linux-kernel - Re: [PATCH 2/2] media: v4l2-core: explicitly clear ioctl input data

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Date:   Thu, 18 Mar 2021 17:07:45 +0200
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        syzbot+142888ffec98ab194028@...kaller.appspotmail.com,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] media: v4l2-core: explicitly clear ioctl input data

Hi Arnd,

Thank you for the patch.

On Thu, Mar 18, 2021 at 02:43:19PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> As seen from a recent syzbot bug report, mistakes in the compat ioctl
> implementation can lead to uninitialized kernel stack data getting used
> as input for driver ioctl handlers.
> 
> The reported bug is now fixed, but it's possible that other related
> bugs are still present or get added in the future. As the drivers need
> to check user input already, the possible impact is fairly low, but it
> might still cause an information leak.
> 
> To be on the safe side, always clear the entire ioctl buffer before
> calling the conversion handler functions that are meant to initialize
> them.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

> ---
>  drivers/media/v4l2-core/v4l2-ioctl.c | 51 ++++++++++++++++------------
>  1 file changed, 29 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
> index 2b1bb68dc27f..6cec92d0972c 100644
> --- a/drivers/media/v4l2-core/v4l2-ioctl.c
> +++ b/drivers/media/v4l2-core/v4l2-ioctl.c
> @@ -3164,12 +3164,23 @@ static int video_get_user(void __user *arg, void *parg,
>  
>  	if (cmd == real_cmd) {
>  		if (copy_from_user(parg, (void __user *)arg, n))
> -			err = -EFAULT;
> -	} else if (in_compat_syscall()) {
> -		err = v4l2_compat_get_user(arg, parg, cmd);
> -	} else {
> -		switch (cmd) {
> +			return -EFAULT;
> +
> +		/* zero out anything we don't copy from userspace */
> +		if (n < _IOC_SIZE(real_cmd))
> +			memset((u8 *)parg + n, 0, _IOC_SIZE(real_cmd) - n);
> +
> +		return 0;
> +	}
> +
> +	/* zero out whole buffer first to deal with missing emulation */
> +	memset(parg, 0, _IOC_SIZE(real_cmd));
> +
> +	if (in_compat_syscall())
> +		return v4l2_compat_get_user(arg, parg, cmd);
> +
>  #if !defined(CONFIG_64BIT) && defined(CONFIG_COMPAT_32BIT_TIME)
> +	switch (cmd) {
>  		case VIDIOC_QUERYBUF_TIME32:
>  		case VIDIOC_QBUF_TIME32:
>  		case VIDIOC_DQBUF_TIME32:
> @@ -3182,28 +3193,24 @@ static int video_get_user(void __user *arg, void *parg,
>  
>  			*vb = (struct v4l2_buffer) {
>  				.index		= vb32.index,
> -					.type		= vb32.type,
> -					.bytesused	= vb32.bytesused,
> -					.flags		= vb32.flags,
> -					.field		= vb32.field,
> -					.timestamp.tv_sec	= vb32.timestamp.tv_sec,
> -					.timestamp.tv_usec	= vb32.timestamp.tv_usec,
> -					.timecode	= vb32.timecode,
> -					.sequence	= vb32.sequence,
> -					.memory		= vb32.memory,
> -					.m.userptr	= vb32.m.userptr,
> -					.length		= vb32.length,
> -					.request_fd	= vb32.request_fd,
> +				.type		= vb32.type,
> +				.bytesused	= vb32.bytesused,
> +				.flags		= vb32.flags,
> +				.field		= vb32.field,
> +				.timestamp.tv_sec	= vb32.timestamp.tv_sec,
> +				.timestamp.tv_usec	= vb32.timestamp.tv_usec,
> +				.timecode	= vb32.timecode,
> +				.sequence	= vb32.sequence,
> +				.memory		= vb32.memory,
> +				.m.userptr	= vb32.m.userptr,
> +				.length		= vb32.length,
> +				.request_fd	= vb32.request_fd,
>  			};
>  			break;
>  		}
> -#endif
> -		}
>  	}
> +#endif
>  
> -	/* zero out anything we don't copy from userspace */
> -	if (!err && n < _IOC_SIZE(real_cmd))
> -		memset((u8 *)parg + n, 0, _IOC_SIZE(real_cmd) - n);
>  	return err;
>  }
>  

-- 
Regards,

Laurent Pinchart