mac80211: fix race in ieee80211_register_hw()

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()  ieee80211_tasklet_handler()
    ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
       |                    |                 |
       |<---phy0----wiphy_register()          |
       |-----iwd if_add---->|                 |
       |                    |<---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()  ieee80211_tasklet_handler()
    ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
       |                    |                 |
       |       alloc_ordered_workqueue()      |
       |                    |                 |
       |              Misc wiphy init.        |
       |                    |                 |
       |<---phy0----wiphy_register()          |
       |-----iwd if_add---->|                 |
       |                    |<---IRQ----(RX packet)
       |                    |                 |
       |            ieee80211_if_add()        |
       |                    |                 |

Cc: [email protected]
Signed-off-by: Sumit Garg <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[Johannes: fix rtnl imbalances]
Signed-off-by: Johannes Berg <[email protected]>

net/mac80211/main.c

@@ -1069,7 +1069,7 @@ 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;
 		}
 	}
 
@@ -1135,7 +1135,7 @@ 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;
@@ -1161,10 +1161,6 @@ 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.
@@ -1217,9 +1213,9 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 		goto fail_flows;
 
 	rtnl_lock();
-
 	result = ieee80211_init_rate_ctrl_alg(local,
 					      hw->rate_control_algorithm);
+	rtnl_unlock();
 	if (result < 0) {
 		wiphy_debug(local->hw.wiphy,
 			    "Failed to initialize rate control algorithm\n");
@@ -1273,6 +1269,12 @@ int ieee80211_register_hw(struct ieee80211_hw *hw)
 		local->sband_allocated |= BIT(band);
 	}
 
+	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)) {
@@ -1312,17 +1314,17 @@ 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);
- fail_rate:
 	rtnl_unlock();
+ fail_rate:
  fail_flows:
 	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);
@@ -1372,8 +1374,8 @@ 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);
 }