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

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Date:   Wed, 30 Nov 2022 17:41:21 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Ulrich Hecht <uli@...nd.eu>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Zheyu Ma <zheyuma97@...il.com>,
        Letu Ren <fantasquex@...il.com>, Helge Deller <deller@....de>,
        Sasha Levin <sashal@...nel.org>,
        "pavel@...x.de" <pavel@...x.de>
Subject: Re: [PATCH 4.9 01/42] fbdev: fb_pm2fb: Avoid potential divide by
 zero error

On Wed, Nov 23, 2022 at 07:34:39AM +0100, Ulrich Hecht wrote:
> 
> > On 09/13/2022 4:07 PM CEST Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
> > 
> >  
> > From: Letu Ren <fantasquex@...il.com>
> > 
> > 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@...il.com>
> > Signed-off-by: Letu Ren <fantasquex@...il.com>
> > Signed-off-by: Helge Deller <deller@....de>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> > Signed-off-by: Sasha Levin <sashal@...nel.org>
> > ---
> >  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.

Good catch, want to send a revert for this to fix it up?

thanks,

greg k-h