[PATCH net 2/5] mptcp: pm: fix ADD_ADDR timer infinite retry on option space insufficient

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

From: Li Xiasong <lixiasong1@huawei.com>

When TCP option space is insufficient (e.g., when sending ADD_ADDR with an
IPv6 address and port while tcp_timestamps is enabled), the original code
jumped to out_unlock without clearing the addr_signal flag. This caused
mptcp_pm_add_timer to keep rescheduling indefinitely, not sending ADD_ADDR,
preventing subsequent addresses in the endpoint list from being announced.

Handle this case by clearing the ADD_ADDR signal and skipping the matching
ADD_ADDR retransmission entry. The skip path cancels the matching timer
(with id check) and advances PM state progression, preserving forward
progress to subsequent PM work.

This cancellation is inherently best-effort. A concurrent add_timer
callback may already be running and may acquire pm.lock before the
cancel path updates entry state. In that case, one final ADD_ADDR
transmit attempt can still be executed.

Once the cancel path sets entry->retrans_times to
ADD_ADDR_RETRANS_MAX, the callback-side retrans_times check suppresses
further ADD_ADDR retransmissions.

Fixes: 00cfd77b9063 ("mptcp: retransmit ADD_ADDR when timeout")
Cc: stable@vger.kernel.org
Signed-off-by: Li Xiasong <lixiasong1@huawei.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/pm.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 46 insertions(+), 10 deletions(-)

diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
index 3c152bf66cd5..3e770c7407e1 100644
--- a/net/mptcp/pm.c
+++ b/net/mptcp/pm.c
@@ -364,7 +364,13 @@ static void mptcp_pm_add_timer(struct timer_list *timer)
 
 	spin_lock_bh(&msk->pm.lock);
 
-	if (!mptcp_pm_should_add_signal_addr(msk)) {
+	/* The cancel path (mptcp_pm_del_add_timer()) can race with this
+	 * callback. Once cancel updates retrans_times to MAX, suppress further
+	 * retransmissions here. If this callback acquires pm.lock first, one
+	 * final transmit attempt is still possible.
+	 */
+	if (entry->retrans_times < ADD_ADDR_RETRANS_MAX &&
+	    !mptcp_pm_should_add_signal_addr(msk)) {
 		pr_debug("retransmit ADD_ADDR id=%d\n", entry->addr.id);
 		mptcp_pm_announce_addr(msk, &entry->addr, false);
 		mptcp_pm_add_addr_send_ack(msk);
@@ -414,8 +420,12 @@ mptcp_pm_del_add_timer(struct mptcp_sock *msk,
 	/* Note: entry might have been removed by another thread.
 	 * We hold rcu_read_lock() to ensure it is not freed under us.
 	 */
-	if (stop_timer)
-		sk_stop_timer_sync(sk, &entry->add_timer);
+	if (stop_timer) {
+		if (check_id)
+			sk_stop_timer(sk, &entry->add_timer);
+		else
+			sk_stop_timer_sync(sk, &entry->add_timer);
+	}
 
 	rcu_read_unlock();
 	return entry;
@@ -882,6 +892,7 @@ bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, const struct sk_buff *skb,
 			      struct mptcp_addr_info *addr, bool *echo,
 			      bool *drop_other_suboptions)
 {
+	bool skip_add_addr = false;
 	int ret = false;
 	u8 add_addr;
 	u8 family;
@@ -903,24 +914,49 @@ bool mptcp_pm_add_addr_signal(struct mptcp_sock *msk, const struct sk_buff *skb,
 	}
 
 	*echo = mptcp_pm_should_add_signal_echo(msk);
-	port = !!(*echo ? msk->pm.remote.port : msk->pm.local.port);
-
-	family = *echo ? msk->pm.remote.family : msk->pm.local.family;
-	if (remaining < mptcp_add_addr_len(family, *echo, port))
-		goto out_unlock;
-
 	if (*echo) {
 		*addr = msk->pm.remote;
 		add_addr = msk->pm.addr_signal & ~BIT(MPTCP_ADD_ADDR_ECHO);
+		port = !!msk->pm.remote.port;
+		family = msk->pm.remote.family;
 	} else {
 		*addr = msk->pm.local;
 		add_addr = msk->pm.addr_signal & ~BIT(MPTCP_ADD_ADDR_SIGNAL);
+		port = !!msk->pm.local.port;
+		family = msk->pm.local.family;
 	}
-	WRITE_ONCE(msk->pm.addr_signal, add_addr);
+
+	if (remaining < mptcp_add_addr_len(family, *echo, port)) {
+		struct net *net = sock_net((struct sock *)msk);
+
+		if (!*drop_other_suboptions)
+			goto out_unlock;
+
+		if (*echo) {
+			MPTCP_INC_STATS(net, MPTCP_MIB_ECHOADDTXDROP);
+		} else {
+			skip_add_addr = true;
+			MPTCP_INC_STATS(net, MPTCP_MIB_ADDADDRTXDROP);
+		}
+		goto drop_signal_mark;
+	}
+
 	ret = true;
 
+drop_signal_mark:
+	WRITE_ONCE(msk->pm.addr_signal, add_addr);
+
 out_unlock:
 	spin_unlock_bh(&msk->pm.lock);
+
+	/* On pure-ACK option-space exhaustion, stop retrying this ADD_ADDR:
+	 * clear the signal bit, cancel the matching retransmission timer, and
+	 * let the PM state machine progress.
+	 */
+	if (skip_add_addr) {
+		mptcp_pm_del_add_timer(msk, addr, true);
+		mptcp_pm_subflow_established(msk);
+	}
 	return ret;
 }
 

-- 
2.53.0