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Subject[21/90] USB: Close usb_find_interface race v3
2.6.31-stable review patch.  If anyone has any objections, please let us know.


From: Russ Dill <>

commit c2d284ee04ab6f6718de2ddcf1b43160e046c41d upstream.

USB drivers that create character devices call usb_register_dev in their
probe function. This associates the usb_interface device with that minor
number and creates the character device and announces it to the world.
However, the driver's probe function is called before the new
usb_interface is added to the driver's klist_devices.

This is a problem because userspace will respond to the character device
creation announcement by opening the character device. The driver's open
function will the call usb_find_interface to find the usb_interface
associated with that minor number. usb_find_interface will walk the
driver's list of devices and find the usb_interface with the matching
minor number.

Because the announcement happens before the usb_interface is added to the
driver's klist_devices, a race condition exists. A straightforward fix
is to walk the list of devices on usb_bus_type instead since the device
is added to that list before the announcement occurs.

bus_find_device calls get_device to bump the reference count on the found
device. It is arguable that the reference count should be dropped by the
caller of usb_find_interface instead of usb_find_interface, however,
the current users of usb_find_interface do not expect this.

The original version of this patch only matched against minor number
instead of driver and minor number. This version matches against both.

Signed-off-by: Russ Dill <>
Signed-off-by: Greg Kroah-Hartman <>

drivers/usb/core/usb.c | 31 ++++++++++++++++---------------
1 file changed, 16 insertions(+), 15 deletions(-)

--- a/drivers/usb/core/usb.c
+++ b/drivers/usb/core/usb.c
@@ -132,7 +132,7 @@ EXPORT_SYMBOL_GPL(usb_altnum_to_altsetti

struct find_interface_arg {
int minor;
- struct usb_interface *interface;
+ struct device_driver *drv;

static int __find_interface(struct device *dev, void *data)
@@ -143,12 +143,10 @@ static int __find_interface(struct devic
if (!is_usb_interface(dev))
return 0;

+ if (dev->driver != arg->drv)
+ return 0;
intf = to_usb_interface(dev);
- if (intf->minor != -1 && intf->minor == arg->minor) {
- arg->interface = intf;
- return 1;
- }
- return 0;
+ return intf->minor == arg->minor;

@@ -156,21 +154,24 @@ static int __find_interface(struct devic
* @drv: the driver whose current configuration is considered
* @minor: the minor number of the desired device
- * This walks the driver device list and returns a pointer to the interface
- * with the matching minor. Note, this only works for devices that share the
- * USB major number.
+ * This walks the bus device list and returns a pointer to the interface
+ * with the matching minor and driver. Note, this only works for devices
+ * that share the USB major number.
struct usb_interface *usb_find_interface(struct usb_driver *drv, int minor)
struct find_interface_arg argb;
- int retval;
+ struct device *dev;

argb.minor = minor;
- argb.interface = NULL;
- /* eat the error, it will be in argb.interface */
- retval = driver_for_each_device(&drv->drvwrap.driver, NULL, &argb,
- __find_interface);
- return argb.interface;
+ argb.drv = &drv->drvwrap.driver;
+ dev = bus_find_device(&usb_bus_type, NULL, &argb, __find_interface);
+ /* Drop reference count from bus_find_device */
+ put_device(dev);
+ return dev ? to_usb_interface(dev) : NULL;

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