[PATCH v2 2/2] dmaengine: fix use-after-free in dma_chan_put() and dma_release_channel()

[Shivank Garg <shivankg@amd.com>]

When dma_device_put() drops the last reference on chan->device->ref,
dma_device_release() runs and may free the dma_device along with its
channels.

Two paths still read that memory after the put:
 - dma_chan_put() reads chan->device->owner via dma_chan_to_owner()
   for the trailing module_put().
 - dma_release_channel() calls dma_chan_put() first, then reads
   chan->device->privatecnt, chan->slave, chan->name and
   chan->dbg_client_name.

KASAN catches the first one:

	slab-use-after-free in dma_chan_put+0x3e6/0x4c0
	Read of size 8 by task insmod/6319
	Freed by task 6319:
	  kfree+0x225/0x470
	  dma_chan_put+0x395/0x4c0
	  dmaengine_put+0xf8/0x160

Cache the module owner in dma_chan_put() before the put so the trailing
module_put() does not need chan->device. In dma_release_channel(), move
dma_chan_put() to the end, after every chan/device read.

Fixes: 8ad342a86359 ("dmaengine: Add reference counting to dma_device struct")
Suggested-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260518-dmaengine-kref-fix-v1-1-4d6125048fb7@amd.com
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
 drivers/dma/dmaengine.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 605bfa477a004cc0b03957ffb85a52308f903441..9c4e206f246864ee185e8d9df96a89014d9e6edf 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -495,10 +495,13 @@ static int dma_chan_get(struct dma_chan *chan)
  */
 static void dma_chan_put(struct dma_chan *chan)
 {
+	struct module *owner;
+
 	/* This channel is not in use, bail out */
 	if (!chan->client_count)
 		return;
 
+	owner = dma_chan_to_owner(chan);
 	chan->client_count--;
 
 	/* This channel is not in use anymore, free it */
@@ -518,7 +521,7 @@ static void dma_chan_put(struct dma_chan *chan)
 	/* This channel is not in use anymore, drop the device ref */
 	if (!chan->client_count)
 		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)
@@ -907,7 +910,6 @@ void dma_release_channel(struct dma_chan *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);
 	/* drop PRIVATE cap enabled by __dma_request_channel() */
 	if (--chan->device->privatecnt == 0)
 		dma_cap_clear(DMA_PRIVATE, chan->device->cap_mask);
@@ -924,6 +926,7 @@ void dma_release_channel(struct dma_chan *chan)
 	kfree(chan->dbg_client_name);
 	chan->dbg_client_name = NULL;
 #endif
+	dma_chan_put(chan);
 	mutex_unlock(&dma_list_mutex);
 }
 EXPORT_SYMBOL_GPL(dma_release_channel);

-- 
2.43.0