[PATCH 1/2] dmaengine: Fix use-after-free on release

[Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>]

Many dmaengine drivers cannot be safely unbound while being used since
they free their device structures in their platform driver ->remove()
callbacks or with devm.  In order to avoid this, the framework allows
drivers to implement ->device_release() and free these structures there
instead.  However, there are use-after-frees in the framework even when
->device_release() is implemented.

For example, the following sequence of commands with the upcoming
dummy-dmac driver triggers a KASAN splat without this patch:

 # insmod dummy-dmac.ko
 # insmod dmatest.ko iterations=1 wait=1 run=1
 # echo dummy-dmac > /sys/bus/platform/drivers/dummy-dmac/unbind
 # rmmod dmatest

 ==================================================================
 BUG: KASAN: slab-use-after-free in dma_chan_put (drivers/dma/dmaengine.c:517)
 Read of size 8 at addr ffff888008b00c78 by task rmmod/1063

 Call Trace:
  dma_chan_put (drivers/dma/dmaengine.c:517)
  dma_release_channel (drivers/dma/dmaengine.c:910)
  cleanup_module (drivers/dma/dmatest.c:1184) dmatest
  ...

 Allocated by task 859:
  kmalloc_trace (mm/slab_common.c:1082)
  dummy_dmac_probe (drivers/dma/dummy-dmac.c:171) dummy_dmac
  platform_probe (drivers/base/platform.c:1405)
  ...

 Freed by task 1063:
  kfree (mm/slab_common.c:1035)
  dummy_dmac_release (drivers/dma/dummy-dmac.c:160) dummy_dmac
  dma_device_put (include/linux/kref.h:65 drivers/dma/dmaengine.c:437)
  dma_chan_put (drivers/dma/dmaengine.c:517)
  dma_release_channel (drivers/dma/dmaengine.c:910)
  cleanup_module (drivers/dma/dmatest.c:1184) dmatest
  ...
 ==================================================================

Fix this by making the framework not touch the dev and client structures
after the last call to dma_device_put().

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
---
 drivers/dma/dmaengine.c | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 826b98284fa1..86b892df8ea1 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -485,13 +485,7 @@ static int dma_chan_get(struct dma_chan *chan)
 	return ret;
 }
 
-/**
- * dma_chan_put - drop a reference to a DMA channel's parent driver module
- * @chan:	channel to release
- *
- * Must be called under dma_list_mutex.
- */
-static void dma_chan_put(struct dma_chan *chan)
+static void __dma_chan_put(struct dma_chan *chan)
 {
 	/* This channel is not in use, bail out */
 	if (!chan->client_count)
@@ -512,9 +506,22 @@ static void dma_chan_put(struct dma_chan *chan)
 		chan->router = NULL;
 		chan->route_data = NULL;
 	}
+}
+
+/**
+ * dma_chan_put - drop a reference to a DMA channel's parent driver module
+ * @chan:	channel to release
+ *
+ * Must be called under dma_list_mutex.
+ */
+static void dma_chan_put(struct dma_chan *chan)
+{
+	struct module *owner = dma_chan_to_owner(chan);
+
+	__dma_chan_put(chan);
 
 	dma_device_put(chan->device);
-	module_put(dma_chan_to_owner(chan));
+	module_put(owner);
 }
 
 enum dma_status dma_sync_wait(struct dma_chan *chan, dma_cookie_t cookie)
@@ -902,10 +909,12 @@ EXPORT_SYMBOL_GPL(dma_request_chan_by_mask);
 
 void dma_release_channel(struct dma_chan *chan)
 {
+	struct module *owner = dma_chan_to_owner(chan);
+
 	mutex_lock(&dma_list_mutex);
 	WARN_ONCE(chan->client_count != 1,
 		  "chan reference count %d != 1\n", chan->client_count);
-	dma_chan_put(chan);
+	__dma_chan_put(chan);
 	/* drop PRIVATE cap enabled by __dma_request_channel() */
 	if (--chan->device->privatecnt == 0)
 		dma_cap_clear(DMA_PRIVATE, chan->device->cap_mask);
@@ -922,6 +931,9 @@ void dma_release_channel(struct dma_chan *chan)
 	kfree(chan->dbg_client_name);
 	chan->dbg_client_name = NULL;
 #endif
+
+	dma_device_put(chan->device);
+	module_put(owner);
 	mutex_unlock(&dma_list_mutex);
 }
 EXPORT_SYMBOL_GPL(dma_release_channel);

-- 
2.34.1