wifi: mac80211: fix BA session teardown race

As previously reported by Alexander, whose commit 69403bad97aa
("wifi: mac80211: sdata can be NULL during AMPDU start") I'm
reverting as part of this commit, there's a race between station
destruction and aggregation setup, where the aggregation setup
can happen while the station is being removed and queue the work
after ieee80211_sta_tear_down_BA_sessions() has already run in
__sta_info_destroy_part1(), and thus the worker will run with a
now freed station. In his case, this manifested in a NULL sdata
pointer, but really there's no guarantee whatsoever.

The real issue seems to be that it's possible at all to have a
situation where this occurs - we want to stop the BA sessions
when doing _part1, but we cannot be sure, and WLAN_STA_BLOCK_BA
isn't necessarily effective since we don't know that the setup
isn't concurrently running and already got past the check.

Simply call ieee80211_sta_tear_down_BA_sessions() again in the
second part of station destruction, since at that point really
nothing else can hold a reference to the station any more.

Also revert the sdata checks since those are just misleading at
this point.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index abcc280acd38..2a61269a4b54 100644
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -1385,6 +1385,20 @@ static void __sta_info_destroy_part2(struct sta_info *sta, bool recalc)
 	 *	 after _part1 and before _part2!
 	 */
 
+	/*
+	 * There's a potential race in _part1 where we set WLAN_STA_BLOCK_BA
+	 * but someone might have just gotten past a check, and not yet into
+	 * queuing the work/creating the data/etc.
+	 *
+	 * Do another round of destruction so that the worker is certainly
+	 * canceled before we later free the station.
+	 *
+	 * Since this is after synchronize_rcu()/synchronize_net() we're now
+	 * certain that nobody can actually hold a reference to the STA and
+	 * be calling e.g. ieee80211_start_tx_ba_session().
+	 */
+	ieee80211_sta_tear_down_BA_sessions(sta, AGG_STOP_DESTROY_STA);
+
 	might_sleep();
 	lockdep_assert_wiphy(local->hw.wiphy);