Re: [Linux-kernel-mentees] [PATCH] media/v4l2-core: Fix kernel-infoleak in video_put_user()

[Laurent Pinchart]

Hi Peilin,

Thank you for the patch.

On Sun, Jul 26, 2020 at 12:44:39PM -0400, Peilin Ye wrote:
> video_put_user() is copying uninitialized stack memory to userspace. Fix
> it by initializing `vb32` using memset().

What makes you think this will fix the issue ? When initializing a
structure at declaration time, the fields that are not explicitly
specified should be initialized to 0 by the compiler. See
https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.cbclx01/strin.htm:

If a structure variable is partially initialized, all the uninitialized
structure members are implicitly initialized to zero no matter what the
storage class of the structure variable is. See the following example:

struct one {
    int a;
    int b;
    int c;
};

void main() {
    struct one z1;         // Members in z1 do not have default initial values.
    static struct one z2;  // z2.a=0, z2.b=0, and z2.c=0.
    struct one z3 = {1};   // z3.a=1, z3.b=0, and z3.c=0.
}

> Reported-and-tested-by: syzbot+79d751604cb6f29fbf59@xxxxxxxxxxxxxxxxxxxxxxxxx
> Link: https://syzkaller.appspot.com/bug?extid=79d751604cb6f29fbf59
> Signed-off-by: Peilin Ye <yepeilin.cs@xxxxxxxxx>
> ---
>  drivers/media/v4l2-core/v4l2-ioctl.c | 32 +++++++++++++++-------------
>  1 file changed, 17 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-ioctl.c b/drivers/media/v4l2-core/v4l2-ioctl.c
> index a556880f225a..08909f58dc80 100644
> --- a/drivers/media/v4l2-core/v4l2-ioctl.c
> +++ b/drivers/media/v4l2-core/v4l2-ioctl.c
> @@ -3210,21 +3210,23 @@ static int video_put_user(void __user *arg, void *parg, unsigned int cmd)
>  	case VIDIOC_DQBUF_TIME32:
>  	case VIDIOC_PREPARE_BUF_TIME32: {
>  		struct v4l2_buffer *vb = parg;
> -		struct v4l2_buffer_time32 vb32 = {
> -			.index		= vb->index,
> -			.type		= vb->type,
> -			.bytesused	= vb->bytesused,
> -			.flags		= vb->flags,
> -			.field		= vb->field,
> -			.timestamp.tv_sec	= vb->timestamp.tv_sec,
> -			.timestamp.tv_usec	= vb->timestamp.tv_usec,
> -			.timecode	= vb->timecode,
> -			.sequence	= vb->sequence,
> -			.memory		= vb->memory,
> -			.m.userptr	= vb->m.userptr,
> -			.length		= vb->length,
> -			.request_fd	= vb->request_fd,
> -		};
> +		struct v4l2_buffer_time32 vb32;
> +
> +		memset(&vb32, 0, sizeof(vb32));
> +
> +		vb32.index	= vb->index;
> +		vb32.type	= vb->type;
> +		vb32.bytesused	= vb->bytesused;
> +		vb32.flags	= vb->flags;
> +		vb32.field	= vb->field;
> +		vb32.timestamp.tv_sec	= vb->timestamp.tv_sec;
> +		vb32.timestamp.tv_usec	= vb->timestamp.tv_usec;
> +		vb32.timecode	= vb->timecode;
> +		vb32.sequence	= vb->sequence;
> +		vb32.memory	= vb->memory;
> +		vb32.m.userptr	= vb->m.userptr;
> +		vb32.length	= vb->length;
> +		vb32.request_fd	= vb->request_fd;
>  
>  		if (copy_to_user(arg, &vb32, sizeof(vb32)))
>  			return -EFAULT;

-- 
Regards,

Laurent Pinchart