wifi: mac80211: Support POWERED_ADDR_CHANGE feature

Adds support in mac80211 for NL80211_EXT_FEATURE_POWERED_ADDR_CHANGE.
The motivation behind this functionality is to fix limitations of
address randomization on frequencies which are disallowed in world
roaming.

The way things work now, if a client wants to randomize their address
per-connection it must power down the device, change the MAC, and
power back up. Here lies a problem since powering down the device
may result in frequencies being disabled (until the regdom is set).
If the desired BSS is on one such frequency the client is unable to
connect once the phy is powered again.

For mac80211 based devices changing the MAC while powered is possible
but currently disallowed (-EBUSY). This patch adds some logic to
allow a MAC change while powered by removing the interface, changing
the MAC, and adding it again. mac80211 will advertise support for
this feature so userspace can determine the best course of action e.g.
disallow address randomization on certain frequencies if not
supported.

There are certain limitations put on this which simplify the logic:
 - No active connection
 - No offchannel work, including scanning.
Signed-off-by: James Prestwood <prestwoj@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/iface.c

@@ -200,15 +200,73 @@ static int ieee80211_verify_mac(struct ieee80211_sub_if_data *sdata, u8 *addr,
 	return ret;
 }
 
+static int ieee80211_can_powered_addr_change(struct ieee80211_sub_if_data *sdata)
+{
+	struct ieee80211_roc_work *roc;
+	struct ieee80211_local *local = sdata->local;
+	struct ieee80211_sub_if_data *scan_sdata;
+	int ret = 0;
+
+	/* To be the most flexible here we want to only limit changing the
+	 * address if the specific interface is doing offchannel work or
+	 * scanning.
+	 */
+	if (netif_carrier_ok(sdata->dev))
+		return -EBUSY;
+
+	mutex_lock(&local->mtx);
+
+	/* First check no ROC work is happening on this iface */
+	list_for_each_entry(roc, &local->roc_list, list) {
+		if (roc->sdata != sdata)
+			continue;
+
+		if (roc->started) {
+			ret = -EBUSY;
+			goto unlock;
+		}
+	}
+
+	/* And if this iface is scanning */
+	if (local->scanning) {
+		scan_sdata = rcu_dereference_protected(local->scan_sdata,
+						       lockdep_is_held(&local->mtx));
+		if (sdata == scan_sdata)
+			ret = -EBUSY;
+	}
+
+	switch (sdata->vif.type) {
+	case NL80211_IFTYPE_STATION:
+	case NL80211_IFTYPE_P2P_CLIENT:
+		/* More interface types could be added here but changing the
+		 * address while powered makes the most sense in client modes.
+		 */
+		break;
+	default:
+		return -EOPNOTSUPP;
+	}
+
+unlock:
+	mutex_unlock(&local->mtx);
+	return ret;
+}
+
 static int ieee80211_change_mac(struct net_device *dev, void *addr)
 {
 	struct ieee80211_sub_if_data *sdata = IEEE80211_DEV_TO_SUB_IF(dev);
+	struct ieee80211_local *local = sdata->local;
 	struct sockaddr *sa = addr;
 	bool check_dup = true;
+	bool live = false;
 	int ret;
 
-	if (ieee80211_sdata_running(sdata))
-		return -EBUSY;
+	if (ieee80211_sdata_running(sdata)) {
+		ret = ieee80211_can_powered_addr_change(sdata);
+		if (ret)
+			return ret;
+
+		live = true;
+	}
 
 	if (sdata->vif.type == NL80211_IFTYPE_MONITOR &&
 	    !(sdata->u.mntr.flags & MONITOR_FLAG_ACTIVE))
@@ -218,6 +276,8 @@ static int ieee80211_change_mac(struct net_device *dev, void *addr)
 	if (ret)
 		return ret;
 
+	if (live)
+		drv_remove_interface(local, sdata);
 	ret = eth_mac_addr(dev, sa);
 
 	if (ret == 0) {
@@ -225,6 +285,12 @@ static int ieee80211_change_mac(struct net_device *dev, void *addr)
 		ether_addr_copy(sdata->vif.bss_conf.addr, sdata->vif.addr);
 	}
 
+	/* Regardless of eth_mac_addr() return we still want to add the
+	 * interface back. This should not fail...
+	 */
+	if (live)
+		WARN_ON(drv_add_interface(local, sdata));
+
 	return ret;
 }
 
@@ -2412,6 +2478,7 @@ int ieee80211_if_add(struct ieee80211_local *local, const char *name,
 			sdata->u.mgd.use_4addr = params->use_4addr;
 
 		ndev->features |= local->hw.netdev_features;
+		ndev->priv_flags |= IFF_LIVE_ADDR_CHANGE;
 		ndev->hw_features |= ndev->features &
 					MAC80211_SUPPORTED_FEATURES_TX;
 

net/mac80211/main.c

@@ -699,6 +699,8 @@ struct ieee80211_hw *ieee80211_alloc_hw_nm(size_t priv_data_len,
 			      NL80211_EXT_FEATURE_CONTROL_PORT_OVER_NL80211_TX_STATUS);
 	wiphy_ext_feature_set(wiphy,
 			      NL80211_EXT_FEATURE_SCAN_FREQ_KHZ);
+	wiphy_ext_feature_set(wiphy,
+			      NL80211_EXT_FEATURE_POWERED_ADDR_CHANGE);
 
 	if (!ops->hw_scan) {
 		wiphy->features |= NL80211_FEATURE_LOW_PRIORITY_SCAN |