rtnetlink: enable IFLA_IF_NETNSID for RTM_NEWLINK

- Backwards Compatibility:
  If userspace wants to determine whether RTM_NEWLINK supports the
  IFLA_IF_NETNSID property they should first send an RTM_GETLINK request
  with IFLA_IF_NETNSID on lo. If either EACCESS is returned or the reply
  does not include IFLA_IF_NETNSID userspace should assume that
  IFLA_IF_NETNSID is not supported on this kernel.
  If the reply does contain an IFLA_IF_NETNSID property userspace
  can send an RTM_NEWLINK with a IFLA_IF_NETNSID property. If they receive
  EOPNOTSUPP then the kernel does not support the IFLA_IF_NETNSID property
  with RTM_NEWLINK. Userpace should then fallback to other means.

- Security:
  Callers must have CAP_NET_ADMIN in the owning user namespace of the
  target network namespace.
Signed-off-by: NChristian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2952,14 +2952,10 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			name_assign_type = NET_NAME_ENUM;
 		}
 
-		dest_net = rtnl_link_get_net(net, tb);
+		dest_net = rtnl_link_get_net_capable(skb, net, tb, CAP_NET_ADMIN);
 		if (IS_ERR(dest_net))
 			return PTR_ERR(dest_net);
 
-		err = -EPERM;
-		if (!netlink_ns_capable(skb, dest_net->user_ns, CAP_NET_ADMIN))
-			goto out;
-
 		if (tb[IFLA_LINK_NETNSID]) {
 			int id = nla_get_s32(tb[IFLA_LINK_NETNSID]);