mptcp: fix lockdep false positive in mptcp_pm_nl_create_listen_socket()

Christoph reports a lockdep splat in the mptcp_subflow_create_socket()
error path, when such function is invoked by
mptcp_pm_nl_create_listen_socket().

Such code path acquires two separates, nested socket lock, with the
internal lock operation lacking the "nested" annotation. Adding that
in sock_release() for mptcp's sake only could be confusing.

Instead just add a new lockclass to the in-kernel msk socket,
re-initializing the lockdep infra after the socket creation.

Fixes: ad21710 ("mptcp: fix locking for in-kernel listener creation")
Cc: stable@vger.kernel.org
Reported-by: Christoph Paasch <cpaasch@apple.com>
Closes: multipath-tcp/mptcp_net-next#354
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Tested-by: Christoph Paasch <cpaasch@apple.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Paolo Abeni

net/mptcp/pm_netlink.c

@@ -997,9 +997,13 @@ static int mptcp_pm_nl_append_new_local_addr(struct pm_nl_pernet *pernet,
 	return ret;
 }
 
+static struct lock_class_key mptcp_slock_keys[2];
+static struct lock_class_key mptcp_keys[2];
+
 static int mptcp_pm_nl_create_listen_socket(struct sock *sk,
 					    struct mptcp_pm_addr_entry *entry)
 {
+	bool is_ipv6 = sk->sk_family == AF_INET6;
 	int addrlen = sizeof(struct sockaddr_in);
 	struct sockaddr_storage addr;
 	struct socket *ssock;
@@ -1016,6 +1020,18 @@ static int mptcp_pm_nl_create_listen_socket(struct sock *sk,
 	if (!newsk)
 		return -EINVAL;
 
+	/* The subflow socket lock is acquired in a nested to the msk one
+	 * in several places, even by the TCP stack, and this msk is a kernel
+	 * socket: lockdep complains. Instead of propagating the _nested
+	 * modifiers in several places, re-init the lock class for the msk
+	 * socket to an mptcp specific one.
+	 */
+	sock_lock_init_class_and_name(newsk,
+				      is_ipv6 ? "mlock-AF_INET6" : "mlock-AF_INET",
+				      &mptcp_slock_keys[is_ipv6],
+				      is_ipv6 ? "msk_lock-AF_INET6" : "msk_lock-AF_INET",
+				      &mptcp_keys[is_ipv6]);
+
 	lock_sock(newsk);
 	ssock = __mptcp_nmpc_socket(mptcp_sk(newsk));
 	release_sock(newsk);