This tries to address a few issues outstanding in the mentioned
patch:
- we explicitly need to reset the timeout timer for mp_fail's sake
- we need to explicitly generate a tcp ack for mp_fail, otherwise
  there are no guarantees for suck option being sent out
- the timeout timer needs handling need some caring, as it's still
  shared between mp_fail and msk socket timeout.
- we can re-use msk->first for msk->fail_ssk, as only the first/mpc
  subflow can fail without reset. That additionally avoid the need
  to clear fail_ssk on the relevant subflow close.
- fail_tout would need some additional annotation. Just to be on the
  safe side move its manipulaiton under the ssk socket lock.

Last 2 paragraph of the squash to commit should be replaced with:

"""
It leverages the fact that only the MPC/first subflow can gracefully
fail to avoid unneeded subflows traversal: the failing subflow can
be only msk->first.

A new 'fail_tout' field is added to the subflow context to record the
MP_FAIL response timeout and use such field to reliably share the
timeout timer between the MP_FAIL event and the MPTCP socket close timeout.

Finally, a new ack is generated to send out MP_FAIL notification as soon
as we hit the relevant condition, instead of waiting a possibly unbound
time for the next data packet.
"""

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
 net/mptcp/pm.c       |  4 +++-
 net/mptcp/protocol.c | 54 ++++++++++++++++++++++++++++++++++++--------
 net/mptcp/protocol.h |  4 ++--
 net/mptcp/subflow.c  | 24 ++++++++++++++++++--
 4 files changed, 72 insertions(+), 14 deletions(-)

diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
index 3c7f07bb124e..45e2a48397b9 100644
--- a/net/mptcp/pm.c
+++ b/net/mptcp/pm.c
@@ -305,13 +305,15 @@ void mptcp_pm_mp_fail_received(struct sock *sk, u64 fail_seq)
 	if (!READ_ONCE(msk->allow_infinite_fallback))
 		return;
 
-	if (!msk->fail_ssk) {
+	if (!subflow->fail_tout) {
 		pr_debug("send MP_FAIL response and infinite map");
 
 		subflow->send_mp_fail = 1;
 		subflow->send_infinite_map = 1;
+		tcp_send_ack(sk);
 	} else {
 		pr_debug("MP_FAIL response received");
+		WRITE_ONCE(subflow->fail_tout, 0);
 	}
 }
 
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index b74c6ca52aee..2910105a99b0 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -500,7 +500,7 @@ static void mptcp_set_timeout(struct sock *sk)
 	__mptcp_set_timeout(sk, tout);
 }
 
-static bool tcp_can_send_ack(const struct sock *ssk)
+static inline bool tcp_can_send_ack(const struct sock *ssk)
 {
 	return !((1 << inet_sk_state_load(ssk)) &
 	       (TCPF_SYN_SENT | TCPF_SYN_RECV | TCPF_TIME_WAIT | TCPF_CLOSE | TCPF_LISTEN));
@@ -2503,24 +2503,56 @@ static void __mptcp_retrans(struct sock *sk)
 		mptcp_reset_timer(sk);
 }
 
+/* schedule the timeout timer for the nearest relevant event: either
+ * close timeout or mp_fail timeout. Both of them could be not
+ * scheduled yet
+ */
+void mptcp_reset_timeout(struct mptcp_sock *msk, unsigned long fail_tout)
+{
+	struct sock *sk = (struct sock *)msk;
+	unsigned long timeout, close_timeout;
+
+	if (!fail_tout && !sock_flag(sk, SOCK_DEAD))
+		return;
+
+	close_timeout = inet_csk(sk)->icsk_mtup.probe_timestamp - tcp_jiffies32 + jiffies + TCP_TIMEWAIT_LEN;
+
+	/* the following is basically time_min(close_timeout, fail_tout) */
+	if (!fail_tout)
+		timeout = close_timeout;
+	else if (!sock_flag(sk, SOCK_DEAD))
+		timeout = fail_tout;
+	else if (time_after(close_timeout, fail_tout))
+		timeout = fail_tout;
+	else
+		timeout = close_timeout;
+
+	sk_reset_timer(sk, &sk->sk_timer, timeout);
+}
+
 static void mptcp_mp_fail_no_response(struct mptcp_sock *msk)
 {
-	struct sock *ssk = msk->fail_ssk;
+	struct sock *ssk = msk->first;
 	bool slow;
 
