mptcp: fix recent failed backports (20260919)

The following patches could not be applied without conflicts in this tree:

- 2b0f561f21b2 ("mptcp: avoid unneeded actions on subflow reset")
- 42064de57fb8 ("mptcp: close race between scheduler and state change")

I backported the following commit to avoid conflicts:

- ab82e996a1fa ("mptcp: hold mptcp socket before calling tcp_done")

Conflicts (if any) have been resolved, and documented in each patch.

Florian Westphal (1):
mptcp: hold mptcp socket before calling tcp_done

Paolo Abeni (2):
mptcp: avoid unneeded actions on subflow reset
mptcp: close race between scheduler and state change

 net/mptcp/protocol.c | 10 +++++-----
 net/mptcp/protocol.h |  3 ++-
 net/mptcp/subflow.c  | 18 +++++++++++++++++-
 3 files changed, 24 insertions(+), 7 deletions(-)

2.55.0

From: Florian Westphal <[email protected]>

commit ab82e996a1fa1b9ae514fa357d9ce8df62321157 upstream.

When processing options from tcp reset path its possible that tcp_done(ssk) drops the last reference on the mptcp socket which results in use-after-free.

Stable-dep-of: 2b0f561f21b2 ("mptcp: avoid unneeded actions on subflow reset") [ Conflicts in subflow.c, because commit 3ba14528684f ("mptcp: avoid
setting TCP_CLOSE state twice") has already been backported and also had the same conflict: this commit here should have been backported first. Fixed now! ]

---
 net/mptcp/subflow.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

index c9cf0fdbfbdc..77cc4f585cd8 100644
@@ -279,11 +279,16 @@ void mptcp_subflow_reset(struct sock *ssk)
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
 	struct sock *sk = subflow->conn;
 
+	/* must hold: tcp_done() could drop last reference on parent */
+	sock_hold(sk);
+
 	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);
+		return; /* worker will put sk for us */
+
+	sock_put(sk);
 }
 
 static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)

2.55.0

This is a note to let you know that I've just added the patch titled

mptcp: hold mptcp socket before calling tcp_done

to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mptcp-hold-mptcp-socket-before-calling-tcp_done.patch
and it can be found in the queue-5.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From [email protected] Sat Sep 19 22:40:23 2026
From: "Matthieu Baerts (NGI0)" <[email protected]>
Date: Sat, 19 Sep 2026 22:40:04 +0200
Subject: mptcp: hold mptcp socket before calling tcp_done
To: [email protected], [email protected], [email protected]
Cc: Florian Westphal <[email protected]>, [email protected], Matthieu Baerts <[email protected]>, Mat Martineau <[email protected]>, Jakub Kicinski <[email protected]>, "Matthieu Baerts (NGI0)" <[email protected]>
Message-ID: <[email protected]>

From: Florian Westphal <[email protected]>

commit ab82e996a1fa1b9ae514fa357d9ce8df62321157 upstream.

When processing options from tcp reset path its possible that tcp_done(ssk) drops the last reference on the mptcp socket which results in use-after-free.

Stable-dep-of: 2b0f561f21b2 ("mptcp: avoid unneeded actions on subflow reset") [ Conflicts in subflow.c, because commit 3ba14528684f ("mptcp: avoid
setting TCP_CLOSE state twice") has already been backported and also had the same conflict: this commit here should have been backported first. Fixed now! ]

---
 net/mptcp/subflow.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

@@ -279,11 +279,16 @@ void mptcp_subflow_reset(struct sock *ss
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
 	struct sock *sk = subflow->conn;
 
+	/* must hold: tcp_done() could drop last reference on parent */
+	sock_hold(sk);
+
 	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);
+		return; /* worker will put sk for us */
+
+	sock_put(sk);
 }
 
 static void subflow_finish_connect(struct sock *sk, const struct sk_buff *skb)

Patches currently in stable-queue which might be from [email protected] are

queue-5.10/mptcp-hold-mptcp-socket-before-calling-tcp_done.patch queue-5.10/mptcp-syncookies-remember-the-request-backup-flag.patch queue-5.10/mptcp-close-race-between-scheduler-and-state-change.patch queue-5.10/mptcp-avoid-unneeded-actions-on-subflow-reset.patch

From: Paolo Abeni <[email protected]>

commit 2b0f561f21b27c40c91ea4975268a06092bd7e9c upstream.

