[MPTCP] [PATCH net 0/2] mptcp: some fallback fixes

[MPTCP] [PATCH net 0/2] mptcp: some fallback fixes

[Paolo Abeni]

[-- Attachment #1: Type: text/plain, Size: 636 bytes --]

pktdrill pointed-out we currently don't handle properly some
fallback scenario for MP_JOIN subflows

The first patch addresses such issue.

Patch 2/2 fixes a related pre-existing issue that is more
evident after 1/2: we could keep using for MPTCP signaling
closed subflows.

Paolo Abeni (2):
  mptcp: fix fallback for MP_JOIN subflows
  mptcp: subflows garbage collection

 net/mptcp/options.c  | 32 +++++++++++++++++++++++++-------
 net/mptcp/protocol.c | 17 +++++++++++++++++
 net/mptcp/protocol.h |  2 ++
 net/mptcp/subflow.c  | 16 ++++++++++++++--
 4 files changed, 58 insertions(+), 9 deletions(-)

-- 
2.26.2

[PATCH net 0/2] mptcp: some fallback fixes

[Paolo Abeni]

pktdrill pointed-out we currently don't handle properly some
fallback scenario for MP_JOIN subflows

The first patch addresses such issue.

Patch 2/2 fixes a related pre-existing issue that is more
evident after 1/2: we could keep using for MPTCP signaling
closed subflows.

Paolo Abeni (2):
  mptcp: fix fallback for MP_JOIN subflows
  mptcp: subflows garbage collection

 net/mptcp/options.c  | 32 +++++++++++++++++++++++++-------
 net/mptcp/protocol.c | 17 +++++++++++++++++
 net/mptcp/protocol.h |  2 ++
 net/mptcp/subflow.c  | 16 ++++++++++++++--
 4 files changed, 58 insertions(+), 9 deletions(-)

-- 
2.26.2

[MPTCP] [PATCH net 1/2] mptcp: fix fallback for MP_JOIN subflows

[Paolo Abeni]

[-- Attachment #1: Type: text/plain, Size: 5130 bytes --]

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: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Reviewed-by: Mat Martineau <mathew.j.martineau(a)linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni(a)redhat.com>
---
 net/mptcp/options.c  | 32 +++++++++++++++++++++++++-------
 net/mptcp/protocol.h |  1 +
 net/mptcp/subflow.c  | 10 ++++++++--
 3 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 888bbbbb3e8a..277f12633fc9 100644
--- a/net/mptcp/options.c
+++ b/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
-	 * established, but not expected for MP_JOIN subflows
+	/* we must process OoO packets before the first subflow is fully
+	 * 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)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 20f04ac85409..0a6e5b3f6ae8 100644
--- a/net/mptcp/protocol.h
+++ b/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,
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 6f035af1c9d2..b1b8028730bf 100644
--- a/net/mptcp/subflow.c
+++ b/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);
-	tcp_done(sk);
+	mptcp_subflow_reset(sk);
 }
 
 struct request_sock_ops mptcp_subflow_request_sock_ops;
-- 
2.26.2

[PATCH net 1/2] mptcp: fix fallback for MP_JOIN subflows

[Paolo Abeni]

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: f296234c98a8 ("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>
---
 net/mptcp/options.c  | 32 +++++++++++++++++++++++++-------
 net/mptcp/protocol.h |  1 +
 net/mptcp/subflow.c  | 10 ++++++++--
 3 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 888bbbbb3e8a..277f12633fc9 100644
--- a/net/mptcp/options.c
+++ b/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
-	 * established, but not expected for MP_JOIN subflows
+	/* we must process OoO packets before the first subflow is fully
+	 * 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)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 20f04ac85409..0a6e5b3f6ae8 100644
--- a/net/mptcp/protocol.h
+++ b/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,
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 6f035af1c9d2..b1b8028730bf 100644
--- a/net/mptcp/subflow.c
+++ b/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);
-	tcp_done(sk);
+	mptcp_subflow_reset(sk);
 }
 
 struct request_sock_ops mptcp_subflow_request_sock_ops;
-- 
2.26.2

[MPTCP] [PATCH net 2/2] mptcp: subflows garbage collection

[Paolo Abeni]

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The msk can close MP_JOIN subflows if the initial handshake
fails. Currently such subflows are kept alive in the
conn_list until the msk itself is closed.

Beyond the wasted memory, we could end-up sending the
DATA_FIN and the DATA_FIN ack on such socket, even after a
reset.