+	if (!ssk)
+		return;
+
 	pr_debug("MP_FAIL doesn't respond, reset the subflow");
 
 	slow = lock_sock_fast(ssk);
 	mptcp_subflow_reset(ssk);
+	WRITE_ONCE(mptcp_subflow_ctx(ssk)->fail_tout, 0);
 	unlock_sock_fast(ssk, slow);
 
-	msk->fail_ssk = NULL;
+	mptcp_reset_timeout(msk, 0);
 }
 
 static void mptcp_worker(struct work_struct *work)
 {
 	struct mptcp_sock *msk = container_of(work, struct mptcp_sock, work);
 	struct sock *sk = &msk->sk.icsk_inet.sk;
+	unsigned long fail_tout;
 	int state;
 
 	lock_sock(sk);
@@ -2557,7 +2589,8 @@ static void mptcp_worker(struct work_struct *work)
 	if (test_and_clear_bit(MPTCP_WORK_RTX, &msk->flags))
 		__mptcp_retrans(sk);
 
-	if (msk->fail_ssk && time_after(jiffies, msk->fail_tout))
+	fail_tout = msk->first ? READ_ONCE(mptcp_subflow_ctx(msk->first)->fail_tout) : 0;
+	if (fail_tout && time_after(jiffies, fail_tout))
 		mptcp_mp_fail_no_response(msk);
 
 unlock:
@@ -2585,8 +2618,6 @@ static int __mptcp_init_sock(struct sock *sk)
 	WRITE_ONCE(msk->csum_enabled, mptcp_is_checksum_enabled(sock_net(sk)));
 	WRITE_ONCE(msk->allow_infinite_fallback, true);
 	msk->recovery = false;
-	msk->fail_ssk = NULL;
-	msk->fail_tout = 0;
 
 	mptcp_pm_data_init(msk);
 
@@ -2807,7 +2838,9 @@ static void __mptcp_destroy_sock(struct sock *sk)
 static void mptcp_close(struct sock *sk, long timeout)
 {
 	struct mptcp_subflow_context *subflow;
+	struct mptcp_sock *msk = mptcp_sk(sk);
 	bool do_cancel_work = false;
+	unsigned long fail_tout = 0;
 
 	lock_sock(sk);
 	sk->sk_shutdown = SHUTDOWN_MASK;
@@ -2825,10 +2858,13 @@ static void mptcp_close(struct sock *sk, long timeout)
 cleanup:
 	/* orphan all the subflows */
 	inet_csk(sk)->icsk_mtup.probe_timestamp = tcp_jiffies32;
-	mptcp_for_each_subflow(mptcp_sk(sk), subflow) {
+	mptcp_for_each_subflow(msk, subflow) {
 		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
 		bool slow = lock_sock_fast_nested(ssk);
 
+		if (ssk == msk->first)
+			fail_tout = subflow->fail_tout;
+
 		sock_orphan(ssk);
 		unlock_sock_fast(ssk, slow);
 	}
@@ -2837,13 +2873,13 @@ static void mptcp_close(struct sock *sk, long timeout)
 	sock_hold(sk);
 	pr_debug("msk=%p state=%d", sk, sk->sk_state);
 	if (mptcp_sk(sk)->token)
-		mptcp_event(MPTCP_EVENT_CLOSED, mptcp_sk(sk), NULL, GFP_KERNEL);
+		mptcp_event(MPTCP_EVENT_CLOSED, msk, NULL, GFP_KERNEL);
 
 	if (sk->sk_state == TCP_CLOSE) {
 		__mptcp_destroy_sock(sk);
 		do_cancel_work = true;
 	} else {
-		sk_reset_timer(sk, &sk->sk_timer, jiffies + TCP_TIMEWAIT_LEN);
+		mptcp_reset_timeout(msk, fail_tout);
 	}
 	release_sock(sk);
 	if (do_cancel_work)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index e420a441c056..1d2d71711872 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -306,8 +306,6 @@ struct mptcp_sock {
 
 	u32 setsockopt_seq;
 	char		ca_name[TCP_CA_NAME_MAX];
-	struct sock	*fail_ssk;
-	unsigned long	fail_tout;
 };
 
 #define mptcp_data_lock(sk) spin_lock_bh(&(sk)->sk_lock.slock)
@@ -483,6 +481,7 @@ struct mptcp_subflow_context {
 	u8	stale_count;
 
 	long	delegated_status;
+	unsigned long	fail_tout;
 
