See http://www.mathyvanhoef.com/2013/11/unmasking-spoofed-mac-address.html

ath9khtc.patch

diff --git a/drivers/net/wireless/ath/ath9k/htc_drv_main.c b/drivers/net/wireless/ath/ath9k/htc_drv_main.c
index d441045..84359c3 100644
--- a/drivers/net/wireless/ath/ath9k/htc_drv_main.c
+++ b/drivers/net/wireless/ath/ath9k/htc_drv_main.c
@@ -147,21 +147,26 @@ static void ath9k_htc_bssid_iter(void *data, u8 *mac, struct ieee80211_vif *vif)
 	struct ath9k_vif_iter_data *iter_data = data;
 	int i;
 
-	for (i = 0; i < ETH_ALEN; i++)
-		iter_data->mask[i] &= ~(iter_data->hw_macaddr[i] ^ mac[i]);
+	if (iter_data->hw_macaddr != NULL) {
+		for (i = 0; i < ETH_ALEN; i++)
+			iter_data->mask[i] &= ~(iter_data->hw_macaddr[i] ^ mac[i]);
+	} else {
+		iter_data->hw_macaddr = mac;
+	}
 }
 
-static void ath9k_htc_set_bssid_mask(struct ath9k_htc_priv *priv,
+static void ath9k_htc_set_mac_bssid_mask(struct ath9k_htc_priv *priv,
 				     struct ieee80211_vif *vif)
 {
 	struct ath_common *common = ath9k_hw_common(priv->ah);
 	struct ath9k_vif_iter_data iter_data;
 
 	/*
-	 * Use the hardware MAC address as reference, the hardware uses it
-	 * together with the BSSID mask when matching addresses.
+	 * Pick the MAC address of the first interface as the new hardware
+	 * MAC address. The hardware will use it together with the BSSID mask
+	 * when matching addresses.
 	 */
-	iter_data.hw_macaddr = common->macaddr;
+	iter_data.hw_macaddr = NULL;
 	memset(&iter_data.mask, 0xff, ETH_ALEN);
 
 	if (vif)
@@ -173,6 +178,10 @@ static void ath9k_htc_set_bssid_mask(struct ath9k_htc_priv *priv,
 		ath9k_htc_bssid_iter, &iter_data);
 
 	memcpy(common->bssidmask, iter_data.mask, ETH_ALEN);
+
+	if (iter_data.hw_macaddr)
+		memcpy(common->macaddr, iter_data.hw_macaddr, ETH_ALEN);
+
 	ath_hw_setbssidmask(common);
 }
 
@@ -1083,7 +1092,7 @@ static int ath9k_htc_add_interface(struct ieee80211_hw *hw,
 		goto out;
 	}
 
-	ath9k_htc_set_bssid_mask(priv, vif);
+	ath9k_htc_set_mac_bssid_mask(priv, vif);
 
 	priv->vif_slot |= (1 << avp->index);
 	priv->nvifs++;
@@ -1148,7 +1157,7 @@ static void ath9k_htc_remove_interface(struct ieee80211_hw *hw,
 
 	ath9k_htc_set_opmode(priv);
 
-	ath9k_htc_set_bssid_mask(priv, vif);
+	ath9k_htc_set_mac_bssid_mask(priv, vif);
 
 	/*
 	 * Stop ANI only if there are no associated station interfaces.
diff --git a/drivers/net/wireless/ath/ath9k/main.c b/drivers/net/wireless/ath/ath9k/main.c
index b6aad69..99ab0aa 100644
--- a/drivers/net/wireless/ath/ath9k/main.c
+++ b/drivers/net/wireless/ath/ath9k/main.c
@@ -885,8 +885,9 @@ void ath9k_calculate_iter_data(struct ieee80211_hw *hw,
 	struct ath_common *common = ath9k_hw_common(ah);
 
 	/*
-	 * Use the hardware MAC address as reference, the hardware uses it
-	 * together with the BSSID mask when matching addresses.
+	 * Pick the MAC address of the first interface as the new hardware
+	 * MAC address. The hardware will use it together with the BSSID mask
+	 * when matching addresses.
 	 */
 	memset(iter_data, 0, sizeof(*iter_data));
 	memset(&iter_data->mask, 0xff, ETH_ALEN);