linux-kernel - Re: [PATCH] fb_pm2: Add a check to avoid potential divide by zero error

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Date:   Mon, 22 Aug 2022 22:05:38 +0200
From:   Helge Deller <deller@....de>
To:     Letu Ren <fantasquex@...il.com>, baihaowen@...zu.com
Cc:     zheyuma97@...il.com, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fb_pm2: Add a check to avoid potential divide by zero
 error

On 8/18/22 12:44, Letu Ren wrote:
> 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@...il.com>
> Signed-off-by: Letu Ren <fantasquex@...il.com>

applied.
Thanks!
Helge

> ---
>  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 d3be2c64f1c0..8fd79deb1e2a 100644
> --- a/drivers/video/fbdev/pm2fb.c
> +++ b/drivers/video/fbdev/pm2fb.c
> @@ -617,6 +617,11 @@ 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));