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net: ensure net_todo_list is processed quickly

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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>
This commit is contained in:
Johannes Berg 2022-04-04 11:38:47 +02:00 committed by Jakub Kicinski
parent 6f2f36e5f9
commit 0b5c21bbc0
3 changed files with 36 additions and 2 deletions
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3
include/linux/netdevice.h
View File

@ -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)

2
net/core/dev.c
View File

@ -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)

33
net/core/rtnetlink.c
View File

@ -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) {