CVE-2021-47391

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

RDMA/cma: Ensure rdma_addr_cancel() happens before issuing more requests

The FSM can run in a circle allowing rdma_resolve_ip() to be called twice on the same id_priv. While this cannot happen without going through the work, it violates the invariant that the same address resolution background request cannot be active twice.

   CPU 1                                  CPU 2

rdma_resolve_addr(): RDMA_CM_IDLE -> RDMA_CM_ADDR_QUERY rdma_resolve_ip(addr_handler) #1

		 process_one_req(): for #1
                      addr_handler():
                        RDMA_CM_ADDR_QUERY -> RDMA_CM_ADDR_BOUND
                        mutex_unlock(&id_priv->handler_mutex);
                        [.. handler still running ..]

rdma_resolve_addr(): RDMA_CM_ADDR_BOUND -> RDMA_CM_ADDR_QUERY rdma_resolve_ip(addr_handler) !! two requests are now on the req_list

rdma_destroy_id(): destroy_id_handler_unlock(): _destroy_id(): cma_cancel_operation(): rdma_addr_cancel()

                      // process_one_req() self removes it
	          spin_lock_bh(&lock);
                       cancel_delayed_work(&req->work);
                   if (!list_empty(&req->list)) == true

  ! rdma_addr_cancel() returns after process_on_req #1 is done

kfree(id_priv)

		 process_one_req(): for #2
                      addr_handler():
                    mutex_lock(&id_priv->handler_mutex);
                        !! Use after free on id_priv

rdma_addr_cancel() expects there to be one req on the list and only cancels the first one. The self-removal behavior of the work only happens after the handler has returned. This yields a situations where the req_list can have two reqs for the same "handle" but rdma_addr_cancel() only cancels the first one.

The second req remains active beyond rdma_destroy_id() and will use-after-free id_priv once it inevitably triggers.

Fix this by remembering if the id_priv has called rdma_resolve_ip() and always cancel before calling it again. This ensures the req_list never gets more than one item in it and doesn't cost anything in the normal flow that never uses this strange error path.