net: ensure net_todo_list is processed quickly

In [1], Will raised a potential issue that the cfg80211 code,
which does (from a locking perspective)

  rtnl_lock()
  wiphy_lock()
  rtnl_unlock()

might be suspectible to ABBA deadlocks, because rtnl_unlock()
calls netdev_run_todo(), which might end up calling rtnl_lock()
again, which could then deadlock (see the comment in the code
added here for the scenario).

Some back and forth and thinking ensued, but clearly this can't
happen if the net_todo_list is empty at the rtnl_unlock() here.
Clearly, the code here cannot actually put an entry on it, and
all other users of rtnl_unlock() will empty it since that will
always go through netdev_run_todo(), emptying the list.

So the only other way to get there would be to add to the list
and then unlock the RTNL without going through rtnl_unlock(),
which is only possible through __rtnl_unlock(). However, this
isn't exported and not used in many places, and none of them
seem to be able to unregister before using it.

Therefore, add a WARN_ON() in the code to ensure this invariant
won't be broken, so that the cfg80211 (or any similar) code
stays safe.

[1] https://lore.kernel.org/r/Yjzpo3TfZxtKPMAG@google.com

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20220404113847.0ee02e4a70da.Ic73d206e217db20fd22dcec14fe5442ca732804b@changeid
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 59e27a2..b6a1e7f 100644 (file)
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -3894,7 +3894,8 @@ void dev_queue_xmit_nit(struct sk_buff *skb, struct net_device *dev);
 extern int             netdev_budget;
 extern unsigned int    netdev_budget_usecs;
 
-/* Called by rtnetlink.c:rtnl_unlock() */
+/* Used by rtnetlink.c:__rtnl_unlock()/rtnl_unlock() */
+extern struct list_head net_todo_list;
 void netdev_run_todo(void);
 
 static inline void __dev_put(struct net_device *dev)

diff --git a/net/core/dev.c b/net/core/dev.c
index 8c6c084..2ec1735 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -9431,7 +9431,7 @@ static int dev_new_index(struct net *net)
 }
 
 /* Delayed registration/unregisteration */
-static LIST_HEAD(net_todo_list);
+LIST_HEAD(net_todo_list);
 DECLARE_WAIT_QUEUE_HEAD(netdev_unregistering_wq);
 
 static void net_set_todo(struct net_device *dev)

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 159c9c6..0e4502d 100644 (file)
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -95,6 +95,39 @@ void __rtnl_unlock(void)
 
        defer_kfree_skb_list = NULL;
 
+       /* Ensure that we didn't actually add any TODO item when __rtnl_unlock()
+        * is used. In some places, e.g. in cfg80211, we have code that will do
+        * something like
+        *   rtnl_lock()
+        *   wiphy_lock()
+        *   ...
+        *   rtnl_unlock()
+        *
+        * and because netdev_run_todo() acquires the RTNL for items on the list
+        * we could cause a situation such as this:
+        * Thread 1                     Thread 2
+        *                                rtnl_lock()
+        *                                unregister_netdevice()
+        *                                __rtnl_unlock()
+        * rtnl_lock()
+        * wiphy_lock()
+        * rtnl_unlock()
+        *   netdev_run_todo()
+        *     __rtnl_unlock()
+        *
+        *     // list not empty now
+        *     // because of thread 2
+        *                                rtnl_lock()
+        *     while (!list_empty(...))
+        *       rtnl_lock()
+        *                                wiphy_lock()
+        * **** DEADLOCK ****
+        *
+        * However, usage of __rtnl_unlock() is rare, and so we can ensure that
+        * it's not used in cases where something is added to do the list.
+        */
+       WARN_ON(!list_empty(&net_todo_list));
+
        mutex_unlock(&rtnl_mutex);
 
        while (head) {