[PATCH] HID: hiddev: Use kref to track struct hiddev lifetime

[Heitor Alves de Siqueira]

If a USB HID device is disconnected while userspace still holds the
hiddev node open, hiddev_disconnect() and hiddev_release() can race on
the embedded existancelock mutex. Syzbot has triggered this with kfree()
happening during the mutex slow path.

Fix by introducing a kref in struct hiddev, and moving kfree() into a
dedicated release callback. This way, struct hiddev will only be freed
after both hiddev_release() and hiddev_disconnect() are done.

Fixes: 6cb4b040795c ("HID: hiddev: fix race between hiddev_disconnect and hiddev_release")
Reported-by: syzbot+563191a4939ddbfe73d4@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=563191a4939ddbfe73d4
Signed-off-by: Heitor Alves de Siqueira <halves@xxxxxxxxxx>
---
 drivers/hid/usbhid/hiddev.c | 36 ++++++++++++++++++++++++------------
 include/linux/hiddev.h      |  1 +
 2 files changed, 25 insertions(+), 12 deletions(-)

diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
index 6378801b22c6..1f2802920bee 100644
--- a/drivers/hid/usbhid/hiddev.c
+++ b/drivers/hid/usbhid/hiddev.c
@@ -46,6 +46,13 @@ struct hiddev_list {
 	struct mutex thread_lock;
 };
 
+static void hiddev_kref_free(struct kref *kref)
+{
+	struct hiddev *hiddev = container_of(kref, struct hiddev, kref);
+
+	kfree(hiddev);
+}
+
 /*
  * Find a report, given the report's type and ID.  The ID can be specified
  * indirectly by REPORT_ID_FIRST (which returns the first report of the given
@@ -227,15 +234,11 @@ static int hiddev_release(struct inode * inode, struct file * file)
 		if (list->hiddev->exist) {
 			hid_hw_close(list->hiddev->hid);
 			hid_hw_power(list->hiddev->hid, PM_HINT_NORMAL);
-		} else {
-			mutex_unlock(&list->hiddev->existancelock);
-			kfree(list->hiddev);
-			vfree(list);
-			return 0;
 		}
 	}
-
 	mutex_unlock(&list->hiddev->existancelock);
+
+	kref_put(&list->hiddev->kref, hiddev_kref_free);
 	vfree(list);
 
 	return 0;
@@ -298,10 +301,20 @@ static int hiddev_open(struct inode *inode, struct file *file)
 	hid = usb_get_intfdata(intf);
 	hiddev = hid->hiddev;
 
+	/*
+	 * kref_get_unless_zero() checks if we've already dropped
+	 * the last reference; bail out early in this case
+	 */
+	if (!hiddev || !kref_get_unless_zero(&hiddev->kref))
+		return -ENODEV;
+
 	mutex_lock(&hiddev->existancelock);
 	res = hiddev->exist ? __hiddev_open(hiddev, file) : -ENODEV;
 	mutex_unlock(&hiddev->existancelock);
 
+	if (res < 0)
+		kref_put(&hiddev->kref, hiddev_kref_free);
+
 	return res;
 }
 
@@ -893,6 +906,7 @@ int hiddev_connect(struct hid_device *hid, unsigned int force)
 	if (!(hiddev = kzalloc_obj(struct hiddev)))
 		return -ENOMEM;
 
+	kref_init(&hiddev->kref);
 	init_waitqueue_head(&hiddev->wait);
 	INIT_LIST_HEAD(&hiddev->list);
 	spin_lock_init(&hiddev->list_lock);
@@ -904,7 +918,7 @@ int hiddev_connect(struct hid_device *hid, unsigned int force)
 	if (retval) {
 		hid_err(hid, "Not able to get a minor for this device\n");
 		hid->hiddev = NULL;
-		kfree(hiddev);
+		kref_put(&hiddev->kref, hiddev_kref_free);
 		return retval;
 	}
 
@@ -933,13 +947,11 @@ void hiddev_disconnect(struct hid_device *hid)
 
 	mutex_lock(&hiddev->existancelock);
 	hiddev->exist = 0;
-
 	if (hiddev->open) {
 		hid_hw_close(hiddev->hid);
 		wake_up_interruptible(&hiddev->wait);
-		mutex_unlock(&hiddev->existancelock);
-	} else {
-		mutex_unlock(&hiddev->existancelock);
-		kfree(hiddev);
 	}
+	mutex_unlock(&hiddev->existancelock);
+
+	kref_put(&hiddev->kref, hiddev_kref_free);
 }
diff --git a/include/linux/hiddev.h b/include/linux/hiddev.h
index 2164c03d2c72..1613c9c37ab7 100644
--- a/include/linux/hiddev.h
+++ b/include/linux/hiddev.h
@@ -24,6 +24,7 @@ struct hiddev {
 	int minor;
 	int exist;
 	int open;
+	struct kref kref;
 	struct mutex existancelock;
 	wait_queue_head_t wait;
 	struct hid_device *hid;

---
base-commit: e43ffb69e0438cddd72aaa30898b4dc446f664f8
change-id: 20260603-hiddev_kref-52f5507d4e42

Best regards,
--  
Heitor Alves de Siqueira <halves@xxxxxxxxxx>