[PATCH net 05/10] mptcp: pm: fix extra_subflows underflow on userspace PM subflow creation

[Matthieu Baerts (NGI0)]

From: Tao Cui <cuitao@xxxxxxxxxx>

The userspace PM increments extra_subflows after __mptcp_subflow_connect()
succeeds, but __mptcp_subflow_connect() calls mptcp_pm_close_subflow()
on failure to roll back the pre-increment done by the kernel PM's fill_*()
helpers. Because the userspace PM hasn't incremented yet at that point,
this decrement is spurious and causes extra_subflows to underflow.

Fix it by aligning the userspace PM with the kernel PM: increment
extra_subflows before calling __mptcp_subflow_connect(), so the existing
error path in subflow.c correctly rolls it back on failure. Also simplify
the error handling by taking pm.lock only when needed for cleanup.

Fixes: 77e4b94a3de6 ("mptcp: update userspace pm infos")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
---
 net/mptcp/pm_userspace.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/net/mptcp/pm_userspace.c b/net/mptcp/pm_userspace.c
index 8cbc1920afb4..0d3a95e676f1 100644
--- a/net/mptcp/pm_userspace.c
+++ b/net/mptcp/pm_userspace.c
@@ -408,19 +408,21 @@ int mptcp_pm_nl_subflow_create_doit(struct sk_buff *skb, struct genl_info *info)
 	local.flags = entry.flags;
 	local.ifindex = entry.ifindex;
 
+	spin_lock_bh(&msk->pm.lock);
+	msk->pm.extra_subflows++;
+	spin_unlock_bh(&msk->pm.lock);
+
 	lock_sock(sk);
 	err = __mptcp_subflow_connect(sk, &local, &addr_r);
 	release_sock(sk);
 
-	if (err)
+	if (err) {
 		GENL_SET_ERR_MSG_FMT(info, "connect error: %d", err);
 
-	spin_lock_bh(&msk->pm.lock);
-	if (err)
+		spin_lock_bh(&msk->pm.lock);
 		mptcp_userspace_pm_delete_local_addr(msk, &entry);
-	else
-		msk->pm.extra_subflows++;
-	spin_unlock_bh(&msk->pm.lock);
+		spin_unlock_bh(&msk->pm.lock);
+	}
 
  create_err:
 	sock_put(sk);

-- 
2.53.0