IB/mad: Fix use after free when destroying MAD agent

Currently, when RMPP MADs are processed while the MAD agent is destroyed,
it could result in use after free of rmpp_recv, as decribed below:

	cpu-0						cpu-1
	-----						-----
ib_mad_recv_done()
 ib_mad_complete_recv()
  ib_process_rmpp_recv_wc()
						unregister_mad_agent()
						 ib_cancel_rmpp_recvs()
						  cancel_delayed_work()
   process_rmpp_data()
    start_rmpp()
     queue_delayed_work(rmpp_recv->cleanup_work)
						  destroy_rmpp_recv()
						   free_rmpp_recv()
     cleanup_work()[1]
      spin_lock_irqsave(&rmpp_recv->agent->lock) <-- use after free

[1] cleanup_work() == recv_cleanup_handler

Fix it by waiting for the MAD agent reference count becoming zero before
calling to ib_cancel_rmpp_recvs().

Fixes: 9a41e38a ("IB/mad: Use IDR for agent IDs")
Link: https://lore.kernel.org/r/20200621104738.54850-2-leon@kernel.orgSigned-off-by: Shay Drory <shayd@mellanox.com>
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/core/mad.c

@@ -509,10 +509,10 @@ static void unregister_mad_agent(struct ib_mad_agent_private *mad_agent_priv)
 	xa_erase(&ib_mad_clients, mad_agent_priv->agent.hi_tid);
 
 	flush_workqueue(port_priv->wq);
-	ib_cancel_rmpp_recvs(mad_agent_priv);
 
 	deref_mad_agent(mad_agent_priv);
 	wait_for_completion(&mad_agent_priv->comp);
+	ib_cancel_rmpp_recvs(mad_agent_priv);
 
 	ib_mad_agent_security_cleanup(&mad_agent_priv->agent);