[PATCH v2 0/2] dmaengine: fix kref underflow and UAF in dma_chan_put()

[PATCH v2 0/2] dmaengine: fix kref underflow and UAF in dma_chan_put()

[Shivank Garg]

Fix bugs related to dma_chan_put().

Signed-off-by: Shivank Garg <shivankg@amd.com>
---
Changes in v2:
- Add patch 2 fixing the dma_chan_put()/dma_release_channel() use-after-free (sashiko) 
- Link to v1: https://lore.kernel.org/r/20260518-dmaengine-kref-fix-v1-1-4d6125048fb7@amd.com

---
Shivank Garg (2):
      dmaengine: Fix device kref underflow in dma_chan_put()
      dmaengine: fix use-after-free in dma_chan_put() and dma_release_channel()

 drivers/dma/dmaengine.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)
---
base-commit: e8c2f9fdadee7cbc75134dc463c1e0d856d6e5c7
change-id: 20260518-dmaengine-kref-fix-7b21acb09455

Best regards,
-- 
Shivank Garg <shivankg@amd.com>

[PATCH v2 1/2] dmaengine: Fix device kref underflow in dma_chan_put()

[Shivank Garg]

dma_chan_get() takes chan->device->ref only on the slow path:

	/* no kref on fast path */
	if (chan->client_count) {
		__module_get(owner);
		chan->client_count++;
		return 0;
	}
	if (!try_module_get(owner))
		return -ENODEV;
	ret = kref_get_unless_zero(&chan->device->ref);

dma_chan_put() drops the ref unconditionally, so every fast-path
get/put pair drops one extra device reference.

The bug fires when two conditions hold together: a non-private
provider has a persistent client holding chan->client_count > 0
and another client cycles dmaengine_get()/dmaengine_put().
When the kref hits zero, the subsequent dma_find_channel() returns
NULL even though the provider module is still loaded.

Fix this by dropping device->ref only on the last put, matching the
single slow-path get.

Fixes: 8ad342a86359 ("dmaengine: Add reference counting to dma_device struct")
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
 drivers/dma/dmaengine.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 405bd2fbb4a3b94fd0bf44526f656f6a19feaad0..605bfa477a004cc0b03957ffb85a52308f903441 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -515,7 +515,9 @@ static void dma_chan_put(struct dma_chan *chan)
 		chan->route_data = NULL;
 	}
 
-	dma_device_put(chan->device);
+	/* 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));
 }
 

-- 
2.43.0

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

[Shivank Garg]

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