USB: Fix race condition when removing host controllers

This patch (as1607) fixes a race that can occur if a USB host
controller is removed while a process is reading the
/sys/kernel/debug/usb/devices file.

The usb_device_read() routine uses the bus->root_hub pointer to
determine whether or not the root hub is registered.  The is not a
valid test, because the pointer is set before the root hub gets
registered and remains set even after the root hub is unregistered and
deallocated.  As a result, usb_device_read() or usb_device_dump() can
access freed memory, causing an oops.

The patch changes the test to use the hcd->rh_registered flag, which
does get set and cleared at the appropriate times.  It also makes sure
to hold the usb_bus_list_lock mutex while setting the flag, so that
usb_device_read() will become aware of new root hubs as soon as they
are registered.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Reported-by: Don Zickus <dzickus@redhat.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/devices.c

@@ -623,7 +623,7 @@ static ssize_t usb_device_read(struct file *file, char __user *buf,
 	/* print devices for all busses */
 	list_for_each_entry(bus, &usb_bus_list, bus_list) {
 		/* recurse through all children of the root hub */
-		if (!bus->root_hub)
+		if (!bus_to_hcd(bus)->rh_registered)
 			continue;
 		usb_lock_device(bus->root_hub);
 		ret = usb_device_dump(&buf, &nbytes, &skip_bytes, ppos,

drivers/usb/core/hcd.c

@@ -1011,10 +1011,7 @@ static int register_root_hub(struct usb_hcd *hcd)
 	if (retval) {
 		dev_err (parent_dev, "can't register root hub for %s, %d\n",
 				dev_name(&usb_dev->dev), retval);
-	}
-	mutex_unlock(&usb_bus_list_lock);
-
-	if (retval == 0) {
+	} else {
 		spin_lock_irq (&hcd_root_hub_lock);
 		hcd->rh_registered = 1;
 		spin_unlock_irq (&hcd_root_hub_lock);
@@ -1023,6 +1020,7 @@ static int register_root_hub(struct usb_hcd *hcd)
 		if (HCD_DEAD(hcd))
 			usb_hc_died (hcd);	/* This time clean up */
 	}
+	mutex_unlock(&usb_bus_list_lock);
 
 	return retval;
 }