net: fou: do not use guehdr after iptunnel_pull_offloads in gue_udp_recv

gue tunnels run iptunnel_pull_offloads on received skbs. This can
determine a possible use-after-free accessing guehdr pointer since
the packet will be 'uncloned' running pskb_expand_head if it is a
cloned gso skb (e.g if the packet has been sent though a veth device)

Fixes: a09a4c8d ("tunnels: Remove encapsulation offloads on decap")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fou.c

@@ -121,6 +121,7 @@ static int gue_udp_recv(struct sock *sk, struct sk_buff *skb)
 	struct guehdr *guehdr;
 	void *data;
 	u16 doffset = 0;
+	u8 proto_ctype;
 
 	if (!fou)
 		return 1;
@@ -212,13 +213,14 @@ static int gue_udp_recv(struct sock *sk, struct sk_buff *skb)
 	if (unlikely(guehdr->control))
 		return gue_control_message(skb, guehdr);
 
+	proto_ctype = guehdr->proto_ctype;
 	__skb_pull(skb, sizeof(struct udphdr) + hdrlen);
 	skb_reset_transport_header(skb);
 
 	if (iptunnel_pull_offloads(skb))
 		goto drop;
 
-	return -guehdr->proto_ctype;
+	return -proto_ctype;
 
 drop:
 	kfree_skb(skb);