Fixes: 43b54c6ee382 ("mptcp: Use full MPTCP-level disconnect state machine")
Reviewed-by: Mat Martineau <mathew.j.martineau(a)linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni(a)redhat.com>
---
 net/mptcp/protocol.c | 17 +++++++++++++++++
 net/mptcp/protocol.h |  1 +
 net/mptcp/subflow.c  |  6 ++++++
 3 files changed, 24 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 5d747c6a610e..b295eb6e9580 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1383,6 +1383,20 @@ static void pm_work(struct mptcp_sock *msk)
 	spin_unlock_bh(&msk->pm.lock);
 }
 
+static void __mptcp_close_subflow(struct mptcp_sock *msk)
+{
+	struct mptcp_subflow_context *subflow, *tmp;
+
+	list_for_each_entry_safe(subflow, tmp, &msk->conn_list, node) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+		if (inet_sk_state_load(ssk) != TCP_CLOSE)
+			continue;
+
+		__mptcp_close_ssk((struct sock *)msk, ssk, subflow, 0);
+	}
+}
+
 static void mptcp_worker(struct work_struct *work)
 {
 	struct mptcp_sock *msk = container_of(work, struct mptcp_sock, work);
@@ -1400,6 +1414,9 @@ static void mptcp_worker(struct work_struct *work)
 	mptcp_clean_una(sk);
 	mptcp_check_data_fin_ack(sk);
 	__mptcp_flush_join_list(msk);
+	if (test_and_clear_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags))
+		__mptcp_close_subflow(msk);
+
 	__mptcp_move_skbs(msk);
 
 	if (msk->pm.status)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 0a6e5b3f6ae8..0c4b8cc64dbc 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -90,6 +90,7 @@
 #define MPTCP_WORK_RTX		2
 #define MPTCP_WORK_EOF		3
 #define MPTCP_FALLBACK_DONE	4
+#define MPTCP_WORK_CLOSE_SUBFLOW 5
 
 struct mptcp_options_received {
 	u64	sndr_key;
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index b1b8028730bf..80f67cb56631 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -272,9 +272,15 @@ static bool subflow_thmac_valid(struct mptcp_subflow_context *subflow)
 
 void mptcp_subflow_reset(struct sock *ssk)
 {
+	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
+	struct sock *sk = subflow->conn;
+
 	tcp_set_state(ssk, TCP_CLOSE);
 	tcp_send_active_reset(ssk, GFP_ATOMIC);
 	tcp_done(ssk);
+	if (!test_and_set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &mptcp_sk(sk)->flags) &&
+	    schedule_work(&mptcp_sk(sk)->work))
+		sock_hold(sk);
 }
 
 static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
-- 
2.26.2

[PATCH net 2/2] mptcp: subflows garbage collection

[Paolo Abeni]

The msk can close MP_JOIN subflows if the initial handshake
fails. Currently such subflows are kept alive in the
conn_list until the msk itself is closed.

Beyond the wasted memory, we could end-up sending the
DATA_FIN and the DATA_FIN ack on such socket, even after a
reset.

Fixes: 43b54c6ee382 ("mptcp: Use full MPTCP-level disconnect state machine")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/protocol.c | 17 +++++++++++++++++
 net/mptcp/protocol.h |  1 +
 net/mptcp/subflow.c  |  6 ++++++
 3 files changed, 24 insertions(+)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 5d747c6a610e..b295eb6e9580 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1383,6 +1383,20 @@ static void pm_work(struct mptcp_sock *msk)
 	spin_unlock_bh(&msk->pm.lock);
 }
 
+static void __mptcp_close_subflow(struct mptcp_sock *msk)
+{
+	struct mptcp_subflow_context *subflow, *tmp;
+
+	list_for_each_entry_safe(subflow, tmp, &msk->conn_list, node) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+
+		if (inet_sk_state_load(ssk) != TCP_CLOSE)
+			continue;
+
+		__mptcp_close_ssk((struct sock *)msk, ssk, subflow, 0);
+	}
+}
+
 static void mptcp_worker(struct work_struct *work)
 {
 	struct mptcp_sock *msk = container_of(work, struct mptcp_sock, work);
@@ -1400,6 +1414,9 @@ static void mptcp_worker(struct work_struct *work)
 	mptcp_clean_una(sk);
 	mptcp_check_data_fin_ack(sk);
 	__mptcp_flush_join_list(msk);
+	if (test_and_clear_bit(MPTCP_WORK_CLOSE_SUBFLOW, &msk->flags))
+		__mptcp_close_subflow(msk);
+
 	__mptcp_move_skbs(msk);
 
