fou: Fix use-after-free in fou_create()

fou_create() publishes struct fou through sk_user_data before adding the
new FOU port to the per-netns list.  If fou_add_to_port_list() fails,
the error path frees fou while it is still reachable through
sk_user_data.  A concurrent receive can then dereference the freed
object in fou_from_sock().

This ordering issue was previously noted in the linked discussion.

The failure is reachable when local port 0 is requested.  Each socket
binds to a different ephemeral port, but fou_cfg_cmp() compares the
requested port 0 and reports -EALREADY once an entry already exists.

Release the tunnel socket before freeing fou so sk_user_data is cleared
first, and defer reclamation with kfree_rcu() to protect concurrent RCU
readers.  This matches the lifetime handling in fou_release().

Fixes: 23461551c0 ("fou: Support for foo-over-udp RX path")
Suggested-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://lore.kernel.org/netdev/20260502031401.3557229-12-kuniyu@google.com/
Cc: stable@vger.kernel.org
Signed-off-by: Xuanqiang Luo <luoxuanqiang@kylinos.cn>
Link: https://patch.msgid.link/20260722083858.182506-1-xuanqiang.luo@linux.dev
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv4/fou_core.c

@@ -629,9 +629,9 @@ static int fou_create(struct net *net, struct fou_cfg *cfg,
 	return 0;
 
 error:
-	kfree(fou);
 	if (sock)
 		udp_tunnel_sock_release(sock->sk);
+	kfree_rcu(fou, rcu);
 
 	return err;
 }