Subject: Re: KMSAN: uninit-value in _copy_to_iter (2)

From: Al Viro <[email-masked]>
Date: Thu, 7 Jun 2018 18:43:55 +0100

On Thu, Jun 07, 2018 at 06:38:48PM +0300, Michael S. Tsirkin wrote:
> #syz test: https://github.com/google/kmsan.git/master d2d741e5d1898dfde1a75ea3d29a9a3e2edf0617
> 
> Subject: vhost: fix info leak
> 
> Fixes: CVE-2018-1118
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index f0be5f35ab28..9beefa6ed1ce 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2345,6 +2345,9 @@ struct vhost_msg_node *vhost_new_msg(struct vhost_virtqueue *vq, int type)
>  	struct vhost_msg_node *node = kmalloc(sizeof *node, GFP_KERNEL);
>  	if (!node)
>  		return NULL;
> +
> +	/* Make sure all padding within the structure is initialized. */
> +	memset(&node->msg, 0, sizeof node->msg);

Umm...  Maybe kzalloc(), then?  You have

struct vhost_msg_node {
  struct vhost_msg msg;
  struct vhost_virtqueue *vq;
  struct list_head node;
};

and that's what, 68 bytes in msg, then either 4 bytes pointer or
4 bytes padding + 8 bytes pointer, then two pointers?  How much
does explicit partial memset() save you here?

>  	node->vq = vq;
>  	node->msg.type = type;
>  	return node;