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.
