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

Laurent Pinchart
2020-07-26T17:30:57Z

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@syzkaller.appspotmail.com
> Link: https://syzkaller.appspot.com/bug?extid=79d751604cb6f29fbf59
> Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
> ---
> 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