Once in a blue moon, the mptcp receive path can recursively call mptcp_data_ready() via state change under unlucky error conditions, and then try to hold the data lock again.

Break the recursion loop explicitly checking for the exceptional condition.

Add a new flag instead of using an existing one like 'closing', to exit early in subflow_state_change(), and explicitly flush the RX queue at reset time.

This avoids unneeded processing to check for available data -- calling get_mapping_status() and more on a dying subflow -- but also in error reporting and worker scheduling.

Note that we must consume the currently peeked skb before invoking mptcp_dss_corruption to avoid consuming it again after the eventual reset has freed it.

[ Note: conflict in protocol.c, because commit e0ca4057e0ec ("mptcp:
micro-optimize __mptcp_move_skb()") is not in this version, and is part of a consequent rx path refactor. The conflict is in the context, and is easy to resolve, "done = true" can be moved along without consequences. Also, the context is slightly different because there is no DEBUG_NET_WARN_ON_ONCE in this version, see the backport commit 12c1676d598e ("mptcp: handle consistently DSS corruption"). Also a conflict in protocol.h, because __unused is at a different number. Decrement the one from this version and add the new flag above. The context is also a bit different with data_avail being an enum, but that's without consequences here.
Also a conflict in subflow.c, because commit 71154bbe4942 ("mptcp:
fallback earlier on simult connection") was not needed in this version, and cause conflicts in the context, but that's without consequences here. Also, a conflict in the context, because commit 81c1d0290160 ("mptcp: consolidate fallback and non fallback state machine") was not needed in this version. ]

---
 net/mptcp/protocol.c |  6 +++---
 net/mptcp/protocol.h |  3 ++-
 net/mptcp/subflow.c  | 11 +++++++++++
 3 files changed, 16 insertions(+), 4 deletions(-)

index 2c6aef813473..292c21713eb7 100644
@@ -581,11 +581,11 @@ static bool __mptcp_move_skbs_from_subflow(struct mptcp_sock *msk,
 			if (unlikely(map_remaining < len))
 				mptcp_dss_corruption(msk, ssk);
 		} else {
-			if (unlikely(!fin))
-				mptcp_dss_corruption(msk, ssk);
-
 			sk_eat_skb(ssk, skb);
 			done = true;
+
+			if (unlikely(!fin))
+				mptcp_dss_corruption(msk, ssk);
 		}
 
 		WRITE_ONCE(tp->copied_seq, seq);

index ac064c44079d..b64a50b22d62 100644
@@ -314,7 +314,8 @@ struct mptcp_subflow_context {
 		mpc_map : 1,
 		backup : 1,
 		rx_eof : 1,
-		can_ack : 1;	    /* only after processing the remote a key */
+		can_ack : 1,	    /* only after processing the remote a key */
+		resetting : 1;	    /* subflow is resetting */
 	enum mptcp_data_avail data_avail;
 	u32	remote_nonce;
 	u64	thmac;

index 77cc4f585cd8..fff70a5b06db 100644
@@ -282,6 +282,10 @@ void mptcp_subflow_reset(struct sock *ssk)
 	/* must hold: tcp_done() could drop last reference on parent */
 	sock_hold(sk);
 
+	subflow->resetting = 1;
+
+	/* No need to delay the actual close for to-be discarded data. */
+	__skb_queue_purge(&ssk->sk_receive_queue);
 	tcp_send_active_reset(ssk, GFP_ATOMIC);
 	tcp_done(ssk);
 	if (!test_and_set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &mptcp_sk(sk)->flags) &&

@@ -1302,6 +1306,13 @@ static void subflow_state_change(struct sock *sk)
 
 	__subflow_state_change(sk);
 
+	/* Rx queue processing is unneeded, error reporting will take place at
+	 * __mptcp_close_ssk() time and subflow reset can't happen in case of
+	 * fallback: subflow_sched_work_if_closed() would be a no-op.
+	 */
+	if (subflow->resetting)
+		return;
+
 	if (subflow_simultaneous_connect(sk)) {
 		mptcp_do_fallback(sk);
 		mptcp_rcv_space_init(mptcp_sk(parent), sk);

2.55.0

This is a note to let you know that I've just added the patch titled

mptcp: avoid unneeded actions on subflow reset

to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mptcp-avoid-unneeded-actions-on-subflow-reset.patch
and it can be found in the queue-5.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From [email protected] Sat Sep 19 22:40:25 2026
From: "Matthieu Baerts (NGI0)" <[email protected]>
Date: Sat, 19 Sep 2026 22:40:05 +0200
Subject: mptcp: avoid unneeded actions on subflow reset
To: [email protected], [email protected], [email protected]
Cc: Paolo Abeni <[email protected]>, [email protected], Xinyang Ge <[email protected]>, "Matthieu Baerts (NGI0)" <[email protected]>, Jakub Kicinski <[email protected]>
Message-ID: <[email protected]>

From: Paolo Abeni <[email protected]>

commit 2b0f561f21b27c40c91ea4975268a06092bd7e9c upstream.

