mptcp: fix fallback for MP_JOIN subflows

Additional/MP_JOIN subflows that do not pass some initial handshake
tests currently causes fallback to TCP. That is an RFC violation:
we should instead reset the subflow and leave the the msk untouched.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/91
Fixes: f296234c98 ("mptcp: Add handling of incoming MP_JOIN requests")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/options.c

@@ -626,6 +626,12 @@ bool mptcp_established_options(struct sock *sk, struct sk_buff *skb,
 	if (unlikely(mptcp_check_fallback(sk)))
 		return false;
 
+	/* prevent adding of any MPTCP related options on reset packet
+	 * until we support MP_TCPRST/MP_FASTCLOSE
+	 */
+	if (unlikely(skb && TCP_SKB_CB(skb)->tcp_flags & TCPHDR_RST))
+		return false;
+
 	if (mptcp_established_options_mp(sk, skb, &opt_size, remaining, opts))
 		ret = true;
 	else if (mptcp_established_options_dss(sk, skb, &opt_size, remaining,
@@ -676,7 +682,7 @@ bool mptcp_synack_options(const struct request_sock *req, unsigned int *size,
 	return false;
 }
 
-static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
+static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
 				    struct mptcp_subflow_context *subflow,
 				    struct sk_buff *skb,
 				    struct mptcp_options_received *mp_opt)
@@ -693,15 +699,20 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
 		    TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq &&
 		    subflow->mp_join && mp_opt->mp_join &&
 		    READ_ONCE(msk->pm.server_side))
-			tcp_send_ack(sk);
+			tcp_send_ack(ssk);
 		goto fully_established;
 	}
 
-	/* we should process OoO packets before the first subflow is fully
+	/* we must process OoO packets before the first subflow is fully
-	 * established, but not expected for MP_JOIN subflows
+	 * established. OoO packets are instead a protocol violation
+	 * for MP_JOIN subflows as the peer must not send any data
+	 * before receiving the forth ack - cfr. RFC 8684 section 3.2.
 	 */
-	if (TCP_SKB_CB(skb)->seq != subflow->ssn_offset + 1)
+	if (TCP_SKB_CB(skb)->seq != subflow->ssn_offset + 1) {
+		if (subflow->mp_join)
+			goto reset;
 		return subflow->mp_capable;
+	}
 
 	if (mp_opt->dss && mp_opt->use_ack) {
 		/* subflows are fully established as soon as we get any
@@ -713,9 +724,12 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
 	}
 
 	/* If the first established packet does not contain MP_CAPABLE + data
-	 * then fallback to TCP
+	 * then fallback to TCP. Fallback scenarios requires a reset for
+	 * MP_JOIN subflows.
 	 */
 	if (!mp_opt->mp_capable) {
+		if (subflow->mp_join)
+			goto reset;
 		subflow->mp_capable = 0;
 		pr_fallback(msk);
 		__mptcp_do_fallback(msk);
@@ -732,12 +746,16 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *sk,
 
 	subflow->pm_notified = 1;
 	if (subflow->mp_join) {
-		clear_3rdack_retransmission(sk);
+		clear_3rdack_retransmission(ssk);
 		mptcp_pm_subflow_established(msk, subflow);
 	} else {
 		mptcp_pm_fully_established(msk);
 	}
 	return true;
+
+reset:
+	mptcp_subflow_reset(ssk);
+	return false;
 }
 
 static u64 expand_ack(u64 old_ack, u64 cur_ack, bool use_64bit)

net/mptcp/protocol.h

@@ -348,6 +348,7 @@ void mptcp_subflow_fully_established(struct mptcp_subflow_context *subflow,
 				     struct mptcp_options_received *mp_opt);
 bool mptcp_subflow_data_available(struct sock *sk);
 void __init mptcp_subflow_init(void);
+void mptcp_subflow_reset(struct sock *ssk);
 
 /* called with sk socket lock held */
 int __mptcp_subflow_connect(struct sock *sk, int ifindex,

net/mptcp/subflow.c

@@ -270,6 +270,13 @@ static bool subflow_thmac_valid(struct mptcp_subflow_context *subflow)
 	return thmac == subflow->thmac;
 }
 
+void mptcp_subflow_reset(struct sock *ssk)
+{
+	tcp_set_state(ssk, TCP_CLOSE);
+	tcp_send_active_reset(ssk, GFP_ATOMIC);
+	tcp_done(ssk);
+}
+
 static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
@@ -342,8 +349,7 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 	return;
 
 do_reset:
-	tcp_send_active_reset(sk, GFP_ATOMIC);
+	mptcp_subflow_reset(sk);
-	tcp_done(sk);
 }
 
 struct request_sock_ops mptcp_subflow_request_sock_ops;