ndisc: Check NS message length before access.

Check message length before accessing "target" field,
as we do for other types.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ndisc.c

@@ -671,6 +671,11 @@ static void ndisc_recv_ns(struct sk_buff *skb)
 	bool inc;
 	int is_router = -1;
 
+	if (skb->len < sizeof(struct nd_msg)) {
+		ND_PRINTK(2, warn, "NS: packet too short\n");
+		return;
+	}
+
 	if (ipv6_addr_is_multicast(&msg->target)) {
 		ND_PRINTK(2, warn, "NS: multicast target address\n");
 		return;