mt76: fix tx status related use-after-free race on station removal

There is a small race window where ongoing tx activity can lead to a skb
getting added to the status tracking idr after that idr has already been
cleaned up, which will keep the wcid linked in the status poll list.
Fix this by only adding status skbs if the wcid pointer is still assigned
in dev->wcid, which gets cleared early by mt76_sta_pre_rcu_remove

Fixes: bd1e3e7b ("mt76: introduce packet_id idr")
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/mac80211.c

@@ -1381,7 +1381,9 @@ void mt76_sta_pre_rcu_remove(struct ieee80211_hw *hw, struct ieee80211_vif *vif,
 	struct mt76_wcid *wcid = (struct mt76_wcid *)sta->drv_priv;
 
 	mutex_lock(&dev->mutex);
+	spin_lock_bh(&dev->status_lock);
 	rcu_assign_pointer(dev->wcid[wcid->idx], NULL);
+	spin_unlock_bh(&dev->status_lock);
 	mutex_unlock(&dev->mutex);
 }
 EXPORT_SYMBOL_GPL(mt76_sta_pre_rcu_remove);

drivers/net/wireless/mediatek/mt76/tx.c

@@ -120,7 +120,7 @@ mt76_tx_status_skb_add(struct mt76_dev *dev, struct mt76_wcid *wcid,
 
 	memset(cb, 0, sizeof(*cb));
 
-	if (!wcid)
+	if (!wcid || !rcu_access_pointer(dev->wcid[wcid->idx]))
 		return MT_PACKET_ID_NO_ACK;
 
 	if (info->flags & IEEE80211_TX_CTL_NO_ACK)