 	if (msk->pm.status)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 0a6e5b3f6ae8..0c4b8cc64dbc 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -90,6 +90,7 @@
 #define MPTCP_WORK_RTX		2
 #define MPTCP_WORK_EOF		3
 #define MPTCP_FALLBACK_DONE	4
+#define MPTCP_WORK_CLOSE_SUBFLOW 5
 
 struct mptcp_options_received {
 	u64	sndr_key;
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index b1b8028730bf..80f67cb56631 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -272,9 +272,15 @@ static bool subflow_thmac_valid(struct mptcp_subflow_context *subflow)
 
 void mptcp_subflow_reset(struct sock *ssk)
 {
+	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
+	struct sock *sk = subflow->conn;
+
 	tcp_set_state(ssk, TCP_CLOSE);
 	tcp_send_active_reset(ssk, GFP_ATOMIC);
 	tcp_done(ssk);
+	if (!test_and_set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &mptcp_sk(sk)->flags) &&
+	    schedule_work(&mptcp_sk(sk)->work))
+		sock_hold(sk);
 }
 
 static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
-- 
2.26.2

[MPTCP] Re: [PATCH net 0/2] mptcp: some fallback fixes

[Jakub Kicinski]

[-- Attachment #1: Type: text/plain, Size: 649 bytes --]

On Fri,  9 Oct 2020 18:59:59 +0200 Paolo Abeni wrote:
> pktdrill pointed-out we currently don't handle properly some
> fallback scenario for MP_JOIN subflows
> 
> The first patch addresses such issue.
> 
> Patch 2/2 fixes a related pre-existing issue that is more
> evident after 1/2: we could keep using for MPTCP signaling
> closed subflows.

Applied, thanks Paolo.

You already have a few of those in the code, but:

+	if (... &&
+	    schedule_work(&mptcp_sk(sk)->work))
+		sock_hold(sk);

isn't this a fairly questionable construct?

You take a reference for the async work to release _after_ you
scheduled the async work? 

Re: [PATCH net 0/2] mptcp: some fallback fixes

[Jakub Kicinski]

On Fri,  9 Oct 2020 18:59:59 +0200 Paolo Abeni wrote:
> pktdrill pointed-out we currently don't handle properly some
> fallback scenario for MP_JOIN subflows
> 
> The first patch addresses such issue.
> 
> Patch 2/2 fixes a related pre-existing issue that is more
> evident after 1/2: we could keep using for MPTCP signaling
> closed subflows.

Applied, thanks Paolo.

You already have a few of those in the code, but:

+	if (... &&
+	    schedule_work(&mptcp_sk(sk)->work))
+		sock_hold(sk);

isn't this a fairly questionable construct?

You take a reference for the async work to release _after_ you
scheduled the async work? 

[MPTCP] Re: [PATCH net 0/2] mptcp: some fallback fixes

[Paolo Abeni]

[-- Attachment #1: Type: text/plain, Size: 1243 bytes --]

On Sat, 2020-10-10 at 11:13 -0700, Jakub Kicinski wrote:
> On Fri,  9 Oct 2020 18:59:59 +0200 Paolo Abeni wrote:
> > pktdrill pointed-out we currently don't handle properly some
> > fallback scenario for MP_JOIN subflows
> > 
> > The first patch addresses such issue.
> > 
> > Patch 2/2 fixes a related pre-existing issue that is more
> > evident after 1/2: we could keep using for MPTCP signaling
> > closed subflows.
> 
> Applied, thanks Paolo.
> 
> You already have a few of those in the code, but:
> 
> +	if (... &&
> +	    schedule_work(&mptcp_sk(sk)->work))
> +		sock_hold(sk);
> 
> isn't this a fairly questionable construct?
> 
> You take a reference for the async work to release _after_ you
> scheduled the async work? 

Thank you for reviewing! Indeed we need to add some comments there:
IIRC that chunk already raised a question in the past.

Afaics, that is safe because the caller (a subflow) held a reference to
sk and sk can't be freed in between the scheduling and the next
sock_hold(). 

We have a pending refactor, targeting the next development cycle, that
will consolidate the workqueue scheduling into an helper. We will add
some comments there to clarify the above.

