Authorizing (or not) your USB devices to connect to the system

(C) 2007 Inaky Perez-Gonzalez <inaky@linux.intel.com> Intel Corporation

This feature allows you to control if a USB device can be used (or

not) in a system. This feature will allow you to implement a lock-down

of USB devices, fully controlled by user space.

As of now, when a USB device is connected it is configured and

its interfaces are immediately made available to the users.  With this

modification, only if root authorizes the device to be configured will

then it be possible to use it.

Usage:

Authorize a device to connect:

$ echo 1 > /sys/bus/usb/devices/DEVICE/authorized

Deauthorize a device:

$ echo 0 > /sys/bus/usb/devices/DEVICE/authorized

Set new devices connected to hostX to be deauthorized by default (ie:

lock down):

$ echo 0 > /sys/bus/usb/devices/usbX/authorized_default

Remove the lock down:

$ echo 1 > /sys/bus/usb/devices/usbX/authorized_default

By default, Wired USB devices are authorized by default to

connect. Wireless USB hosts deauthorize by default all new connected

devices (this is so because we need to do an authentication phase

before authorizing).

Example system lockdown (lame)

-----------------------

Imagine you want to implement a lockdown so only devices of type XYZ

can be connected (for example, it is a kiosk machine with a visible

USB port):

boot up

rc.local ->

 for host in /sys/bus/usb/devices/usb*

 do

    echo 0 > $host/authorized_default

 done

Hookup an script to udev, for new USB devices

 if device_is_my_type $DEV

 then

   echo 1 > $device_path/authorized

 done

Now, device_is_my_type() is where the juice for a lockdown is. Just

checking if the class, type and protocol match something is the worse

security verification you can make (or the best, for someone willing

to break it). If you need something secure, use crypto and Certificate

Authentication or stuff like that. Something simple for an storage key

could be:

function device_is_my_type()

{

   echo 1 > authorized		# temporarily authorize it

                                # FIXME: make sure none can mount it

   mount DEVICENODE /mntpoint

   sum=$(md5sum /mntpoint/.signature)

   if [ $sum = $(cat /etc/lockdown/keysum) ]

   then

        echo "We are good, connected"

        umount /mntpoint

        # Other stuff so others can use it

   else

        echo 0 > authorized

   fi

}

Of course, this is lame, you'd want to do a real certificate

verification stuff with PKI, so you don't depend on a shared secret,

etc, but you get the idea. Anybody with access to a device gadget kit

can fake descriptors and device info. Don't trust that. You are

welcome.

Interface authorization

-----------------------

There is a similar approach to allow or deny specific USB interfaces.

That allows to block only a subset of an USB device.

Authorize an interface:

$ echo 1 > /sys/bus/usb/devices/INTERFACE/authorized

Deauthorize an interface:

$ echo 0 > /sys/bus/usb/devices/INTERFACE/authorized

The default value for new interfaces

on a particular USB bus can be changed, too.

Allow interfaces per default:

$ echo 1 > /sys/bus/usb/devices/usbX/interface_authorized_default

Deny interfaces per default:

$ echo 0 > /sys/bus/usb/devices/usbX/interface_authorized_default

Per default the interface_authorized_default bit is 1.

So all interfaces would authorized per default.

Note:

If a deauthorized interface will be authorized so the driver probing must

be triggered manually by writing INTERFACE to /sys/bus/usb/drivers_probe

For drivers that need multiple interfaces all needed interfaces should be

authroized first. After that the drivers should be probed.

This avoids side effects.