[PATCH 4/4] USB: legousbtower: fix use-after-free on disconnect race

[Johan Hovold <johan@kernel.org>]

mutex_unlock() may access the mutex structure after releasing the lock
and therefore cannot be used to manage lifetime of objects directly
(unlike spinlocks and refcounts). [1][2]

Use a kref to release the driver data to avoid use-after-free in
mutex_unlock() when release() races with disconnect().

[1] a51749ab34d9 ("locking/mutex: Document that mutex_unlock() is
                   non-atomic")
[2] 2b9d9e0a9ba0 ("locking/mutex: Clarify that mutex_unlock(), and most
                   other sleeping locks, can still use the lock object
                   after it's unlocked")

Fixes: 18bcbcfe9ca2 ("USB: misc: legousbtower: semaphore to mutex")
Cc: stable@vger.kernel.org	# 2.6.25
Cc: Daniel Walker <dwalker@mvista.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/legousbtower.c | 37 +++++++++++++++++----------------
 1 file changed, 19 insertions(+), 18 deletions(-)

diff --git a/drivers/usb/misc/legousbtower.c b/drivers/usb/misc/legousbtower.c
index 052ffc2e71ee..18dd4115befb 100644
--- a/drivers/usb/misc/legousbtower.c
+++ b/drivers/usb/misc/legousbtower.c
@@ -185,6 +185,7 @@ MODULE_DEVICE_TABLE(usb, tower_table);
 
 /* Structure to hold all of our device specific stuff */
 struct lego_usb_tower {
+	struct kref		kref;
 	struct mutex		lock;		/* locks this structure */
 	struct usb_device	*udev;		/* save off the usb device pointer */
 	unsigned char		minor;		/* the starting minor number for this device */
@@ -220,7 +221,6 @@ struct lego_usb_tower {
 /* local function prototypes */
 static ssize_t tower_read(struct file *file, char __user *buffer, size_t count, loff_t *ppos);
 static ssize_t tower_write(struct file *file, const char __user *buffer, size_t count, loff_t *ppos);
-static inline void tower_delete(struct lego_usb_tower *dev);
 static int tower_open(struct inode *inode, struct file *file);
 static int tower_release(struct inode *inode, struct file *file);
 static __poll_t tower_poll(struct file *file, poll_table *wait);
@@ -286,8 +286,10 @@ static inline void lego_usb_tower_debug_data(struct device *dev,
 /*
  *	tower_delete
  */
-static inline void tower_delete(struct lego_usb_tower *dev)
+static inline void tower_delete(struct kref *kref)
 {
+	struct lego_usb_tower *dev = container_of(kref, struct lego_usb_tower, kref);
+
 	/* free data structures */
 	usb_free_urb(dev->interrupt_in_urb);
 	usb_free_urb(dev->interrupt_out_urb);
@@ -381,6 +383,8 @@ static int tower_open(struct inode *inode, struct file *file)
 
 	dev->open_count = 1;
 
+	kref_get(&dev->kref);
+
 unlock_exit:
 	mutex_unlock(&dev->lock);
 
@@ -404,14 +408,8 @@ static int tower_release(struct inode *inode, struct file *file)
 
 	mutex_lock(&dev->lock);
 
-	if (dev->disconnected) {
-		/* the device was unplugged before the file was released */
-
-		/* unlock here as tower_delete frees dev */
-		mutex_unlock(&dev->lock);
-		tower_delete(dev);
-		goto exit;
-	}
+	if (dev->disconnected)
+		goto out_unlock;
 
 	/* wait until write transfer is finished */
 	if (dev->interrupt_out_busy) {
@@ -425,7 +423,9 @@ static int tower_release(struct inode *inode, struct file *file)
 
 	dev->open_count = 0;
 
+out_unlock:
 	mutex_unlock(&dev->lock);
+	kref_put(&dev->kref, tower_delete);
 exit:
 	return retval;
 }
@@ -752,6 +752,7 @@ static int tower_probe(struct usb_interface *interface, const struct usb_device_
 	if (!dev)
 		goto exit;
 
+	kref_init(&dev->kref);
 	mutex_init(&dev->lock);
 	dev->udev = usb_get_dev(udev);
 	spin_lock_init(&dev->read_buffer_lock);
@@ -828,7 +829,7 @@ static int tower_probe(struct usb_interface *interface, const struct usb_device_
 	return retval;
 
 error:
-	tower_delete(dev);
+	kref_put(&dev->kref, tower_delete);
 	return retval;
 }
 
@@ -856,18 +857,18 @@ static void tower_disconnect(struct usb_interface *interface)
 
 	mutex_lock(&dev->lock);
 
-	/* if the device is not opened, then we clean up right now */
-	if (!dev->open_count) {
-		mutex_unlock(&dev->lock);
-		tower_delete(dev);
-	} else {
-		dev->disconnected = 1;
+	dev->disconnected = 1;
+
+	if (dev->open_count) {
 		/* wake up pollers */
 		wake_up_interruptible_all(&dev->read_wait);
 		wake_up_interruptible_all(&dev->write_wait);
-		mutex_unlock(&dev->lock);
 	}
 
+	mutex_unlock(&dev->lock);
+
+	kref_put(&dev->kref, tower_delete);
+
 	dev_info(&interface->dev, "LEGO USB Tower #%d now disconnected\n",
 		 (minor - LEGO_USB_TOWER_MINOR_BASE));
 }
-- 
2.53.0