[PATCH] wifi: cfg80211: avoid holding rtnl_mutex across all cfg80211_leave() calls

reg_check_chans_work() holds rtnl_mutex for the entire duration of
iterating over all registered devices and calling cfg80211_leave() on
each invalid wdev. cfg80211_leave() can be slow (disconnect, stop AP,
leave mesh), causing rtnl_mutex starvation when many wireless interfaces
are present. This results in tasks waiting for rtnl_mutex for longer
than hung_task_timeout_secs:

  INFO: task hung in inet_rtm_newaddr
  INFO: task hung in inet6_rtm_newaddr
  INFO: task hung in nsim_destroy

Fix by walking cfg80211_rdev_list under RCU and acquiring rtnl per-device,
so other rtnl waiters get a chance to run between devices.

I could not add the Fixes: tag because this patch addresses three separate
hung task reports whose cause bisections all failed, making it impossible
to identify a single introducing commit.

Reported-by: syzbot+adeb8550754921fece20@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=adeb8550754921fece20
Reported-by: syzbot+101224300649c3eb8af4@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=101224300649c3eb8af4
Reported-by: syzbot+8141dcbd23a8f857798a@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=8141dcbd23a8f857798a
Signed-off-by: Ömer Mete Kaya <redacted>
---
 net/wireless/reg.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/net/wireless/reg.c b/net/wireless/reg.c
index a8336baf85dc..a2e0d1cf8317 100644
--- a/net/wireless/reg.c
+++ b/net/wireless/reg.c

@@ -2466,12 +2466,21 @@ static void reg_check_chans_work(struct work_struct *work)
 	struct cfg80211_registered_device *rdev;
 
 	pr_debug("Verifying active interfaces after reg change\n");
-	rtnl_lock();
-
-	for_each_rdev(rdev)
+	/*
+	 * Acquire rtnl per-device instead of holding it for the entire loop;
+	 * cfg80211_leave() can be slow and starve other rtnl waiters otherwise.
+	 * wiphy_unregister() holds rtnl across list_del_rcu() + synchronize_rcu(),
+	 * so rdev cannot be freed while we hold rtnl_lock() below.
+	 */
+	rcu_read_lock();
+	list_for_each_entry_rcu(rdev, &cfg80211_rdev_list, list) {
+		rcu_read_unlock();
+		rtnl_lock();
 		reg_leave_invalid_chans(&rdev->wiphy);
-
-	rtnl_unlock();
+		rtnl_unlock();
+		rcu_read_lock();
+	}
+	rcu_read_unlock();
 }
 
 void reg_check_channels(void)

-- 
2.55.0