[AF_NETLINK]: Fix DoS in netlink_rcv_skb()

From: Martin Murray <murrayma@citi.umich.edu>

Sanity check nlmsg_len during netlink_rcv_skb.  An nlmsg_len == 0 can
cause infinite loop in kernel, effectively DoSing machine.  Noted by
Matin Murray.
Signed-off-by: NChris Wright <chrisw@sous-sol.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -1422,7 +1422,7 @@ static int netlink_rcv_skb(struct sk_buff *skb, int (*cb)(struct sk_buff *,
 	while (skb->len >= nlmsg_total_size(0)) {
 		nlh = (struct nlmsghdr *) skb->data;
 
-		if (skb->len < nlh->nlmsg_len)
+		if (nlh->nlmsg_len < NLMSG_HDRLEN || skb->len < nlh->nlmsg_len)
 			return 0;
 
 		total_len = min(NLMSG_ALIGN(nlh->nlmsg_len), skb->len);