[PATCH v5 0/4] dmaengine: fix kref underflow and UAF in dma_chan_put()

[PATCH v5 0/4] dmaengine: fix kref underflow and UAF in dma_chan_put()

[Shivank Garg]

Fix bugs related to dma_chan_put(), found while testing with SDXI[1].

[1]: https://lore.kernel.org/dmaengine/20260605-sdxi-base-v3-0-4d38ca2bdffe@amd.com

Signed-off-by: Shivank Garg <shivankg@amd.com>
---
Changes in v5:
- Use int return type and add __must_check for dma_device_get() (Frank)
- Drop the comment above synchronize_rcu() and note in the commit message
  that this will delay for grace period dma_list_mutex held (Logan)
- Link to v4: https://lore.kernel.org/r/20260818-dmaengine-kref-fix-v4-0-c6ef991462a0@amd.com

Changes in v4:
- Add dma_device_get() helper (Frank)
- Drop dma_chan_put() move change (Frank)
- Link to v3: https://lore.kernel.org/r/20260816-dmaengine-kref-fix-v3-0-7e76187145df@amd.com

Changes in v3:
- Add patch 3: add synchronize_rcu() to wait for RCU readers to prevent
  use-after-free. (Sashiko)
- Link to v2: https://lore.kernel.org/r/20260526-dmaengine-kref-fix-v2-0-3df60afac01d@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 (4):
      dmaengine: add dma_device_get() helper
      dmaengine: Fix device kref underflow in dma_chan_put()
      dmaengine: fix use-after-free in dma_chan_put() and dma_release_channel()
      dmaengine: wait for RCU readers before releasing dma_device

 drivers/dma/dmaengine.c | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)
---
base-commit: a4ff2be345d0abc943da8dd8da98151843b750dc
change-id: 20260518-dmaengine-kref-fix-7b21acb09455

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

[PATCH v5 1/4] dmaengine: add dma_device_get() helper

[Shivank Garg]

Add dma_device_get() helper to match dma_device_put() to make code
symmetric. It wraps open-coded kref_get_unless_zero() and asserts that
dma_list_mutex is held, matching its put counterpart.

No functional change intended.

Suggested-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
 drivers/dma/dmaengine.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 9049171df857..cb3b7bccce1f 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -433,6 +433,12 @@ static void dma_device_release(struct kref *ref)
 		device->device_release(device);
 }
 
+static int __must_check dma_device_get(struct dma_device *device)
+{
+	lockdep_assert_held(&dma_list_mutex);
+	return kref_get_unless_zero(&device->ref);
+}
+
 static void dma_device_put(struct dma_device *device)
 {
 	lockdep_assert_held(&dma_list_mutex);
@@ -460,8 +466,7 @@ static int dma_chan_get(struct dma_chan *chan)
 	if (!try_module_get(owner))
 		return -ENODEV;
 
-	ret = kref_get_unless_zero(&chan->device->ref);
-	if (!ret) {
+	if (!dma_device_get(chan->device)) {
 		ret = -ENODEV;
 		goto module_put_out;
 	}

-- 
2.43.0

[PATCH v5 2/4] 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;
	if (!dma_device_get(chan->device)) { // calls kref_get_unless_zero()

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")
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
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 cb3b7bccce1f..f2760b039d93 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -520,7 +520,9 @@ 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 v5 3/4] 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.

dma_chan_put() then still reads chan->device->owner via
dma_chan_to_owner() for the trailing module_put(). KASAN catches it:

	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.

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
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
 drivers/dma/dmaengine.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index f2760b039d93..9214b9272a55 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -500,10 +500,13 @@ 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 */
@@ -523,7 +526,7 @@ 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)

-- 
2.43.0

[PATCH v5 4/4] dmaengine: wait for RCU readers before releasing dma_device

[Shivank Garg]

dma_issue_pending_all() walks the dma_device_list with
list_for_each_entry_rcu() under rcu_read_lock(). dma_device_release()
unlinks the device with list_del_rcu() and then calls
device->device_release() (which in many drivers, such as plx_dma.c,
directly calls kfree()).

Because there is no grace period between unlinking the device and
freeing it, concurrent RCU readers in dma_issue_pending_all() can
access the device after it has been freed.

The lockless walk originally relied on clients holding a dmaengine
reference to pin the provider module, and therefore the device, for as
long as they might traverse the list. Commit 8ad342a86359 ("dmaengine:
Add reference counting to dma_device struct") decoupled the dma_device
lifetime from the module reference, so the device can now be released
while a reader is still walking the list.

Add synchronize_rcu() before the device is freed, so RCU readers are
guaranteed to have finished. Keep it unconditional: providers that do
not implement device_release() free the device themselves once
dma_async_device_unregister() returns. This call will delay for a grace
period with dma_list_mutex held, which is safe and only teardown path is
delayed.

Fixes: 2ba05622b8b1 ("dmaengine: provide a common 'issue_pending_all' implementation")
Suggested-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260526-dmaengine-kref-fix-v2-0-3df60afac01d@amd.com
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Shivank Garg <shivankg@amd.com>
---
 drivers/dma/dmaengine.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/dma/dmaengine.c b/drivers/dma/dmaengine.c
index 9214b9272a55..f9bc3e69fe58 100644
--- a/drivers/dma/dmaengine.c
+++ b/drivers/dma/dmaengine.c
@@ -428,6 +428,7 @@ static void dma_device_release(struct kref *ref)
 
 	list_del_rcu(&device->global_node);
 	dma_channel_rebalance();
+	synchronize_rcu();
 
 	if (device->device_release)
 		device->device_release(device);

-- 
2.43.0