[PATCH RFC] media: mc, vivid: fix use-after-free in media_request_release()

A use-after-free bug occurs in media_request_release() when the vivid
driver is unbound while user-space still holds open file descriptors for
media requests. The root cause is that media_request objects do not hold a
reference to the media_device (or its container, v4l2_device), allowing the
device to be freed prematurely.

When the device is unbound, vivid_remove() unregisters the media device and
drops the reference to the v4l2_device. If there are no other open video
nodes, the v4l2_device is released, freeing the vivid_dev structure which
embeds the media_device. Later, when user-space closes the request file
descriptor, media_request_release() accesses the freed media_device to call
mdev->ops->req_free and decrement mdev->num_requests, leading to a KASAN
slab-use-after-free crash.

To fix this, implement the req_alloc and req_free callbacks in the vivid
driver to take and drop a reference to the v4l2_device (using
v4l2_device_get() and v4l2_device_put()), ensuring the device remains alive
as long as there are active requests.

Additionally, in mc-request.c, move the atomic_dec(&mdev->num_requests)
call before mdev->ops->req_free(req) in media_request_release(). This is
necessary because req_free() might drop the last reference to the
v4l2_device, causing the media_device to be freed immediately. Accessing
mdev->num_requests after req_free() would result in another use-after-free.

BUG: KASAN: slab-use-after-free in media_request_release
drivers/media/mc/mc-request.c:74 [inline]
BUG: KASAN: slab-use-after-free in kref_put include/linux/kref.h:65
[inline]
BUG: KASAN: slab-use-after-free in media_request_put+0xb5/0x1e0
drivers/media/mc/mc-request.c:83
Read of size 8 at addr ffff88818de003d0 by task syz-executor478/5852

Call Trace:
 media_request_release drivers/media/mc/mc-request.c:74 [inline]
 kref_put include/linux/kref.h:65 [inline]
 media_request_put+0xb5/0x1e0 drivers/media/mc/mc-request.c:83
 media_request_close+0x38/0x50 drivers/media/mc/mc-request.c:91
 __fput+0x418/0xa50 fs/file_table.c:512
 fput_close_sync+0x11f/0x240 fs/file_table.c:617
 __do_sys_close fs/open.c:1511 [inline]
 __se_sys_close fs/open.c:1496 [inline]
 __x64_sys_close+0x7e/0x110 fs/open.c:1496

Fixes: 10905d70d788 ("media: media-request: implement media requests")
Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=3910ee87c4ba0a51e3f9
Link: https://syzkaller.appspot.com/ai_job?id=3e0de0f5-6fdc-4089-9f0c-90a26880ecf4
To: "Hans Verkuil" <[email protected]>
To: "Laurent Pinchart" <[email protected]>
To: <[email protected]>
To: "Mauro Carvalho Chehab" <[email protected]>
To: "Sakari Ailus" <[email protected]>
To: "Hans Verkuil" <[email protected]>
Cc: <[email protected]>

---
diff --git a/drivers/media/mc/mc-request.c b/drivers/media/mc/mc-request.c
index 13e776488..54bb08e65 100644
--- a/drivers/media/mc/mc-request.c
+++ b/drivers/media/mc/mc-request.c
@@ -71,11 +71,11 @@ static void media_request_release(struct kref *kref)
 
 	media_request_clean(req);
 
+	atomic_dec(&mdev->num_requests);
 	if (mdev->ops->req_free)
 		mdev->ops->req_free(req);
 	else
 		kfree(req);
-	atomic_dec(&mdev->num_requests);
 }
 
 void media_request_put(struct media_request *req)
diff --git a/drivers/media/test-drivers/vivid/vivid-core.c b/drivers/media/test-drivers/vivid/vivid-core.c
index 62cfb5feb..2374f4865 100644
--- a/drivers/media/test-drivers/vivid/vivid-core.c
+++ b/drivers/media/test-drivers/vivid/vivid-core.c
@@ -869,6 +869,27 @@ static void vivid_dev_release(struct v4l2_device *v4l2_dev)
 }
 
 #ifdef CONFIG_MEDIA_CONTROLLER
+static struct media_request *vivid_req_alloc(struct media_device *mdev)
+{
+	struct vivid_dev *dev = container_of(mdev, struct vivid_dev, mdev);
+	struct media_request *req;
+
+	req = kzalloc_obj(*req);
+	if (!req)
+		return NULL;
+
+	v4l2_device_get(&dev->v4l2_dev);
+	return req;
+}
+
+static void vivid_req_free(struct media_request *req)
+{
+	struct vivid_dev *dev = container_of(req->mdev, struct vivid_dev, mdev);
+
+	kfree(req);
+	v4l2_device_put(&dev->v4l2_dev);
+}
+
 static int vivid_req_validate(struct media_request *req)
 {
 	struct vivid_dev *dev = container_of(req->mdev, struct vivid_dev, mdev);
@@ -881,6 +902,8 @@ static int vivid_req_validate(struct media_request *req)
 }
 
 static const struct media_device_ops vivid_media_ops = {
+	.req_alloc = vivid_req_alloc,
+	.req_free = vivid_req_free,
 	.req_validate = vivid_req_validate,
 	.req_queue = vb2_request_queue,
 };


base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f
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