net: Verify permission to link_net in newlink

When applicable verify that the caller has permisson to the underlying
network namespace for a newly created network device.

Similary checks exist for the network namespace a network device will
be created in.

Fixes: 317f4810e45e ("rtnl: allow to create device with IFLA_LINK_NETNSID set")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2134,6 +2134,9 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh)
 				err =  -EINVAL;
 				goto out;
 			}
+			err = -EPERM;
+			if (!netlink_ns_capable(skb, link_net->user_ns, CAP_NET_ADMIN))
+				goto out;
 		}
 
 		dev = rtnl_create_link(link_net ? : dest_net, ifname,