Re: [PATCH net-next v11 09/23] ovpn: implement basic RX path (UDP)

[Antonio Quartulli <antonio@openvpn.net>]

On 29/11/2024 14:20, Sabrina Dubroca wrote:
> 2024-11-27, 02:40:02 +0100, Antonio Quartulli wrote:
>> On 26/11/2024 09:49, Antonio Quartulli wrote:
>> [...]
>>>>
>>>> The potential issue is tricky since we create it patch-by-patch.
>>>>
>>>> Up to this patch the socket releasing procedure looks solid and
>>>> reliable. E.g. the P2P netdev destroying:
>>>>
>>>>     ovpn_netdev_notifier_call(NETDEV_UNREGISTER)
>>>>       ovpn_peer_release_p2p
>>>>         ovpn_peer_del_p2p
>>>>           ovpn_peer_put
>>>>             ovpn_peer_release_kref
>>>>               ovpn_peer_release
>>>>                 ovpn_socket_put
>>>>                   ovpn_socket_release_kref
>>>>                     ovpn_socket_detach
>>>>                       ovpn_udp_socket_detach
>>>>                         setup_udp_tunnel_sock
>>>>     netdev_run_todo
>>>>       rcu_barrier  <- no running ovpn_udp_encap_recv after this point
>>>>       free_netdev
>>>>
>>>> After the setup_udp_tunnel_sock() call no new ovpn_udp_encap_recv()
>>>> will be spawned. And after the rcu_barrier() all running
>>>> ovpn_udp_encap_recv() will be done. All good.
>>>>
>>>
>>> ok
>>>
>>>> Then, the following patch 'ovpn: implement TCP transport' disjoin
>>>> ovpn_socket_release_kref() and ovpn_socket_detach() by scheduling
>>>> the socket detach function call:
>>>>
>>>>     ovpn_socket_release_kref
>>>>       ovpn_socket_schedule_release
>>>>         schedule_work(&sock->work)
>>>>
>>>> And long time after the socket will be actually detached:
>>>>
>>>>     ovpn_socket_release_work
>>>>       ovpn_socket_detach
>>>>         ovpn_udp_socket_detach
>>>>           setup_udp_tunnel_sock
>>>>
>>>> And until this detaching will take a place, UDP handler can call
>>>> ovpn_udp_encap_recv() whatever number of times.
>>>>
>>>> So, we can end up with this scenario:
>>>>
>>>>     ovpn_netdev_notifier_call(NETDEV_UNREGISTER)
>>>>       ovpn_peer_release_p2p
>>>>         ovpn_peer_del_p2p
>>>>           ovpn_peer_put
>>>>             ovpn_peer_release_kref
>>>>               ovpn_peer_release
>>>>                 ovpn_socket_put
>>>>                   ovpn_socket_release_kref
>>>>                     ovpn_socket_schedule_release
>>>>                       schedule_work(&sock->work)
>>>>     netdev_run_todo
>>>>       rcu_barrier
>>>>       free_netdev
>>>>
>>>>     ovpn_udp_encap_recv  <- called for an incoming UDP packet
>>>>       ovpn_from_udp_sock <- returns pointer to freed memory
>>>>       // Any access to ovpn pointer is the use-after-free
>>>>
>>>>     ovpn_socket_release_work  <- kernel finally ivoke the work
>>>>       ovpn_socket_detach
>>>>         ovpn_udp_socket_detach
>>>>           setup_udp_tunnel_sock
>>>>
>>>> To address the issue, I see two possible solutions:
>>>> 1. flush the workqueue somewhere before the netdev release
>>>
>>> yes! This is what I was missing. This will also solve the "how can the
>>> module wait for all workers to be done before unloading?"
>>>
>>
>> Actually there might be even a simpler solution: each ovpn_socket will hold
>> a reference to an ovpn_peer (TCP) or to an ovpn_priv (UDP).
>> I can simply increase the refcounter those objects while they are referenced
>> by the socket and decrease it when the socket is fully released (in the
>> detach() function called by the worker).
>>
>> This way the netdev cannot be released until all socket (and all peers) are
>> gone.
>>
>> This approach doesn't require any local workqueue or any other special
>> coordination as we'll just force the whole cleanup to happen in a specific
>> order.
>>
>> Does it make sense?
> 
> This dependency between refcounts worries me. I'm already having a
> hard time remembering how all objects interact together.
> 
> And since ovpn_peer_release already calls ovpn_socket_put, you'd get a
> refcount loop if ovpn_socket now also has a ref on the peer, no?

You're right.
Therefore I started playing with the following approach:
* implement ovpn_peer_remove() that is invoked by ovpn_peer_del(), i.e. 
when ovpn wants to remove the peer from its state
* ovpn_peer_remove() will do all kind of cleanup and unhash, including 
calling ovpn_socket_put()
* in turn, when the socket is released from all other contexts, it will 
also call ovpn_peer_put() and allow the peer to be free'd for good.

On one hand it sounds a bit clumsy, but on the other hand it allows each 
component to keep relying on any reference it is holding until the end.

The only downside is that we will start shutting down a peer and then 
keep it around until any reference is dropped. But it should work.

Regards,

-- 
Antonio Quartulli
OpenVPN Inc.