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>

1 files changed, 33 insertions, 0 deletions

diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 159c9c61e6af..0e4502d641eb 100644
--- 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) {