cn: verify msg->len before making callback

The struct cn_msg len field comes from userspace and needs to be
validated. More logical to do so here where the cn_msg pointer is
pulled out of the sk_buff than the callback which is passed cn_msg *
and might assume no validation is needed.

Reported-by: Dan Carpenter <[e-mail]>
Acked-by: Evgeniy Polyakov <[e-mail]>
Signed-off-by: David Fries <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/connector/connector.c

···
  */
 static int cn_call_callback(struct sk_buff *skb)
 {
+	struct nlmsghdr *nlh;
 	struct cn_callback_entry *i, *cbq = NULL;
 	struct cn_dev *dev = &cdev;
 	struct cn_msg *msg = nlmsg_data(nlmsg_hdr(skb));
 	struct netlink_skb_parms *nsp = &NETLINK_CB(skb);
 	int err = -ENODEV;
+
+	/* verify msg->len is within skb */
+	nlh = nlmsg_hdr(skb);
+	if (nlh->nlmsg_len < NLMSG_HDRLEN + sizeof(struct cn_msg) + msg->len)
+		return -EINVAL;
 
 	spin_lock_bh(&dev->cbdev->queue_lock);
 	list_for_each_entry(i, &dev->cbdev->queue_list, callback_entry) {