[PATCH 2/3] dmaengine: idxd: Prevent use after free on completion memory

[Reinette Chatre]

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: 63c14ae6c161 ("dmaengine: idxd: refactor wq driver enable/disable operations")
Suggested-by: Dave Jiang <dave.jiang@xxxxxxxxx>
Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
---
 drivers/dma/idxd/device.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma/idxd/device.c b/drivers/dma/idxd/device.c
index fcd03d29a941..b4d7bb923a40 100644
--- a/drivers/dma/idxd/device.c
+++ b/drivers/dma/idxd/device.c
@@ -1408,11 +1408,11 @@ 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;
-- 
2.34.1