mptcp: handle correctly disconnect() failures

Currently the mptcp code has assumes that disconnect() can fail only
at mptcp_sendmsg_fastopen() time - to avoid a deadlock scenario - and
don't even bother returning an error code.

Soon mptcp_disconnect() will handle more error conditions: let's track
them explicitly.

As a bonus, explicitly annotate TCP-level disconnect as not failing:
the mptcp code never blocks for event on the subflows.

Fixes: 7d80334 ("mptcp: fix deadlock in fastopen error path")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Tested-by: Christoph Paasch <cpaasch@apple.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>

net/mptcp/protocol.c

@@ -1727,7 +1727,13 @@ static int mptcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
 		if (ret && ret != -EINPROGRESS && ret != -ERESTARTSYS && ret != -EINTR)
 			*copied_syn = 0;
 	} else if (ret && ret != -EINPROGRESS) {
-		mptcp_disconnect(sk, 0);
+		/* The disconnect() op called by tcp_sendmsg_fastopen()/
+		 * __inet_stream_connect() can fail, due to looking check,
+		 * see mptcp_disconnect().
+		 * Attempt it again outside the problematic scope.
+		 */
+		if (!mptcp_disconnect(sk, 0))
+			sk->sk_socket->state = SS_UNCONNECTED;
 	}
 	inet_sk(sk)->defer_connect = 0;
 
@@ -2389,7 +2395,10 @@ static void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 
 	need_push = (flags & MPTCP_CF_PUSH) && __mptcp_retransmit_pending_data(sk);
 	if (!dispose_it) {
-		tcp_disconnect(ssk, 0);
+		/* The MPTCP code never wait on the subflow sockets, TCP-level
+		 * disconnect should never fail
+		 */
+		WARN_ON_ONCE(tcp_disconnect(ssk, 0));
 		msk->subflow->state = SS_UNCONNECTED;
 		mptcp_subflow_ctx_reset(subflow);
 		release_sock(ssk);
@@ -2812,7 +2821,7 @@ void mptcp_subflow_shutdown(struct sock *sk, struct sock *ssk, int how)
 			break;
 		fallthrough;
 	case TCP_SYN_SENT:
-		tcp_disconnect(ssk, O_NONBLOCK);
+		WARN_ON_ONCE(tcp_disconnect(ssk, O_NONBLOCK));
 		break;
 	default:
 		if (__mptcp_check_fallback(mptcp_sk(sk))) {
@@ -3075,11 +3084,10 @@ static int mptcp_disconnect(struct sock *sk, int flags)
 
 	/* We are on the fastopen error path. We can't call straight into the
 	 * subflows cleanup code due to lock nesting (we are already under
-	 * msk->firstsocket lock). Do nothing and leave the cleanup to the
-	 * caller.
+	 * msk->firstsocket lock).
 	 */
 	if (msk->fastopening)
-		return 0;
+		return -EBUSY;
 
 	mptcp_listen_inuse_dec(sk);
 	inet_sk_state_store(sk, TCP_CLOSE);