Once in a blue moon, the mptcp receive path can recursively call mptcp_data_ready() via state change under unlucky error conditions, and then try to hold the data lock again.

Break the recursion loop explicitly checking for the exceptional condition.

Add a new flag instead of using an existing one like 'closing', to exit early in subflow_state_change(), and explicitly flush the RX queue at reset time.

This avoids unneeded processing to check for available data -- calling get_mapping_status() and more on a dying subflow -- but also in error reporting and worker scheduling.

Note that we must consume the currently peeked skb before invoking mptcp_dss_corruption to avoid consuming it again after the eventual reset has freed it.

[ Note: conflict in protocol.c, because commit e0ca4057e0ec ("mptcp:
micro-optimize __mptcp_move_skb()") is not in this version, and is part of a consequent rx path refactor. The conflict is in the context, and is easy to resolve, "done = true" can be moved along without consequences. Also, the context is slightly different because there is no DEBUG_NET_WARN_ON_ONCE in this version, see the backport commit 12c1676d598e ("mptcp: handle consistently DSS corruption"). Also a conflict in protocol.h, because __unused is at a different number. Decrement the one from this version and add the new flag above. The context is also a bit different with data_avail being an enum, but that's without consequences here.
Also a conflict in subflow.c, because commit 71154bbe4942 ("mptcp:
fallback earlier on simult connection") was not needed in this version, and cause conflicts in the context, but that's without consequences here. Also, a conflict in the context, because commit 81c1d0290160 ("mptcp: consolidate fallback and non fallback state machine") was not needed in this version. ]

---
 net/mptcp/protocol.c |    6 +++---
 net/mptcp/protocol.h |    3 ++-
 net/mptcp/subflow.c  |   11 +++++++++++
 3 files changed, 16 insertions(+), 4 deletions(-)

@@ -581,11 +581,11 @@ static bool __mptcp_move_skbs_from_subfl
 			if (unlikely(map_remaining < len))
 				mptcp_dss_corruption(msk, ssk);
 		} else {
-			if (unlikely(!fin))
-				mptcp_dss_corruption(msk, ssk);
-
 			sk_eat_skb(ssk, skb);
 			done = true;
+
+			if (unlikely(!fin))
+				mptcp_dss_corruption(msk, ssk);
 		}
 
 		WRITE_ONCE(tp->copied_seq, seq);

@@ -314,7 +314,8 @@ struct mptcp_subflow_context {
 		mpc_map : 1,
 		backup : 1,
 		rx_eof : 1,
-		can_ack : 1;	    /* only after processing the remote a key */
+		can_ack : 1,	    /* only after processing the remote a key */
+		resetting : 1;	    /* subflow is resetting */
 	enum mptcp_data_avail data_avail;
 	u32	remote_nonce;
 	u64	thmac;

@@ -282,6 +282,10 @@ void mptcp_subflow_reset(struct sock *ss
 	/* must hold: tcp_done() could drop last reference on parent */
 	sock_hold(sk);
 
+	subflow->resetting = 1;
+
+	/* No need to delay the actual close for to-be discarded data. */
+	__skb_queue_purge(&ssk->sk_receive_queue);
 	tcp_send_active_reset(ssk, GFP_ATOMIC);
 	tcp_done(ssk);
 	if (!test_and_set_bit(MPTCP_WORK_CLOSE_SUBFLOW, &mptcp_sk(sk)->flags) &&

@@ -1302,6 +1306,13 @@ static void subflow_state_change(struct
 
 	__subflow_state_change(sk);
 
+	/* Rx queue processing is unneeded, error reporting will take place at
+	 * __mptcp_close_ssk() time and subflow reset can't happen in case of
+	 * fallback: subflow_sched_work_if_closed() would be a no-op.
+	 */
+	if (subflow->resetting)
+		return;
+
 	if (subflow_simultaneous_connect(sk)) {
 		mptcp_do_fallback(sk);
 		mptcp_rcv_space_init(mptcp_sk(parent), sk);

