cfg80211: fix deadlock

When removing an interface with nl80211, cfg80211 will
deadlock in the netdev notifier because we're already
holding rdev->mtx and try to acquire it again to verify
the scan has been done.

This bug was introduced by my patch
"cfg80211: check for and abort dangling scan requests".

To fix this, move the dangling scan request check into
wiphy_unregister(). This will not be able to catch all
cases right away, but if the scan problem happens with
a manual ifdown or so it will be possible to remedy it
by removing the module/device.

Additionally, add comments about the deadlock scenario.

Reported-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Tested-by: Christian Lamparter <chunkeey@web.de>
Tested-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 18 insertions, 12 deletions

diff --git a/net/wireless/core.c b/net/wireless/core.c
index bc99e4ec746..69a185ba9ff 100644
--- a/net/wireless/core.c
+++ b/net/wireless/core.c
@@ -586,9 +586,14 @@ void wiphy_unregister(struct wiphy *wiphy)
          * get to lock contention here if userspace issues a command
          * that identified the hardware by wiphy index.
          */
-        mutex_lock(&rdev->mtx);
-        /* unlock again before freeing */
-        mutex_unlock(&rdev->mtx);
+        cfg80211_lock_rdev(rdev);
+
+        if (WARN_ON(rdev->scan_req)) {
+                rdev->scan_req->aborted = true;
+                ___cfg80211_scan_done(rdev);
+        }
+
+        cfg80211_unlock_rdev(rdev);
 
         cfg80211_debugfs_rdev_del(rdev);
 
@@ -605,7 +610,6 @@ void wiphy_unregister(struct wiphy *wiphy)
 
         flush_work(&rdev->scan_done_wk);
         cancel_work_sync(&rdev->conn_work);
-        kfree(rdev->scan_req);
         flush_work(&rdev->event_work);
 }
 EXPORT_SYMBOL(wiphy_unregister);
@@ -653,6 +657,11 @@ static int cfg80211_netdev_notifier_call(struct notifier_block * nb,
 
         switch (state) {
         case NETDEV_REGISTER:
+                /*
+                 * NB: cannot take rdev->mtx here because this may be
+                 * called within code protected by it when interfaces
+                 * are added with nl80211.
+                 */
                 mutex_init(&wdev->mtx);
                 INIT_LIST_HEAD(&wdev->event_list);
                 spin_lock_init(&wdev->event_lock);
@@ -730,13 +739,11 @@ static int cfg80211_netdev_notifier_call(struct notifier_block * nb,
 #endif
                 break;
         case NETDEV_UNREGISTER:
-                cfg80211_lock_rdev(rdev);
-
-                if (WARN_ON(rdev->scan_req && rdev->scan_req->dev == dev)) {
-                        rdev->scan_req->aborted = true;
-                        ___cfg80211_scan_done(rdev);
-                }
-
+                /*
+                 * NB: cannot take rdev->mtx here because this may be
+                 * called within code protected by it when interfaces
+                 * are removed with nl80211.
+                 */
                 mutex_lock(&rdev->devlist_mtx);
                 /*
                  * It is possible to get NETDEV_UNREGISTER
@@ -755,7 +762,6 @@ static int cfg80211_netdev_notifier_call(struct notifier_block * nb,
 #endif
                 }
                 mutex_unlock(&rdev->devlist_mtx);
-                cfg80211_unlock_rdev(rdev);
                 break;
         case NETDEV_PRE_UP:
                 if (!(wdev->wiphy->interface_modes & BIT(wdev->iftype)))