Re: [PATCH] video: fbdev: cirrusfb: check pixclock to avoid divide by zero

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Oct 25, 2021 at 02:01:30PM -0500, George Kennedy wrote:
> Do a sanity check on pixclock value before using it as a divisor.
> 
> Syzkaller reported a divide error in cirrusfb_check_pixclock.
> 
> divide error: 0000 [#1] SMP KASAN PTI
> CPU: 0 PID: 14938 Comm: cirrusfb_test Not tainted 5.15.0-rc6 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2
> RIP: 0010:cirrusfb_check_var+0x6f1/0x1260
> 
> Call Trace:
>  fb_set_var+0x398/0xf90
>  do_fb_ioctl+0x4b8/0x6f0
>  fb_ioctl+0xeb/0x130
>  __x64_sys_ioctl+0x19d/0x220
>  do_syscall_64+0x3a/0x80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Signed-off-by: George Kennedy <george.kennedy@oracle.com>
> ---
>  drivers/video/fbdev/cirrusfb.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
> index 93802ab..099ddcb 100644
> --- a/drivers/video/fbdev/cirrusfb.c
> +++ b/drivers/video/fbdev/cirrusfb.c
> @@ -477,6 +477,9 @@ static int cirrusfb_check_pixclock(const struct fb_var_screeninfo *var,
>  	struct cirrusfb_info *cinfo = info->par;
>  	unsigned maxclockidx = var->bits_per_pixel >> 3;
>  
> +	if (!var->pixclock)
> +		return -EINVAL;

Shouldn't you be checking further up the call chain where this got set
to 0?

What logic allows this to be a valid value?  What about all other fb
drivers?

thanks,

greg k-h