 	);
 
@@ -663,6 +662,7 @@ void mptcp_get_options(const struct sk_buff *skb,
 
 void mptcp_finish_connect(struct sock *sk);
 void __mptcp_set_connected(struct sock *sk);
+void mptcp_reset_timeout(struct mptcp_sock *msk, unsigned long fail_tout);
 static inline bool mptcp_is_fully_established(struct sock *sk)
 {
 	return inet_sk_state_load(sk) == TCP_ESTABLISHED &&
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 98b12a9c4eb5..040901c1f40c 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1158,6 +1158,27 @@ static bool subflow_can_fallback(struct mptcp_subflow_context *subflow)
 		return !subflow->fully_established;
 }
 
+static void mptcp_subflow_fail(struct mptcp_sock *msk, struct sock *ssk)
+{
+	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
+	unsigned long fail_tout;
+
+	/* grecefull failure can happen only on the MPC subflow */
+	if (WARN_ON_ONCE(ssk != READ_ONCE(msk->first)))
+		return;
+
+	/* we don't need extreme accuracy here, use a zero fail_tout as special
+	 * value meaning no fail timeout at all;
+	 */
+	fail_tout = jiffies + TCP_RTO_MAX;
+	if (!fail_tout)
+		fail_tout = 1;
+	WRITE_ONCE(subflow->fail_tout, fail_tout);
+	tcp_send_ack(ssk);
+
+	mptcp_reset_timeout(msk, subflow->fail_tout);
+}
+
 static bool subflow_check_data_avail(struct sock *ssk)
 {
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
@@ -1233,8 +1254,7 @@ static bool subflow_check_data_avail(struct sock *ssk)
 				while ((skb = skb_peek(&ssk->sk_receive_queue)))
 					sk_eat_skb(ssk, skb);
 			} else {
-				msk->fail_ssk = ssk;
-				msk->fail_tout = jiffies + TCP_RTO_MAX;
+				mptcp_subflow_fail(msk, ssk);
 			}
 			WRITE_ONCE(subflow->data_avail, MPTCP_SUBFLOW_NODATA);
 			return true;
-- 
2.35.3

