usb: misc: legousbtower: Fix NULL pointer deference

This patch fixes a NULL pointer dereference caused by a race codition in
the probe function of the legousbtower driver. It re-structures the
probe function to only register the interface after successfully reading
the board's firmware ID.

The probe function does not deregister the usb interface after an error
receiving the devices firmware ID. The device file registered
(/dev/usb/legousbtower%d) may be read/written globally before the probe
function returns. When tower_delete is called in the probe function
(after an r/w has been initiated), core dev structures are deleted while
the file operation functions are still running. If the 0 address is
mappable on the machine, this vulnerability can be used to create a
Local Priviege Escalation exploit via a write-what-where condition by
remapping dev->interrupt_out_buffer in tower_write. A forged USB device
and local program execution would be required for LPE. The USB device
would have to delay the control message in tower_probe and accept
the control urb in tower_open whilst guest code initiated a write to the
device file as tower_delete is called from the error in tower_probe.

This bug has existed since 2003. Patch tested by emulated device.

Reported-by: James Patrick-Evans <james@jmp-e.com>
Tested-by: James Patrick-Evans <james@jmp-e.com>
Signed-off-by: James Patrick-Evans <james@jmp-e.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
