else block never gets executed. Which
> eventually causes panic in fast_imageblit.
> 
> Add this extra check in the conditions to fix this breakage.
> 
> #syz-test: https://github.com/torvalds/linux.git e0dccc3b76fb 
> 
> ---
>  drivers/video/fbdev/core/fbcon.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 1a9aa12cf886..d026f3845b60 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2591,14 +2591,13 @@ static unsigned long fbcon_getxy(struct vc_data *vc, unsigned long pos,
>  {
>  	unsigned long ret;
>  	int x, y;
> +	unsigned long offset = (pos - vc->vc_origin) / 2;
> +	x = offset % vc->vc_cols;
> +	y = offset / vc->vc_cols;
> +	ret = pos + (vc->vc_cols - x) * 2;
>  
> -	if (pos >= vc->vc_origin && pos < vc->vc_scr_end) {
> -		unsigned long offset = (pos - vc->vc_origin) / 2;
> -
> -		x = offset % vc->vc_cols;
> -		y = offset / vc->vc_cols;
> -		ret = pos + (vc->vc_cols - x) * 2;
> -	} else {
> +	if (!pos >= vc->vc_origin || !pos < vc->vc_scr_end ||
> +	                             !ret < vc->vc_scr_end) {

These are precendence bugs.  The ! will be done before the >=.  Write it
as:

	if (pos < vc->vc_origin || pos >= vc->vc_scr_end ||
	    ret >= vc->vc_scr_end) {

>  		/* Should not happen */
>  		x = y = 0;
>  		ret = vc->vc_origin;

regards,
dan carpenter