[PATCH] mac80211: fix race in ieee80211_register_hw()

[Sumit Garg]

A race condition leading to a kernel crash is observed during invocation
of ieee80211_register_hw() on a dragonboard410c device having wcn36xx
driver built as a loadable module along with a wifi manager in user-space
waiting for a wifi device (wlanX) to be active.

Sequence diagram for a particular kernel crash scenario:

    user-space  ieee80211_register_hw()  RX IRQ
    +++++++++++++++++++++++++++++++++++++++++++++
       |                    |             |
       |<---wlan0---wiphy_register()      |
       |----start wlan0---->|             |
       |                    |<---IRQ---(RX packet)
       |              Kernel crash        |
       |              due to unallocated  |
       |              workqueue.          |
       |                    |             |
       |       alloc_ordered_workqueue()  |
       |                    |             |
       |              Misc wiphy init.    |
       |                    |             |
       |            ieee80211_if_add()    |
       |                    |             |

As evident from above sequence diagram, this race condition isn't specific
to a particular wifi driver but rather the initialization sequence in
ieee80211_register_hw() needs to be fixed. So re-order the initialization
sequence and the updated sequence diagram would look like:

    user-space  ieee80211_register_hw()  RX IRQ
    +++++++++++++++++++++++++++++++++++++++++++++
       |                    |             |
       |       alloc_ordered_workqueue()  |
       |                    |             |
       |              Misc wiphy init.    |
       |                    |             |
       |<---wlan0---wiphy_register()      |
       |----start wlan0---->|             |
       |                    |<---IRQ---(RX packet)
       |                    |             |
       |            ieee80211_if_add()    |
       |                    |             |

Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
---
 net/mac80211/main.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/net/mac80211/main.c b/net/mac80211/main.c
index 4c2b5ba..4ca62fc 100644
--- a/net/mac80211/main.c
+++ b/net/mac80211/main.c
@@ -1051,7 +1051,7 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 		local->hw.wiphy->signal_type = CFG80211_SIGNAL_TYPE_UNSPEC;
 		if (hw->max_signal <= 0) {
 			result = -EINVAL;
-			goto fail_wiphy_register;
+			goto fail_workqueue;
 		}
 	}
 
@@ -1113,7 +1113,7 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 
 	result = ieee80211_init_cipher_suites(local);
 	if (result < 0)
-		goto fail_wiphy_register;
+		goto fail_workqueue;
 
 	if (!local->ops->remain_on_channel)
 		local->hw.wiphy->max_remain_on_channel_duration = 5000;
@@ -1139,10 +1139,6 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 
 	local->hw.wiphy->max_num_csa_counters = IEEE80211_MAX_CSA_COUNTERS_NUM;
 
-	result = wiphy_register(local->hw.wiphy);
-	if (result < 0)
-		goto fail_wiphy_register;
-
 	/*
 	 * We use the number of queues for feature tests (QoS, HT) internally
 	 * so restrict them appropriately.
@@ -1254,6 +1250,14 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 		local->sband_allocated |= BIT(band);
 	}
 
+	rtnl_unlock();
+
+	result = wiphy_register(local->hw.wiphy);
+	if (result < 0)
+		goto fail_wiphy_register;
+
+	rtnl_lock();
+
 	/* add one default STA interface if supported */
 	if (local->hw.wiphy->interface_modes & BIT(NL80211_IFTYPE_STATION) &&
 	    !ieee80211_hw_check(hw, NO_AUTO_VIF)) {
@@ -1293,6 +1297,8 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 #if defined(CONFIG_INET) || defined(CONFIG_IPV6)
  fail_ifa:
 #endif
+	wiphy_unregister(local->hw.wiphy);
+ fail_wiphy_register:
 	rtnl_lock();
 	rate_control_deinitialize(local);
 	ieee80211_remove_interfaces(local);
@@ -1302,8 +1308,6 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 	ieee80211_led_exit(local);
 	destroy_workqueue(local->workqueue);
  fail_workqueue:
-	wiphy_unregister(local->hw.wiphy);
- fail_wiphy_register:
 	if (local->wiphy_ciphers_allocated)
 		kfree(local->hw.wiphy->cipher_suites);
 	kfree(local->int_scan_req);
@@ -1353,8 +1357,8 @@ void ieee80211_unregister_hw(struct ieee80211_hw *hw)
 	skb_queue_purge(&local->skb_queue_unreliable);
 	skb_queue_purge(&local->skb_queue_tdls_chsw);
 
-	destroy_workqueue(local->workqueue);
 	wiphy_unregister(local->hw.wiphy);
+	destroy_workqueue(local->workqueue);
 	ieee80211_led_exit(local);
 	kfree(local->int_scan_req);
 }
-- 
2.7.4