USB: prevent char device open/deregister race

This patch (as908) adds central protection in usbcore for the
prototypical race between opening and unregistering a char device.
The spinlock used to protect the minor-numbers array is replaced with
an rwsem, which can remain locked across a call to a driver's open()
method.  This guarantees that open() and deregister() will be mutually
exclusive.

The private locks currently used in several individual drivers for
this purpose are no longer necessary, and the patch removes them.  The
following USB drivers are affected: usblcd, idmouse, auerswald,
legousbtower, sisusbvga/sisusb, ldusb, adutux, iowarrior, and
usb-skeleton.

As a side effect of this change, usb_deregister_dev() must not be
called while holding a lock that is acquired by open().  Unfortunately
a number of drivers do this, but luckily the solution is simple: call
usb_deregister_dev() before acquiring the lock.

In addition to these changes (and their consequent code
simplifications), the patch fixes a use-after-free bug in adutux and a
race between open() and release() in iowarrior.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 8 insertions, 18 deletions

diff --git a/drivers/usb/misc/iowarrior.c b/drivers/usb/misc/iowarrior.c
index 3bb33f7bfa36..28548d186712 100644
--- a/drivers/usb/misc/iowarrior.c
+++ b/drivers/usb/misc/iowarrior.c
@@ -100,8 +100,6 @@ struct iowarrior {
 /*--------------*/
 /*    globals   */
 /*--------------*/
-/* prevent races between open() and disconnect() */
-static DECLARE_MUTEX(disconnect_sem);
 
 /*
  *  USB spec identifies 5 second timeouts.
@@ -600,22 +598,18 @@ static int iowarrior_open(struct inode *inode, struct file *file)
 
 	subminor = iminor(inode);
 
-	/* prevent disconnects */
-	down(&disconnect_sem);
-
 	interface = usb_find_interface(&iowarrior_driver, subminor);
 	if (!interface) {
 		err("%s - error, can't find device for minor %d", __FUNCTION__,
 		    subminor);
-		retval = -ENODEV;
-		goto out;
+		return -ENODEV;
 	}
 
 	dev = usb_get_intfdata(interface);
-	if (!dev) {
-		retval = -ENODEV;
-		goto out;
-	}
+	if (!dev)
+		return -ENODEV;
+
+	mutex_lock(&dev->mutex);
 
 	/* Only one process can open each device, no sharing. */
 	if (dev->opened) {
@@ -636,7 +630,7 @@ static int iowarrior_open(struct inode *inode, struct file *file)
 	retval = 0;
 
 out:
-	up(&disconnect_sem);
+	mutex_unlock(&dev->mutex);
 	return retval;
 }
 
@@ -868,19 +862,16 @@ static void iowarrior_disconnect(struct usb_interface *interface)
 	struct iowarrior *dev;
 	int minor;
 
-	/* prevent races with open() */
-	down(&disconnect_sem);
-
 	dev = usb_get_intfdata(interface);
 	usb_set_intfdata(interface, NULL);
 
-	mutex_lock(&dev->mutex);
-
 	minor = dev->minor;
 
 	/* give back our minor */
 	usb_deregister_dev(interface, &iowarrior_class);
 
+	mutex_lock(&dev->mutex);
+
 	/* prevent device read, write and ioctl */
 	dev->present = 0;
 
@@ -898,7 +889,6 @@ static void iowarrior_disconnect(struct usb_interface *interface)
 		/* no process is using the device, cleanup now */
 		iowarrior_delete(dev);
 	}
-	up(&disconnect_sem);
 
 	dev_info(&interface->dev, "I/O-Warror #%d now disconnected\n",
 		 minor - IOWARRIOR_MINOR_BASE);