mac80211_hwsim: fix locking when iterating radios during ns exit

The cleanup of radios during namespace exit has recently been reworked
to directly delete a radio while temporarily releasing the spinlock,
fixing a race condition between the work-queue execution and namespace
exits. However, the temporary unlock allows unsafe modifications on the
iterated list, resulting in a potential crash when continuing the
iteration of additional radios.

Move radios about to destroy to a temporary list, and clean that up
after releasing the spinlock once iteration is complete.

Fixes: 8cfd36a ("mac80211_hwsim: fix use-after-free bug in hwsim_exit_net")
Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

drivers/net/wireless/mac80211_hwsim.c

@@ -3646,6 +3646,7 @@ static __net_init int hwsim_init_net(struct net *net)
 static void __net_exit hwsim_exit_net(struct net *net)
 {
 	struct mac80211_hwsim_data *data, *tmp;
+	LIST_HEAD(list);
 
 	spin_lock_bh(&hwsim_radio_lock);
 	list_for_each_entry_safe(data, tmp, &hwsim_radios, list) {
@@ -3656,17 +3657,19 @@ static void __net_exit hwsim_exit_net(struct net *net)
 		if (data->netgroup == hwsim_net_get_netgroup(&init_net))
 			continue;
 
-		list_del(&data->list);
+		list_move(&data->list, &list);
 		rhashtable_remove_fast(&hwsim_radios_rht, &data->rht,
 				       hwsim_rht_params);
 		hwsim_radios_generation++;
-		spin_unlock_bh(&hwsim_radio_lock);
+	}
+	spin_unlock_bh(&hwsim_radio_lock);
+
+	list_for_each_entry_safe(data, tmp, &list, list) {
+		list_del(&data->list);
 		mac80211_hwsim_del_radio(data,
 					 wiphy_name(data->hw->wiphy),
 					 NULL);
-		spin_lock_bh(&hwsim_radio_lock);
 	}
-	spin_unlock_bh(&hwsim_radio_lock);
 
 	ida_simple_remove(&hwsim_netgroup_ida, hwsim_net_get_netgroup(net));
 }