CVE-2025-21823: batman-adv: Drop unmanaged ELP metric worker

In the Linux kernel, the following vulnerability has been resolved:

batman-adv: Drop unmanaged ELP metric worker

The ELP worker needs to calculate new metric values for all neighbors "reachable" over an interface. Some of the used metric sources require locks which might need to sleep. This sleep is incompatible with the RCU list iterator used for the recorded neighbors. The initial approach to work around of this problem was to queue another work item per neighbor and then run this in a new context.

Even when this solved the RCU vs mightsleep() conflict, it has a major problems: Nothing was stopping the work item in case it is not needed anymore - for example because one of the related interfaces was removed or the batman-adv module was unloaded - resulting in potential invalid memory accesses.

Directly canceling the metric worker also has various problems:

cancelworksync for a to-be-deactivated interface is called with rtnllock held. But the code in the ELP metric worker also tries to use rtnllock() - which will never return in this case. This also means that cancelworksync would never return because it is waiting for the worker to finish. iterating over the neighbor list for the to-be-deactivated interface is currently done using the RCU specific methods. Which means that it is possible to miss items when iterating over it without the associated spinlock - a behaviour which is acceptable for a periodic metric check but not for a cleanup routine (which must "stop" all still running workers)

The better approch is to get rid of the per interface neighbor metric worker and handle everything in the interface worker. The original problems are solved by:

creating a list of neighbors which require new metric information inside the RCU protected context, gathering the metric according to the new list outside the RCU protected context only use rcutrylock inside metric gathering code to avoid a deadlock when the canceldelayedworksync is called in the interface removal code (which is called with the rtnllock held)