Re: [PATCH 4.9 01/42] fbdev: fb_pm2fb: Avoid potential divide by zero error

[Ulrich Hecht]

> On 09/13/2022 4:07 PM CEST Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> 
>  
> From: Letu Ren <fantasquex@xxxxxxxxx>
> 
> commit 19f953e7435644b81332dd632ba1b2d80b1e37af upstream.
> 
> In `do_fb_ioctl()` of fbmem.c, if cmd is FBIOPUT_VSCREENINFO, var will be
> copied from user, then go through `fb_set_var()` and
> `info->fbops->fb_check_var()` which could may be `pm2fb_check_var()`.
> Along the path, `var->pixclock` won't be modified. This function checks
> whether reciprocal of `var->pixclock` is too high. If `var->pixclock` is
> zero, there will be a divide by zero error. So, it is necessary to check
> whether denominator is zero to avoid crash. As this bug is found by
> Syzkaller, logs are listed below.
> 
> divide error in pm2fb_check_var
> Call Trace:
>  <TASK>
>  fb_set_var+0x367/0xeb0 drivers/video/fbdev/core/fbmem.c:1015
>  do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1110
>  fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1189
> 
> Reported-by: Zheyu Ma <zheyuma97@xxxxxxxxx>
> Signed-off-by: Letu Ren <fantasquex@xxxxxxxxx>
> Signed-off-by: Helge Deller <deller@xxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  drivers/video/fbdev/pm2fb.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/video/fbdev/pm2fb.c b/drivers/video/fbdev/pm2fb.c
> index 9b32b9fc44a5c..50b569d047b10 100644
> --- a/drivers/video/fbdev/pm2fb.c
> +++ b/drivers/video/fbdev/pm2fb.c
> @@ -619,6 +619,11 @@ static int pm2fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
>  		return -EINVAL;
>  	}
>  
> +	if (!var->pixclock) {
> +		DPRINTK("pixclock is zero\n");
> +		return -EINVAL;
> +	}
> +
>  	if (PICOS2KHZ(var->pixclock) > PM2_MAX_PIXCLOCK) {
>  		DPRINTK("pixclock too high (%ldKHz)\n",
>  			PICOS2KHZ(var->pixclock));
> -- 
> 2.35.1

This is a duplicate, the same patch has already been applied in 4.9.327 (0f1174f4972ea9fad6becf8881d71adca8e9ca91), so the above snippet of code is now in there twice.

Doesn't make a difference in functionality in this case, I just happened to notice it when reviewing backports from 4.9 for the CIP 4.4-stable tree.

CU
Uli