rt2x00: Fix for race condition while update beacon

The patch "Implement set_tim callback for all drivers" can cause kernel
oops in rt73usb_write_beacon. The oops is caused by one of the following
race conditions:
* In case of two near calls to set_tim: rt2x00lib_beacondone_iter is
cleaning the beacon skb, whereas rt73usb_write_beacon is still using it.
* In case of two near updates of beacon: first as the result of set_tim
and second as the result of a call from an application (e.g. hostapd).
This patch fixes the race condition by rearranging the update logic and
guarding rt2x00_intf->beacon->skb with a mutex.

Signed-off-by: Igor Perminov <igor.perminov@inbox.ru>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2x00.h

@@ -333,6 +333,11 @@ struct rt2x00_intf {
 	 */
 	u8 bssid[ETH_ALEN];
 
+	/*
+	 * beacon->skb must be protected with the mutex.
+	 */
+	struct mutex beacon_skb_mutex;
+
 	/*
 	 * Entry in the beacon queue which belongs to
 	 * this interface. Each interface has its own

drivers/net/wireless/rt2x00/rt2x00dev.c

@@ -186,7 +186,6 @@ static void rt2x00lib_intf_scheduled(struct work_struct *work)
 static void rt2x00lib_beacondone_iter(void *data, u8 *mac,
 				      struct ieee80211_vif *vif)
 {
-	struct rt2x00_dev *rt2x00dev = data;
 	struct rt2x00_intf *intf = vif_to_intf(vif);
 
 	if (vif->type != NL80211_IFTYPE_AP &&
@@ -195,12 +194,6 @@ static void rt2x00lib_beacondone_iter(void *data, u8 *mac,
 	    vif->type != NL80211_IFTYPE_WDS)
 		return;
 
-	/*
-	 * Clean up the beacon skb.
-	 */
-	rt2x00queue_free_skb(rt2x00dev, intf->beacon->skb);
-	intf->beacon->skb = NULL;
-
 	spin_lock(&intf->lock);
 	intf->delayed_flags |= DELAYED_UPDATE_BEACON;
 	spin_unlock(&intf->lock);

drivers/net/wireless/rt2x00/rt2x00mac.c

@@ -274,6 +274,7 @@ int rt2x00mac_add_interface(struct ieee80211_hw *hw,
 
 	spin_lock_init(&intf->lock);
 	spin_lock_init(&intf->seqlock);
+	mutex_init(&intf->beacon_skb_mutex);
 	intf->beacon = entry;
 
 	if (conf->type == NL80211_IFTYPE_AP)

drivers/net/wireless/rt2x00/rt2x00queue.c

@@ -503,14 +503,25 @@ int rt2x00queue_update_beacon(struct rt2x00_dev *rt2x00dev,
 	if (unlikely(!intf->beacon))
 		return -ENOBUFS;
 
+	mutex_lock(&intf->beacon_skb_mutex);
+
+	/*
+	 * Clean up the beacon skb.
+	 */
+	rt2x00queue_free_skb(rt2x00dev, intf->beacon->skb);
+	intf->beacon->skb = NULL;
+
 	if (!enable_beacon) {
 		rt2x00dev->ops->lib->kill_tx_queue(rt2x00dev, QID_BEACON);
+		mutex_unlock(&intf->beacon_skb_mutex);
 		return 0;
 	}
 
 	intf->beacon->skb = ieee80211_beacon_get(rt2x00dev->hw, vif);
-	if (!intf->beacon->skb)
+	if (!intf->beacon->skb) {
+		mutex_unlock(&intf->beacon_skb_mutex);
 		return -ENOMEM;
+	}
 
 	/*
 	 * Copy all TX descriptor information into txdesc,
@@ -548,6 +559,8 @@ int rt2x00queue_update_beacon(struct rt2x00_dev *rt2x00dev,
 	rt2x00dev->ops->lib->write_beacon(intf->beacon);
 	rt2x00dev->ops->lib->kick_tx_queue(rt2x00dev, QID_BEACON);
 
+	mutex_unlock(&intf->beacon_skb_mutex);
+
 	return 0;
 }