udp: ipv4: fix potential use after free in udp_v4_early_demux()

pskb_may_pull() can reallocate skb->head, we need to move the
initialization of iph and uh pointers after its call.

Fixes: 421b3885bf6d ("udp: ipv4: Add udp early demux")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1909,17 +1909,20 @@ static struct sock *__udp4_lib_demux_lookup(struct net *net,
 
 void udp_v4_early_demux(struct sk_buff *skb)
 {
-	const struct iphdr *iph = ip_hdr(skb);
-	const struct udphdr *uh = udp_hdr(skb);
+	struct net *net = dev_net(skb->dev);
+	const struct iphdr *iph;
+	const struct udphdr *uh;
 	struct sock *sk;
 	struct dst_entry *dst;
-	struct net *net = dev_net(skb->dev);
 	int dif = skb->dev->ifindex;
 
 	/* validate the packet */
 	if (!pskb_may_pull(skb, skb_transport_offset(skb) + sizeof(struct udphdr)))
 		return;
 
+	iph = ip_hdr(skb);
+	uh = udp_hdr(skb);
+
 	if (skb->pkt_type == PACKET_BROADCAST ||
 	    skb->pkt_type == PACKET_MULTICAST)
 		sk = __udp4_lib_mcast_demux_lookup(net, uh->dest, iph->daddr,