[PATCH v1 3/3] media: sun4i-csi: add notifier unbind callback to drop the source subdev

[Cong Nguyen <congnt264@gmail.com>]

sun4i_csi_notify_ops only implements .bound and .complete. When the remote
sensor's subdevice goes away (e.g. its module is unloaded), the V4L2 async
core unbinds and frees it, but the driver keeps the stale pointer in
csi->src_subdev and leaves the video node registered.

A subsequent VIDIOC_STREAMON reaches sun4i_csi_start_streaming(), which
calls v4l2_subdev_call(csi->src_subdev, video, s_stream, 1) on the freed
subdev, resulting in a use-after-free.

Add an .unbind callback that unregisters the video device so userspace can
no longer start streaming, and clears csi->src_subdev. Unregistering the
already-unregistered video device again in sun4i_csi_remove() is harmless
(vb2_video_unregister_device() is a no-op when it is not registered).

Fixes: 577bbf23b758 ("media: sunxi: Add A10 CSI driver")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4
Signed-off-by: Cong Nguyen <congnt264@gmail.com>
---
 .../media/platform/sunxi/sun4i-csi/sun4i_csi.c  | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/media/platform/sunxi/sun4i-csi/sun4i_csi.c b/drivers/media/platform/sunxi/sun4i-csi/sun4i_csi.c
index a8711336a754..6610ada1c06d 100644
--- a/drivers/media/platform/sunxi/sun4i-csi/sun4i_csi.c
+++ b/drivers/media/platform/sunxi/sun4i-csi/sun4i_csi.c
@@ -122,8 +122,25 @@ static int sun4i_csi_notify_complete(struct v4l2_async_notifier *notifier)
 	return ret;
 }
 
+static void sun4i_csi_notify_unbind(struct v4l2_async_notifier *notifier,
+				    struct v4l2_subdev *subdev,
+				    struct v4l2_async_connection *asd)
+{
+	struct sun4i_csi *csi = container_of(notifier, struct sun4i_csi,
+					     notifier);
+
+	/*
+	 * The remote subdev is being freed. Tear down the video node so
+	 * userspace can no longer reach sun4i_csi_start_streaming() and
+	 * dereference the now dangling source subdev, and drop the pointer.
+	 */
+	vb2_video_unregister_device(&csi->vdev);
+	csi->src_subdev = NULL;
+}
+
 static const struct v4l2_async_notifier_operations sun4i_csi_notify_ops = {
 	.bound		= sun4i_csi_notify_bound,
+	.unbind		= sun4i_csi_notify_unbind,
 	.complete	= sun4i_csi_notify_complete,
 };
 
-- 
2.25.1