linux-kernel - Re: [PATCH] DMA: Fix broken device refcounting

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Date:	Sat, 27 Oct 2007 12:12:37 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Haavard Skinnemoen <hskinnemoen@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Nelson, Shannon" <shannon.nelson@...el.com>
Subject: Re: [PATCH] DMA: Fix broken device refcounting

On Sat, 2007-10-27 at 06:49 -0700, Haavard Skinnemoen wrote:
> On Fri, 26 Oct 2007 09:36:17 -0700
> Dan Williams <dan.j.williams@...el.com> wrote:
> 
> > @@ -221,7 +220,6 @@ void dma_chan_cleanup(struct kref *kref)
> >  {
> >       struct dma_chan *chan = container_of(kref, struct dma_chan, refcount);
> >       chan->device->device_free_chan_resources(chan);
> > -     kref_put(&chan->device->refcount, dma_async_device_cleanup);
> >  }
> >  EXPORT_SYMBOL(dma_chan_cleanup);
> 
> While I can't see any problems with the rest of the patch, I think this
> part is wrong for the same reasons removing the kref_put() from the
> class device cleanup function is. I don't see any constraint that
> guarantees that dma_chan_cleanup() will always be called before
> dma_dev_release(), which means that "chan" may have been freed before
> this function gets a chance to run. Please correct me if I'm wrong.

Absolutely right, the driver, not dmaengine, frees the memory so there
must be a per channel reference on the device to hold off the driver's
remove routine.
> 
> Håvard

So how about this...

---snip---
dmaengine: Fix broken device refcounting

From: Haavard Skinnemoen <hskinnemoen@...el.com>

When a DMA device is unregistered, its reference count is decremented
twice for each channel: Once dma_class_dev_release() and once in
dma_chan_cleanup(). This may result in the DMA device driver's
remove() function completing before all channels have been cleaned
up, causing lots of use-after-free fun.

Fix it by incrementing the device's reference count twice for each
channel during registration.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@...el.com>
[dan.j.williams@...el.com: kill unnecessary client refcounting]
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
---

 drivers/dma/dmaengine.c |   17 ++++++-----------
 1 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 84257f7..ec7e871 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -186,10 +186,9 @@ static void dma_client_chan_alloc(struct dma_client *client)
 				/* we are done once this client rejects
 				 * an available resource
 				 */
-				if (ack == DMA_ACK) {
+				if (ack == DMA_ACK)
 					dma_chan_get(chan);
-					kref_get(&device->refcount);
-				} else if (ack == DMA_NAK)
+				else if (ack == DMA_NAK)
 					return;
 			}
 		}
@@ -276,11 +275,8 @@ static void dma_clients_notify_removed(struct dma_chan *chan)
 		/* client was holding resources for this channel so
 		 * free it
 		 */
-		if (ack == DMA_ACK) {
+		if (ack == DMA_ACK)
 			dma_chan_put(chan);
-			kref_put(&chan->device->refcount,
-				dma_async_device_cleanup);
-		}
 	}
 
 	mutex_unlock(&dma_list_mutex);
@@ -320,11 +316,8 @@ void dma_async_client_unregister(struct dma_client *client)
 			ack = client->event_callback(client, chan,
 				DMA_RESOURCE_REMOVED);
 
-			if (ack == DMA_ACK) {
+			if (ack == DMA_ACK)
 				dma_chan_put(chan);
-				kref_put(&chan->device->refcount,
-					dma_async_device_cleanup);
-			}
 		}
 
 	list_del(&client->global_node);
@@ -401,6 +394,8 @@ int dma_async_device_register(struct dma_device *device)
 			goto err_out;
 		}
 
+		/* One for the channel, one of the class device */
+		kref_get(&device->refcount);
 		kref_get(&device->refcount);
 		kref_init(&chan->refcount);
 		chan->slow_ref = 0;
-
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