udpv6: fix possible user after free in error handler

Before derefencing the encap pointer, commit e7cc0824 ("udp: Support
for error handlers of tunnels with arbitrary destination port") checks
for a NULL value, but the two fetch operation can race with removal.
Fix the above using a single access.
Also fix a couple of type annotations, to make sparse happy.

Fixes: e7cc0824 ("udp: Support for error handlers of tunnels with arbitrary destination port")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/udp.c

@@ -420,17 +420,19 @@ EXPORT_SYMBOL(udpv6_encap_enable);
  */
 static int __udp6_lib_err_encap_no_sk(struct sk_buff *skb,
 				      struct inet6_skb_parm *opt,
-				      u8 type, u8 code, int offset, u32 info)
+				      u8 type, u8 code, int offset, __be32 info)
 {
 	int i;
 
 	for (i = 0; i < MAX_IPTUN_ENCAP_OPS; i++) {
 		int (*handler)(struct sk_buff *skb, struct inet6_skb_parm *opt,
-			       u8 type, u8 code, int offset, u32 info);
+			       u8 type, u8 code, int offset, __be32 info);
+		const struct ip6_tnl_encap_ops *encap;
 
-		if (!ip6tun_encaps[i])
+		encap = rcu_dereference(ip6tun_encaps[i]);
+		if (!encap)
 			continue;
-		handler = rcu_dereference(ip6tun_encaps[i]->err_handler);
+		handler = encap->err_handler;
 		if (handler && !handler(skb, opt, type, code, offset, info))
 			return 0;
 	}