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

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

commit 116a1b9f1cb769b83e5adff323f977a62b1dcb2e upstream.

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: 9a41e38a467c ("IB/mad: Use IDR for agent IDs")
Link: https://lore.kernel.org/r/20200621104738.54850-2-leon@kernel.org
Signed-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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>