Patches currently in stable-queue which might be from [email protected] are

queue-5.10/mptcp-hold-mptcp-socket-before-calling-tcp_done.patch queue-5.10/mptcp-syncookies-remember-the-request-backup-flag.patch queue-5.10/mptcp-close-race-between-scheduler-and-state-change.patch queue-5.10/mptcp-avoid-unneeded-actions-on-subflow-reset.patch

From: Paolo Abeni <[email protected]>

commit 42064de57fb83231fcc89663a94885f228a1ee53 upstream.

The mptcp scheduler may race with subflow sockets state change: data transmission on the selected socket may fail and a later release could try to use mss_now reset to 0 for a divide operation.

Address the issue by explicitly checking for the critical scenario.

[ Note: moved the mss_now check in previous places before commit
d9ca1de8c0cd ("mptcp: move page frag allocation in mptcp_sendmsg()") which is not in this version. ]

---
 net/mptcp/protocol.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 292c21713eb7..fdebc451e23a 100644
@@ -1381,7 +1381,7 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	}
 
 	mptcp_set_timeout(sk, ssk);
-	if (copied) {
+	if (copied && mss_now) {
 		tcp_push(ssk, msg->msg_flags, mss_now, tcp_sk(ssk)->nonagle,
 			 size_goal);
 

@@ -1891,7 +1891,7 @@ static void mptcp_worker(struct work_struct *work)
 		if (!mptcp_ext_cache_refill(msk))
 			break;
 	}
-	if (copied) {
+	if (copied && mss_now) {
 		tcp_push(ssk, msg.msg_flags, mss_now, tcp_sk(ssk)->nonagle,
 			 size_goal);
 		WRITE_ONCE(msk->allow_infinite_fallback, false);

2.55.0

This is a note to let you know that I've just added the patch titled

mptcp: close race between scheduler and state change

to the 5.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mptcp-close-race-between-scheduler-and-state-change.patch
and it can be found in the queue-5.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From [email protected] Sat Sep 19 22:40:32 2026
From: "Matthieu Baerts (NGI0)" <[email protected]>
Date: Sat, 19 Sep 2026 22:40:06 +0200
Subject: mptcp: close race between scheduler and state change
To: [email protected], [email protected], [email protected]
Cc: Paolo Abeni <[email protected]>, [email protected], Shardul Bankar <[email protected]>, Xinyang Ge <[email protected]>, "Matthieu Baerts (NGI0)" <[email protected]>, Jakub Kicinski <[email protected]>
Message-ID: <[email protected]>

From: Paolo Abeni <[email protected]>

commit 42064de57fb83231fcc89663a94885f228a1ee53 upstream.

The mptcp scheduler may race with subflow sockets state change: data transmission on the selected socket may fail and a later release could try to use mss_now reset to 0 for a divide operation.

Address the issue by explicitly checking for the critical scenario.

[ Note: moved the mss_now check in previous places before commit
d9ca1de8c0cd ("mptcp: move page frag allocation in mptcp_sendmsg()") which is not in this version. ]

---
 net/mptcp/protocol.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

@@ -1381,7 +1381,7 @@ restart:
 	}
 
 	mptcp_set_timeout(sk, ssk);
-	if (copied) {
+	if (copied && mss_now) {
 		tcp_push(ssk, msg->msg_flags, mss_now, tcp_sk(ssk)->nonagle,
 			 size_goal);
 

@@ -1891,7 +1891,7 @@ static void mptcp_worker(struct work_str
 		if (!mptcp_ext_cache_refill(msk))
 			break;
 	}
-	if (copied) {
+	if (copied && mss_now) {
 		tcp_push(ssk, msg.msg_flags, mss_now, tcp_sk(ssk)->nonagle,
 			 size_goal);
 		WRITE_ONCE(msk->allow_infinite_fallback, false);

Patches currently in stable-queue which might be from [email protected] are

queue-5.10/mptcp-hold-mptcp-socket-before-calling-tcp_done.patch queue-5.10/mptcp-syncookies-remember-the-request-backup-flag.patch queue-5.10/mptcp-close-race-between-scheduler-and-state-change.patch queue-5.10/mptcp-avoid-unneeded-actions-on-subflow-reset.patch