cfg80211: fix netdev registration deadlock

If register_netdevice() fails after having called cfg80211's
netdev notifier (cfg80211_netdev_notifier_call) it will call
the notifier again with UNREGISTER. This would then lock the
wiphy mutex because we're marked as registered, which causes
a deadlock.

Fix this by separately keeping track of whether or not we're
in the middle of registering to also skip the notifier call
on this unregister.

Reported-by: syzbot+2ae0ca9d7737ad1a62b7@syzkaller.appspotmail.com
Fixes: a05829a ("cfg80211: avoid holding the RTNL when calling the driver")
Link: https://lore.kernel.org/r/20210201192048.ed8bad436737.I7cae042c44b15f80919a285799a15df467e9d42d@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -5299,6 +5299,8 @@ static inline void wiphy_unlock(struct wiphy *wiphy)
  * @wiphy: pointer to hardware description
  * @iftype: interface type
  * @registered: is this wdev already registered with cfg80211
+ * @registering: indicates we're doing registration under wiphy lock
+ *	for the notifier
  * @list: (private) Used to collect the interfaces
  * @netdev: (private) Used to reference back to the netdev, may be %NULL
  * @identifier: (private) Identifier used in nl80211 to identify this
@@ -5382,7 +5384,7 @@ struct wireless_dev {
 
 	struct mutex mtx;
 
-	bool use_4addr, is_running, registered;
+	bool use_4addr, is_running, registered, registering;
 
 	u8 address[ETH_ALEN] __aligned(sizeof(u16));
 

net/wireless/core.c

@@ -1346,6 +1346,7 @@ int cfg80211_register_netdevice(struct net_device *dev)
 
 	/* we'll take care of this */
 	wdev->registered = true;
+	wdev->registering = true;
 	ret = register_netdevice(dev);
 	if (ret)
 		goto out;
@@ -1361,6 +1362,7 @@ int cfg80211_register_netdevice(struct net_device *dev)
 	cfg80211_register_wdev(rdev, wdev);
 	ret = 0;
 out:
+	wdev->registering = false;
 	if (ret)
 		wdev->registered = false;
 	return ret;
@@ -1403,7 +1405,7 @@ static int cfg80211_netdev_notifier_call(struct notifier_block *nb,
 		 * It is possible to get NETDEV_UNREGISTER multiple times,
 		 * so check wdev->registered.
 		 */
-		if (wdev->registered) {
+		if (wdev->registered && !wdev->registering) {
 			wiphy_lock(&rdev->wiphy);
 			_cfg80211_unregister_wdev(wdev, false);
 			wiphy_unlock(&rdev->wiphy);