nl80211: use __cfg80211_rdev_from_attrs for testmode

To fix the testmode cross-namespace access problem,
use __cfg80211_rdev_from_attrs() to get the device
instead of open-coding similar functionality.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -5230,21 +5230,18 @@ static int nl80211_testmode_dump(struct sk_buff *skb,
 				  nl80211_policy);
 		if (err)
 			return err;
-		if (nl80211_fam.attrbuf[NL80211_ATTR_WIPHY]) {
-			phy_idx = nla_get_u32(
-				nl80211_fam.attrbuf[NL80211_ATTR_WIPHY]);
-		} else {
-			struct net_device *netdev;
 
-			err = get_rdev_dev_by_ifindex(sock_net(skb->sk),
-						      nl80211_fam.attrbuf,
-						      &rdev, &netdev);
-			if (err)
-				return err;
-			dev_put(netdev);
-			phy_idx = rdev->wiphy_idx;
-			cfg80211_unlock_rdev(rdev);
+		mutex_lock(&cfg80211_mutex);
+		rdev = __cfg80211_rdev_from_attrs(sock_net(skb->sk),
+						  nl80211_fam.attrbuf);
+		if (IS_ERR(rdev)) {
+			mutex_unlock(&cfg80211_mutex);
+			return PTR_ERR(rdev);
 		}
+		phy_idx = rdev->wiphy_idx;
+		rdev = NULL;
+		mutex_unlock(&cfg80211_mutex);
+
 		if (nl80211_fam.attrbuf[NL80211_ATTR_TESTDATA])
 			cb->args[1] =
 				(long)nl80211_fam.attrbuf[NL80211_ATTR_TESTDATA];