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>

1 files changed, 27 insertions, 11 deletions

diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index b31a627ff97f..c6c0df4bbd2c 100644
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -55,19 +55,29 @@ static int sta_info_hash_del(struct ieee80211_local *local,
         return -ENOENT;
 }
 
-struct sta_info *sta_info_get(struct ieee80211_local *local, u8 *addr)
+/* must hold local->sta_lock */
+static struct sta_info *__sta_info_find(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_info_get(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 = __sta_info_find(local, addr);
+        if (sta)
+                __sta_info_get(sta);
         read_unlock_bh(&local->sta_lock);
 
         return sta;
@@ -110,8 +120,8 @@ void sta_info_put(struct sta_info *sta)
 EXPORT_SYMBOL(sta_info_put);
 
 
-struct sta_info * sta_info_add(struct ieee80211_local *local,
-                               struct net_device *dev, u8 *addr, gfp_t gfp)
+struct sta_info *sta_info_add(struct ieee80211_local *local,
+                              struct net_device *dev, u8 *addr, gfp_t gfp)
 {
         struct sta_info *sta;
         int i;
@@ -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);