mac80211: atomically check whether STA exists already

When a STA structure is added, it is often checked whether it
already exists before adding it. This, however, isn't done
atomically so there is a race condition that could lead to two
STA structures being added with the same MAC address. This
patch changes sta_info_add() to return an ERR_PTR in case
of failure and adds the failure mode -EEXIST when the STA
already exists.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/cfg.c

@@ -562,13 +562,6 @@ static int ieee80211_add_station(struct wiphy *wiphy, struct net_device *dev,
 	if (!netif_running(dev))
 		return -ENETDOWN;
 
-	/* XXX: get sta belonging to dev */
-	sta = sta_info_get(local, mac);
-	if (sta) {
-		sta_info_put(sta);
-		return -EEXIST;
-	}
-
 	if (params->vlan) {
 		sdata = IEEE80211_DEV_TO_SUB_IF(params->vlan);
 
@@ -579,8 +572,8 @@ static int ieee80211_add_station(struct wiphy *wiphy, struct net_device *dev,
 		sdata = IEEE80211_DEV_TO_SUB_IF(dev);
 
 	sta = sta_info_add(local, dev, mac, GFP_KERNEL);
-	if (!sta)
-		return -ENOMEM;
+	if (IS_ERR(sta))
+		return PTR_ERR(sta);
 
 	sta->dev = sdata->dev;
 	if (sdata->vif.type == IEEE80211_IF_TYPE_VLAN ||

net/mac80211/ieee80211.c

@@ -838,8 +838,8 @@ int ieee80211_if_update_wds(struct net_device *dev, u8 *remote_addr)
 
 	/* Create STA entry for the new peer */
 	sta = sta_info_add(local, dev, remote_addr, GFP_KERNEL);
-	if (!sta)
-		return -ENOMEM;
+	if (IS_ERR(sta))
+		return PTR_ERR(sta);
 
 	sta->flags |= WLAN_STA_AUTHORIZED;
 

net/mac80211/ieee80211_sta.c

@@ -1807,9 +1807,9 @@ static void ieee80211_rx_mgmt_assoc_resp(struct ieee80211_sub_if_data *sdata,
 	if (!sta) {
 		struct ieee80211_sta_bss *bss;
 		sta = sta_info_add(local, dev, ifsta->bssid, GFP_KERNEL);
-		if (!sta) {
+		if (IS_ERR(sta)) {
 			printk(KERN_DEBUG "%s: failed to add STA entry for the"
-			       " AP\n", dev->name);
+			       " AP (error %ld)\n", dev->name, PTR_ERR(sta));
 			return;
 		}
 		bss = ieee80211_rx_bss_get(dev, ifsta->bssid,
@@ -3820,7 +3820,7 @@ struct sta_info * ieee80211_ibss_add_sta(struct net_device *dev,
 	       wiphy_name(local->hw.wiphy), print_mac(mac, addr), dev->name);
 
 	sta = sta_info_add(local, dev, addr, GFP_ATOMIC);
-	if (!sta)
+	if (IS_ERR(sta))
 		return NULL;
 
 	sta->flags |= WLAN_STA_AUTHORIZED;

net/mac80211/sta_info.c

@@ -55,19 +55,29 @@ static int sta_info_hash_del(struct ieee80211_local *local,
 	return -ENOENT;
 }
 
+/* must hold local->sta_lock */
+static struct sta_info *__sta_info_find(struct ieee80211_local *local,
+					u8 *addr)
+{
+	struct sta_info *sta;
+
+	sta = local->sta_hash[STA_HASH(addr)];
+	while (sta) {
+		if (compare_ether_addr(sta->addr, addr) == 0)
+			break;
+		sta = sta->hnext;
+	}
+	return sta;
+}
+
 struct sta_info *sta_info_get(struct ieee80211_local *local, u8 *addr)
 {
 	struct sta_info *sta;
 
 	read_lock_bh(&local->sta_lock);
-	sta = local->sta_hash[STA_HASH(addr)];
-	while (sta) {
-		if (memcmp(sta->addr, addr, ETH_ALEN) == 0) {
+	sta = __sta_info_find(local, addr);
+	if (sta)
 		__sta_info_get(sta);
-			break;
-		}
-		sta = sta->hnext;
-	}
 	read_unlock_bh(&local->sta_lock);
 
 	return sta;
@@ -119,7 +129,7 @@ struct sta_info * sta_info_add(struct ieee80211_local *local,
 
 	sta = kzalloc(sizeof(*sta), gfp);
 	if (!sta)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	kref_init(&sta->kref);
 
@@ -128,7 +138,7 @@ struct sta_info * sta_info_add(struct ieee80211_local *local,
 	if (!sta->rate_ctrl_priv) {
 		rate_control_put(sta->rate_ctrl);
 		kfree(sta);
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 	}
 
 	memcpy(sta->addr, addr, ETH_ALEN);
@@ -158,9 +168,15 @@ struct sta_info * sta_info_add(struct ieee80211_local *local,
 	}
 	skb_queue_head_init(&sta->ps_tx_buf);
 	skb_queue_head_init(&sta->tx_filtered);
-	__sta_info_get(sta);	/* sta used by caller, decremented by
-				 * sta_info_put() */
 	write_lock_bh(&local->sta_lock);
+	/* mark sta as used (by caller) */
+	__sta_info_get(sta);
+	/* check if STA exists already */
+	if (__sta_info_find(local, addr)) {
+		write_unlock_bh(&local->sta_lock);
+		sta_info_put(sta);
+		return ERR_PTR(-EEXIST);
+	}
 	list_add(&sta->list, &local->sta_list);
 	local->num_sta++;
 	sta_info_hash_add(local, sta);