[PATCH 2/4] USB: idmouse: 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: 54d2bc068fd2 ("USB: fix locking in idmouse")
Cc: stable@vger.kernel.org	# 2.6.24
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/misc/idmouse.c | 45 +++++++++++++++++++-------------------
 1 file changed, 22 insertions(+), 23 deletions(-)

diff --git a/drivers/usb/misc/idmouse.c b/drivers/usb/misc/idmouse.c
index 0f6b3464c2d6..3e37adf2bb57 100644
--- a/drivers/usb/misc/idmouse.c
+++ b/drivers/usb/misc/idmouse.c
@@ -63,6 +63,7 @@ MODULE_DEVICE_TABLE(usb, idmouse_table);
 
 /* structure to hold all of our device specific stuff */
 struct usb_idmouse {
+	struct kref kref;
 
 	struct usb_device *udev; /* save off the usb device pointer */
 	struct usb_interface *interface; /* the interface for this device */
@@ -209,8 +210,10 @@ static int idmouse_resume(struct usb_interface *intf)
 	return 0;
 }
 
-static inline void idmouse_delete(struct usb_idmouse *dev)
+static inline void idmouse_delete(struct kref *kref)
 {
+	struct usb_idmouse *dev = container_of(kref, struct usb_idmouse, kref);
+
 	kfree(dev->bulk_in_buffer);
 	kfree(dev);
 }
@@ -254,6 +257,8 @@ static int idmouse_open(struct inode *inode, struct file *file)
 		/* increment our usage count for the driver */
 		++dev->open;
 
+		kref_get(&dev->kref);
+
 		/* save our object in the file's private structure */
 		file->private_data = dev;
 
@@ -277,16 +282,11 @@ static int idmouse_release(struct inode *inode, struct file *file)
 
 	/* lock our device */
 	mutex_lock(&dev->lock);
-
 	--dev->open;
+	mutex_unlock(&dev->lock);
+
+	kref_put(&dev->kref, idmouse_delete);
 
-	if (!dev->present) {
-		/* the device was unplugged before the file was released */
-		mutex_unlock(&dev->lock);
-		idmouse_delete(dev);
-	} else {
-		mutex_unlock(&dev->lock);
-	}
 	return 0;
 }
 
@@ -334,6 +334,7 @@ static int idmouse_probe(struct usb_interface *interface,
 	if (dev == NULL)
 		return -ENOMEM;
 
+	kref_init(&dev->kref);
 	mutex_init(&dev->lock);
 	dev->udev = udev;
 	dev->interface = interface;
@@ -342,8 +343,7 @@ static int idmouse_probe(struct usb_interface *interface,
 	result = usb_find_bulk_in_endpoint(iface_desc, &endpoint);
 	if (result) {
 		dev_err(&interface->dev, "Unable to find bulk-in endpoint.\n");
-		idmouse_delete(dev);
-		return result;
+		goto err_put_kref;
 	}
 
 	dev->orig_bi_size = usb_endpoint_maxp(endpoint);
@@ -351,8 +351,8 @@ static int idmouse_probe(struct usb_interface *interface,
 	dev->bulk_in_endpointAddr = endpoint->bEndpointAddress;
 	dev->bulk_in_buffer = kmalloc(IMGSIZE + dev->bulk_in_size, GFP_KERNEL);
 	if (!dev->bulk_in_buffer) {
-		idmouse_delete(dev);
-		return -ENOMEM;
+		result = -ENOMEM;
+		goto err_put_kref;
 	}
 
 	/* allow device read, write and ioctl */
@@ -364,14 +364,18 @@ static int idmouse_probe(struct usb_interface *interface,
 	if (result) {
 		/* something prevented us from registering this device */
 		dev_err(&interface->dev, "Unable to allocate minor number.\n");
-		idmouse_delete(dev);
-		return result;
+		goto err_put_kref;
 	}
 
 	/* be noisy */
 	dev_info(&interface->dev,"%s now attached\n",DRIVER_DESC);
 
 	return 0;
+
+err_put_kref:
+	kref_put(&dev->kref, idmouse_delete);
+
+	return result;
 }
 
 static void idmouse_disconnect(struct usb_interface *interface)
@@ -387,14 +391,9 @@ static void idmouse_disconnect(struct usb_interface *interface)
 	/* prevent device read, write and ioctl */
 	dev->present = 0;
 
-	/* if the device is opened, idmouse_release will clean this up */
-	if (!dev->open) {
-		mutex_unlock(&dev->lock);
-		idmouse_delete(dev);
-	} else {
-		/* unlock */
-		mutex_unlock(&dev->lock);
-	}
+	mutex_unlock(&dev->lock);
+
+	kref_put(&dev->kref, idmouse_delete);
 
 	dev_info(&interface->dev, "disconnected\n");
 }
-- 
2.53.0