udp: Fix UDP short packet false positive

The UDP header pointer assignment must happen after calling
pskb_may_pull().  As pskb_may_pull() can potentially alter the SKB
buffer.

This was exposted by running multicast traffic through the NIU driver,
as it won't prepull the protocol headers into the linear area on
receive.
Signed-off-by: NJesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1231,7 +1231,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		   int proto)
 {
 	struct sock *sk;
-	struct udphdr *uh = udp_hdr(skb);
+	struct udphdr *uh;
 	unsigned short ulen;
 	struct rtable *rt = (struct rtable*)skb->dst;
 	__be32 saddr = ip_hdr(skb)->saddr;
@@ -1244,6 +1244,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 	if (!pskb_may_pull(skb, sizeof(struct udphdr)))
 		goto drop;		/* No space for header. */
 
+	uh   = udp_hdr(skb);
 	ulen = ntohs(uh->len);
 	if (ulen > skb->len)
 		goto short_packet;