mac80211: fix iflist_mtx/mtx locking in radar detection

The scan code creates an iflist_mtx -> mtx locking dependency,
and a few other places, notably radar detection, were creating
the opposite dependency, causing lockdep to complain. As scan
and radar detection are mutually exclusive, the deadlock can't
really happen in practice, but it's still bad form.

A similar issue exists in the monitor mode code, but this is
only used by channel-context drivers right now and those have
to have hardware scan, so that also can't happen.

Still, fix these issues by making some of the channel context
code require the mtx to be held rather than acquiring it, thus
allowing the monitor/radar callers to keep the iflist_mtx->mtx
lock ordering.

While at it, also fix access to the local->scanning variable
in the radar code, and document that radar_detect_enabled is
now properly protected by the mtx.

All this would now introduce an ABBA deadlock between the DFS
work cancelling and local->mtx, so change the locking there a
bit to not need to use cancel_delayed_work_sync() but be able
to just use cancel_delayed_work(). The work is also safely
stopped/removed when the interface is stopped, so no extra
changes are needed.

Reported-by: Kalle Valo <kvalo@qca.qualcomm.com>
Tested-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/net/mac80211/util.c b/net/mac80211/util.c
index 656648b9b9d3..ed93504d24b8 100644
--- a/net/mac80211/util.c
+++ b/net/mac80211/util.c
@@ -2315,9 +2315,14 @@ void ieee80211_dfs_cac_cancel(struct ieee80211_local *local)
 	struct ieee80211_sub_if_data *sdata;
 	struct cfg80211_chan_def chandef;
 
+	mutex_lock(&local->mtx);
 	mutex_lock(&local->iflist_mtx);
 	list_for_each_entry(sdata, &local->interfaces, list) {
-		cancel_delayed_work_sync(&sdata->dfs_cac_timer_work);
+		/* it might be waiting for the local->mtx, but then
+		 * by the time it gets it, sdata->wdev.cac_started
+		 * will no longer be true
+		 */
+		cancel_delayed_work(&sdata->dfs_cac_timer_work);
 
 		if (sdata->wdev.cac_started) {
 			chandef = sdata->vif.bss_conf.chandef;
@@ -2329,6 +2334,7 @@ void ieee80211_dfs_cac_cancel(struct ieee80211_local *local)
 		}
 	}
 	mutex_unlock(&local->iflist_mtx);
+	mutex_unlock(&local->mtx);
 }
 
 void ieee80211_dfs_radar_detected_work(struct work_struct *work)