xfrm: nat_keepalive: avoid double free on send error

nat_keepalive_send() frees the keepalive skb whenever the IPv4 or IPv6
send helper reports an error.

That cleanup is only correct before the skb is handed to the output
path. Once ip_build_and_send_pkt() or ip6_xmit() takes ownership, the
networking stack may already have consumed the skb before returning an
error, so freeing it again is unsafe.

Handle the pre-handoff failure cases inside nat_keepalive_send_ipv4()
and nat_keepalive_send_ipv6(), where the caller still owns the skb, and
keep nat_keepalive_send() responsible only for family dispatch and the
unsupported-family cleanup path.

Fixes: f531d13 ("xfrm: support sending NAT keepalives in ESP in UDP states")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Qianyu Luo <qianyuluo3@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Author: DoLoveCat

net/xfrm/xfrm_nat_keepalive.c

@@ -55,8 +55,10 @@ static int nat_keepalive_send_ipv4(struct sk_buff *skb,
 			   ka->encap_sport, sock_net_uid(net, NULL));
 
 	rt = ip_route_output_key(net, &fl4);
-	if (IS_ERR(rt))
+	if (IS_ERR(rt)) {
+		kfree_skb(skb);
 		return PTR_ERR(rt);
+	}
 
 	skb_dst_set(skb, &rt->dst);
 
@@ -101,6 +103,7 @@ static int nat_keepalive_send_ipv6(struct sk_buff *skb,
 	dst = ip6_dst_lookup_flow(net, sk, &fl6, NULL);
 	if (IS_ERR(dst)) {
 		local_unlock_nested_bh(&nat_keepalive_sk_ipv6.bh_lock);
+		kfree_skb(skb);
 		return PTR_ERR(dst);
 	}
 
@@ -118,7 +121,6 @@ static void nat_keepalive_send(struct nat_keepalive *ka)
 					sizeof(struct ipv6hdr)) +
 				    sizeof(struct udphdr);
 	const u8 nat_ka_payload = 0xFF;
-	int err = -EAFNOSUPPORT;
 	struct sk_buff *skb;
 	struct udphdr *uh;
 
@@ -140,16 +142,17 @@ static void nat_keepalive_send(struct nat_keepalive *ka)
 
 	switch (ka->family) {
 	case AF_INET:
-		err = nat_keepalive_send_ipv4(skb, ka);
+		nat_keepalive_send_ipv4(skb, ka);
 		break;
 #if IS_ENABLED(CONFIG_IPV6)
 	case AF_INET6:
-		err = nat_keepalive_send_ipv6(skb, ka, uh);
+		nat_keepalive_send_ipv6(skb, ka, uh);
 		break;
 #endif
-	}
-	if (err)
+	default:
 		kfree_skb(skb);
+		break;
+	}
 }
 
 struct nat_keepalive_work_ctx {