[PATCH net] ovpn: tcp - don't deref NULL sk_socket member after tcp_close()

[Antonio Quartulli <antonio@openvpn.net>]

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.

Link: https://lore.kernel.org/netdev/176996279620.3109699.15382994681575380467@eldamar.lan/
Link: https://github.com/OpenVPN/ovpn-net-next/issues/29
Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
---
 drivers/net/ovpn/tcp.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ovpn/tcp.c b/drivers/net/ovpn/tcp.c
index f0b4e07ba924..ec2bbc28c196 100644
--- a/drivers/net/ovpn/tcp.c
+++ b/drivers/net/ovpn/tcp.c
@@ -199,7 +199,19 @@ void ovpn_tcp_socket_detach(struct ovpn_socket *ovpn_sock)
 	sk->sk_data_ready = peer->tcp.sk_cb.sk_data_ready;
 	sk->sk_write_space = peer->tcp.sk_cb.sk_write_space;
 	sk->sk_prot = peer->tcp.sk_cb.prot;
-	sk->sk_socket->ops = peer->tcp.sk_cb.ops;
+
+	/* tcp_close() may race this function and could set
+	 * sk->sk_socket to NULL. It does so by invoking
+	 * sock_orphan(), which holds sk_callback_lock before
+	 * doing the assignment.
+	 *
+	 * For this reason we acquire the same lock to avoid
+	 * sk_socket to disappear under our feet
+	 */
+	write_lock_bh(&sk->sk_callback_lock);
+	if (sk->sk_socket)
+		sk->sk_socket->ops = peer->tcp.sk_cb.ops;
+	write_unlock_bh(&sk->sk_callback_lock);
 
 	rcu_assign_sk_user_data(sk, NULL);
 }
-- 
2.52.0