[PATCH] video: fbdev: kyrofb: fix a DoS bug by restricting user input

[Sam Ravnborg]

Hi 
On Tue, Jul 13, 2021 at 12:51:14PM +0000, Zheyu Ma wrote:
> The user can pass in any value to the driver through the 'ioctl'
> interface. The driver dost not check, which may cause DoS bugs.
> 
> Fix this by checking if the divisor is 0

This fix is trying to avoid the situation on a too low layer.
Could you please try to look at validating the inputs on a much higher
level, in kyro_dev_overlay_viewport_set().
Then the checks are much more obvious and you are more likely to catch a
number of similar bugs that the test just fails to trigger for now.

	Sam

> 
> The following log reveals it:
> 
> divide error: 0000 [#1] PREEMPT SMP KASAN PTI
> RIP: 0010:SetOverlayViewPort+0x133/0x5f0 drivers/video/fbdev/kyro/STG4000OverlayDevice.c:476
> Call Trace:
>  kyro_dev_overlay_viewport_set drivers/video/fbdev/kyro/fbdev.c:378 [inline]
>  kyrofb_ioctl+0x2eb/0x330 drivers/video/fbdev/kyro/fbdev.c:603
>  do_fb_ioctl+0x1f3/0x700 drivers/video/fbdev/core/fbmem.c:1171
>  fb_ioctl+0xeb/0x130 drivers/video/fbdev/core/fbmem.c:1185
>  vfs_ioctl fs/ioctl.c:48 [inline]
>  __do_sys_ioctl fs/ioctl.c:753 [inline]
>  __se_sys_ioctl fs/ioctl.c:739 [inline]
>  __x64_sys_ioctl+0x19b/0x220 fs/ioctl.c:739
>  do_syscall_64+0x32/0x80 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> 
> Signed-off-by: Zheyu Ma <zheyuma97 at gmail.com>
> ---
>  drivers/video/fbdev/kyro/STG4000OverlayDevice.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/video/fbdev/kyro/STG4000OverlayDevice.c b/drivers/video/fbdev/kyro/STG4000OverlayDevice.c
> index 9fde0e3b69ec..29d692fe5e75 100644
> --- a/drivers/video/fbdev/kyro/STG4000OverlayDevice.c
> +++ b/drivers/video/fbdev/kyro/STG4000OverlayDevice.c
> @@ -407,6 +407,9 @@ int SetOverlayViewPort(volatile STG4000REG __iomem *pSTGReg,
>  		ulVertDecFactor = 1;
>  	}
>  
> +	if ((ulDest + 1) == 0)
> +		return -EINVAL;
> +
>  	ulDacYScale = ((ulSrc - 1) * 2048) / (ulDest + 1);
>  
>  	tmp = STG_READ_REG(DACOverlayVtDec);	/* Decimation */
> @@ -471,6 +474,9 @@ int SetOverlayViewPort(volatile STG4000REG __iomem *pSTGReg,
>  		 */
>  		ulScaleLeft = ulSrcLeft;
>  
> +		if ((ulRight - ulLeft + 2) == 0)
> +			return -EINVAL;
> +
>  		/* shift fxscale until it is in the range of the scaler */
>  		ulhDecim = 0;
>  		ulScale = (((ulSrcRight - ulSrcLeft) - 1) << (11 - ulhDecim)) / (ulRight - ulLeft + 2);
> -- 
> 2.17.6