ovpn: tcp - don't deref NULL sk_socket member after tcp_close()

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

ovpn: tcp - don't deref NULL sk_socket member after tcp_close()

When deleting a peer in case of keepalive expiration, the peer is

removed from the OpenVPN hashtable and is temporary inserted in a

"release list" for further processing.

This happens in:

ovpn_peer_keepalive_work()

unlock_ovpn(release_list)

This processing includes detaching from the socket being used to

talk to this peer, by restoring its original proto and socket

ops/callbacks.

In case of TCP it may happen that, while the peer is sitting in

the release list, userspace decides to close the socket.

This will result in a concurrent execution of:

tcp_close(sk)

__tcp_close(sk)

sock_orphan(sk)

sk_set_socket(sk, NULL)

The last function call will set sk->sk_socket to NULL.

When the releasing routine is resumed, ovpn_tcp_socket_detach()

will attempt to dereference sk->sk_socket to restore its original

ops member. This operation will crash due to sk->sk_socket being NULL.

Fix this race condition by testing-and-accessing

sk->sk_socket atomically under sk->sk_callback_lock.