On Wed, 2022-06-15 at 11:16 +0200, Paolo Abeni wrote:
> This tries to address a few issues outstanding in the mentioned
> patch:
> - we explicitly need to reset the timeout timer for mp_fail's sake
> - we need to explicitly generate a tcp ack for mp_fail, otherwise
>   there are no guarantees for suck option being sent out
> - the timeout timer needs handling need some caring, as it's still
>   shared between mp_fail and msk socket timeout.
> - we can re-use msk->first for msk->fail_ssk, as only the first/mpc
>   subflow can fail without reset. That additionally avoid the need
>   to clear fail_ssk on the relevant subflow close.
> - fail_tout would need some additional annotation. Just to be on the
>   safe side move its manipulaiton under the ssk socket lock.
> 
> Last 2 paragraph of the squash to commit should be replaced with:
> 
> """
> It leverages the fact that only the MPC/first subflow can gracefully
> fail to avoid unneeded subflows traversal: the failing subflow can
> be only msk->first.
> 
> A new 'fail_tout' field is added to the subflow context to record the
> MP_FAIL response timeout and use such field to reliably share the
> timeout timer between the MP_FAIL event and the MPTCP socket close timeout.
> 
> Finally, a new ack is generated to send out MP_FAIL notification as soon
> as we hit the relevant condition, instead of waiting a possibly unbound
> time for the next data packet.
> """
> 
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
>  net/mptcp/pm.c       |  4 +++-
>  net/mptcp/protocol.c | 54 ++++++++++++++++++++++++++++++++++++--------
>  net/mptcp/protocol.h |  4 ++--
>  net/mptcp/subflow.c  | 24 ++++++++++++++++++--
>  4 files changed, 72 insertions(+), 14 deletions(-)
> 
> diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
> index 3c7f07bb124e..45e2a48397b9 100644
> --- a/net/mptcp/pm.c
> +++ b/net/mptcp/pm.c
> @@ -305,13 +305,15 @@ void mptcp_pm_mp_fail_received(struct sock *sk, u64 fail_seq)
>  	if (!READ_ONCE(msk->allow_infinite_fallback))
>  		return;
>  
> -	if (!msk->fail_ssk) {
> +	if (!subflow->fail_tout) {
>  		pr_debug("send MP_FAIL response and infinite map");
>  
>  		subflow->send_mp_fail = 1;
>  		subflow->send_infinite_map = 1;
> +		tcp_send_ack(sk);
>  	} else {
>  		pr_debug("MP_FAIL response received");
> +		WRITE_ONCE(subflow->fail_tout, 0);
>  	}
>  }
>  
> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
> index b74c6ca52aee..2910105a99b0 100644
> --- a/net/mptcp/protocol.c
> +++ b/net/mptcp/protocol.c
> @@ -500,7 +500,7 @@ static void mptcp_set_timeout(struct sock *sk)
>  	__mptcp_set_timeout(sk, tout);
>  }
>  
> -static bool tcp_can_send_ack(const struct sock *ssk)
> +static inline bool tcp_can_send_ack(const struct sock *ssk)
>  {
>  	return !((1 << inet_sk_state_load(ssk)) &
>  	       (TCPF_SYN_SENT | TCPF_SYN_RECV | TCPF_TIME_WAIT | TCPF_CLOSE | TCPF_LISTEN));
> @@ -2503,24 +2503,56 @@ static void __mptcp_retrans(struct sock *sk)
>  		mptcp_reset_timer(sk);
>  }
>  
> +/* schedule the timeout timer for the nearest relevant event: either
> + * close timeout or mp_fail timeout. Both of them could be not
> + * scheduled yet
> + */
> +void mptcp_reset_timeout(struct mptcp_sock *msk, unsigned long fail_tout)
> +{
> +	struct sock *sk = (struct sock *)msk;
> +	unsigned long timeout, close_timeout;
> +
> +	if (!fail_tout && !sock_flag(sk, SOCK_DEAD))
> +		return;
> +
> +	close_timeout = inet_csk(sk)->icsk_mtup.probe_timestamp - tcp_jiffies32 + jiffies + TCP_TIMEWAIT_LEN;
> +
> +	/* the following is basically time_min(close_timeout, fail_tout) */
> +	if (!fail_tout)
> +		timeout = close_timeout;
> +	else if (!sock_flag(sk, SOCK_DEAD))
> +		timeout = fail_tout;
> +	else if (time_after(close_timeout, fail_tout))
> +		timeout = fail_tout;
> +	else
> +		timeout = close_timeout;
> +
> +	sk_reset_timer(sk, &sk->sk_timer, timeout);
> +}
> +
>  static void mptcp_mp_fail_no_response(struct mptcp_sock *msk)
>  {
> -	struct sock *ssk = msk->fail_ssk;
> +	struct sock *ssk = msk->first;
>  	bool slow;
>  
> +	if (!ssk)
> +		return;
> +
>  	pr_debug("MP_FAIL doesn't respond, reset the subflow");
>  
>  	slow = lock_sock_fast(ssk);
>  	mptcp_subflow_reset(ssk);
> +	WRITE_ONCE(mptcp_subflow_ctx(ssk)->fail_tout, 0);
>  	unlock_sock_fast(ssk, slow);
>  
> -	msk->fail_ssk = NULL;
> +	mptcp_reset_timeout(msk, 0);
>  }
>  
>  static void mptcp_worker(struct work_struct *work)
>  {
>  	struct mptcp_sock *msk = container_of(work, struct mptcp_sock, work);
>  	struct sock *sk = &msk->sk.icsk_inet.sk;
> +	unsigned long fail_tout;
>  	int state;
>  
>  	lock_sock(sk);
> @@ -2557,7 +2589,8 @@ static void mptcp_worker(struct work_struct *work)
>  	if (test_and_clear_bit(MPTCP_WORK_RTX, &msk->flags))
>  		__mptcp_retrans(sk);
>  
> -	if (msk->fail_ssk && time_after(jiffies, msk->fail_tout))
> +	fail_tout = msk->first ? READ_ONCE(mptcp_subflow_ctx(msk->first)->fail_tout) : 0;

For the records, the above apparently trigger an UaF in the CI debug
build on pktdrill tests.

AFAICS that is cause by the listener mptcp socket being destroyed while
the MPC is still in accept queue: the MPC socket is freed, but msk-
>first is not updated accordingly.

Note that the above issue is not related to this patch-series, but it
makes it much more easy to reproduce.

/P