xfrm Fix use after free in __xfrm6_udp_encap_rcv.

A recent patch changed xfrm6_udp_encap_rcv to not
free the skb itself anymore but fogot the case
where xfrm4_udp_encap_rcv is called subsequently.

Fix this by moving the call to xfrm4_udp_encap_rcv
from __xfrm6_udp_encap_rcv to xfrm6_udp_encap_rcv.

Fixes: 221ddb72 ("xfrm: Support GRO for IPv6 ESP in UDP encapsulation")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/xfrm4_input.c

@@ -159,7 +159,6 @@ static int __xfrm4_udp_encap_rcv(struct sock *sk, struct sk_buff *skb, bool pull
 	/* process ESP */
 	return 0;
 }
-EXPORT_SYMBOL(xfrm4_udp_encap_rcv);
 
 /* If it's a keepalive packet, then just eat it.
  * If it's an encapsulated packet, then pass it to the
@@ -184,6 +183,7 @@ int xfrm4_udp_encap_rcv(struct sock *sk, struct sk_buff *skb)
 
 	return ret;
 }
+EXPORT_SYMBOL(xfrm4_udp_encap_rcv);
 
 struct sk_buff *xfrm4_gro_udp_encap_rcv(struct sock *sk, struct list_head *head,
 					struct sk_buff *skb)
@@ -223,6 +223,7 @@ struct sk_buff *xfrm4_gro_udp_encap_rcv(struct sock *sk, struct list_head *head,
 
 	return NULL;
 }
+EXPORT_SYMBOL(xfrm4_gro_udp_encap_rcv);
 
 int xfrm4_rcv(struct sk_buff *skb)
 {

net/ipv6/xfrm6_input.c

@@ -80,9 +80,6 @@ static int __xfrm6_udp_encap_rcv(struct sock *sk, struct sk_buff *skb, bool pull
 	__be32 *udpdata32;
 	u16 encap_type;
 
-	if (skb->protocol == htons(ETH_P_IP))
-		return xfrm4_udp_encap_rcv(sk, skb);
-
 	encap_type = READ_ONCE(up->encap_type);
 	/* if this is not encapsulated socket, then just return now */
 	if (!encap_type)
@@ -169,6 +166,9 @@ int xfrm6_udp_encap_rcv(struct sock *sk, struct sk_buff *skb)
 {
 	int ret;
 
+	if (skb->protocol == htons(ETH_P_IP))
+		return xfrm4_udp_encap_rcv(sk, skb);
+
 	ret = __xfrm6_udp_encap_rcv(sk, skb, true);
 	if (!ret)
 		return xfrm6_rcv_encap(skb, IPPROTO_ESP, 0,
@@ -190,6 +190,9 @@ struct sk_buff *xfrm6_gro_udp_encap_rcv(struct sock *sk, struct list_head *head,
 	struct sk_buff *pp = NULL;
 	int ret;
 
+	if (skb->protocol == htons(ETH_P_IP))
+		return xfrm4_gro_udp_encap_rcv(sk, head, skb);
+
 	offset = offset - sizeof(struct udphdr);
 
 	if (!pskb_pull(skb, offset))