dmaengine: idxd: Prevent use after free on completion memory

On driver unload any pending descriptors are flushed at the
time the interrupt is freed:
idxd_dmaengine_drv_remove() ->
	drv_disable_wq() ->
		idxd_wq_free_irq() ->
			idxd_flush_pending_descs().

If there are any descriptors present that need to be flushed this
flow triggers a "not present" page fault as below:

 BUG: unable to handle page fault for address: ff391c97c70c9040
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page

The address that triggers the fault is the address of the
descriptor that was freed moments earlier via:
drv_disable_wq()->idxd_wq_free_resources()

Fix the use after free by freeing the descriptors after any possible
usage. This is done after idxd_wq_reset() to ensure that the memory
remains accessible during possible completion writes by the device.

Fixes: 63c14ae ("dmaengine: idxd: refactor wq driver enable/disable operations")
Suggested-by: Dave Jiang <[email protected]>
Signed-off-by: Reinette Chatre <[email protected]>
Reviewed-by: Dave Jiang <[email protected]>
Reviewed-by: Fenghua Yu <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/6c4657d9cff0a0a00501a7b928297ac966e9ec9d.1670452419.git.reinette.chatre@intel.com
Signed-off-by: Vinod Koul <[email protected]>

Author: rchatre

drivers/dma/idxd/device.c

@@ -1407,11 +1407,11 @@ void drv_disable_wq(struct idxd_wq *wq)
 		dev_warn(dev, "Clients has claim on wq %d: %d\n",
 			 wq->id, idxd_wq_refcount(wq));
 
-	idxd_wq_free_resources(wq);
 	idxd_wq_unmap_portal(wq);
 	idxd_wq_drain(wq);
 	idxd_wq_free_irq(wq);
 	idxd_wq_reset(wq);
+	idxd_wq_free_resources(wq);
 	percpu_ref_exit(&wq->wq_active);
 	wq->type = IDXD_WQT_NONE;
 	wq->client_count = 0;