Thanks,

Paolo

Re: [PATCH net 0/2] mptcp: some fallback fixes

[Paolo Abeni]

On Sat, 2020-10-10 at 11:13 -0700, Jakub Kicinski wrote:
> On Fri,  9 Oct 2020 18:59:59 +0200 Paolo Abeni wrote:
> > pktdrill pointed-out we currently don't handle properly some
> > fallback scenario for MP_JOIN subflows
> > 
> > The first patch addresses such issue.
> > 
> > Patch 2/2 fixes a related pre-existing issue that is more
> > evident after 1/2: we could keep using for MPTCP signaling
> > closed subflows.
> 
> Applied, thanks Paolo.
> 
> You already have a few of those in the code, but:
> 
> +	if (... &&
> +	    schedule_work(&mptcp_sk(sk)->work))
> +		sock_hold(sk);
> 
> isn't this a fairly questionable construct?
> 
> You take a reference for the async work to release _after_ you
> scheduled the async work? 

Thank you for reviewing! Indeed we need to add some comments there:
IIRC that chunk already raised a question in the past.

Afaics, that is safe because the caller (a subflow) held a reference to
sk and sk can't be freed in between the scheduling and the next
sock_hold(). 

We have a pending refactor, targeting the next development cycle, that
will consolidate the workqueue scheduling into an helper. We will add
some comments there to clarify the above.

Thanks,

Paolo

[MPTCP] [PATCH net 1/2] mptcp: fix fallback for MP_JOIN subflows

[Paolo Abeni]

[-- Attachment #1: Type: text/plain, Size: 5073 bytes --]

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: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: Paolo Abeni <pabeni(a)redhat.com>
---
 net/mptcp/options.c  | 32 +++++++++++++++++++++++++-------
 net/mptcp/protocol.h |  1 +
 net/mptcp/subflow.c  | 10 ++++++++--
 3 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 4055769e4fde..ad2bc95dd638 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -657,6 +657,12 @@ 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,
@@ -711,7 +717,7 @@ 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)
@@ -728,15 +734,20 @@ 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
-	 * established, but not expected for MP_JOIN subflows
+	/* we must process OoO packets before the first subflow is fully
+	 * 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
@@ -748,9 +759,12 @@ 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);
@@ -767,12 +781,16 @@ 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)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index aa0ab18d2e57..de5219fd3570 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -369,6 +369,7 @@ void mptcp_subflow_shutdown(struct sock *sk, struct sock *ssk, int how);
 void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 		       struct mptcp_subflow_context *subflow,
 		       long timeout);
+void mptcp_subflow_reset(struct sock *ssk);
 
 /* called with sk socket lock held */
 int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 5ca8032e0d24..d2eb5f2abe5a 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -271,6 +271,13 @@ 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);
@@ -343,8 +350,7 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 	return;
 
 do_reset:
-	tcp_send_active_reset(sk, GFP_ATOMIC);
-	tcp_done(sk);
+	mptcp_subflow_reset(sk);
 }
 
 struct request_sock_ops mptcp_subflow_request_sock_ops;
-- 
2.26.2

[MPTCP] [PATCH net 1/2] mptcp: fix fallback for MP_JOIN subflows

[Paolo Abeni]

[-- Attachment #1: Type: text/plain, Size: 5073 bytes --]

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: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Signed-off-by: Paolo Abeni <pabeni(a)redhat.com>
---
 net/mptcp/options.c  | 32 +++++++++++++++++++++++++-------
 net/mptcp/protocol.h |  1 +
 net/mptcp/subflow.c  | 10 ++++++++--
 3 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 4055769e4fde..ad2bc95dd638 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -657,6 +657,12 @@ 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,
@@ -711,7 +717,7 @@ 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)
@@ -728,15 +734,20 @@ 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
-	 * established, but not expected for MP_JOIN subflows
+	/* we must process OoO packets before the first subflow is fully
+	 * 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
@@ -748,9 +759,12 @@ 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);
@@ -767,12 +781,16 @@ 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)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index aa0ab18d2e57..de5219fd3570 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -369,6 +369,7 @@ void mptcp_subflow_shutdown(struct sock *sk, struct sock *ssk, int how);
 void __mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 		       struct mptcp_subflow_context *subflow,
 		       long timeout);
+void mptcp_subflow_reset(struct sock *ssk);
 
 /* called with sk socket lock held */
 int __mptcp_subflow_connect(struct sock *sk, const struct mptcp_addr_info *loc,
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 5ca8032e0d24..d2eb5f2abe5a 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -271,6 +271,13 @@ 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);
@@ -343,8 +350,7 @@ static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)
 	return;
 
 do_reset:
-	tcp_send_active_reset(sk, GFP_ATOMIC);
-	tcp_done(sk);
+	mptcp_subflow_reset(sk);
 }
 
 struct request_sock_ops mptcp_subflow_request_sock_ops;
-- 
2.26.2