rtnl: protect do_setlink from IFLA_XDP_ATTACHED

The IFLA_XDP_ATTACHED nested attribute is meant for read-only, and while
do_setlink properly ignores it, it should be more paranoid and reject
commands that try to set it.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2109,6 +2109,10 @@ static int do_setlink(const struct sk_buff *skb,
 		if (err < 0)
 			goto errout;
 
+		if (xdp[IFLA_XDP_ATTACHED]) {
+			err = -EINVAL;
+			goto errout;
+		}
 		if (xdp[IFLA_XDP_FD]) {
 			err = dev_change_xdp_fd(dev,
 						nla_get_s32(xdp[IFLA_XDP_FD]));