[NETLINK]: Fix processing of fib_lookup netlink messages

The receive path for fib_lookup netlink messages is lacking sanity
checks for header and payload and is thus vulnerable to malformed
netlink messages causing illegal memory references.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_frontend.c

@@ -544,12 +544,16 @@ static void nl_fib_input(struct sock *sk, int len)
 	struct sk_buff *skb = NULL;
         struct nlmsghdr *nlh = NULL;
 	struct fib_result_nl *frn;
-	int err;
 	u32 pid;     
 	struct fib_table *tb;
 
-	skb = skb_recv_datagram(sk, 0, 0, &err);
+	skb = skb_dequeue(&sk->sk_receive_queue);
 	nlh = (struct nlmsghdr *)skb->data;
+	if (skb->len < NLMSG_SPACE(0) || skb->len < nlh->nlmsg_len ||
+	    nlh->nlmsg_len < NLMSG_LENGTH(sizeof(*frn))) {
+		kfree_skb(skb);
+		return;
+	}
 
 	frn = (struct fib_result_nl *) NLMSG_DATA(nlh);
 	tb = fib_get_table(frn->tb_id_in);