133 lines
3.9 KiB
ReStructuredText
133 lines
3.9 KiB
ReStructuredText
==============================================================
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Authorizing (or not) your USB devices to connect to the system
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==============================================================
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Copyright (C) 2007 Inaky Perez-Gonzalez <inaky@linux.intel.com> Intel Corporation
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This feature allows you to control if a USB device can be used (or
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not) in a system. This feature will allow you to implement a lock-down
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of USB devices, fully controlled by user space.
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As of now, when a USB device is connected it is configured and
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its interfaces are immediately made available to the users. With this
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modification, only if root authorizes the device to be configured will
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then it be possible to use it.
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Usage
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=====
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Authorize a device to connect::
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$ echo 1 > /sys/bus/usb/devices/DEVICE/authorized
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De-authorize a device::
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$ echo 0 > /sys/bus/usb/devices/DEVICE/authorized
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Set new devices connected to hostX to be deauthorized by default (ie:
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lock down)::
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$ echo 0 > /sys/bus/usb/devices/usbX/authorized_default
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Remove the lock down::
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$ echo 1 > /sys/bus/usb/devices/usbX/authorized_default
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By default, Wired USB devices are authorized by default to
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connect. Wireless USB hosts deauthorize by default all new connected
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devices (this is so because we need to do an authentication phase
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before authorizing). Writing "2" to the authorized_default attribute
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causes kernel to only authorize by default devices connected to internal
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USB ports.
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Example system lockdown (lame)
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------------------------------
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Imagine you want to implement a lockdown so only devices of type XYZ
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can be connected (for example, it is a kiosk machine with a visible
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USB port)::
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boot up
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rc.local ->
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for host in /sys/bus/usb/devices/usb*
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do
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echo 0 > $host/authorized_default
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done
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Hookup an script to udev, for new USB devices::
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if device_is_my_type $DEV
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then
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echo 1 > $device_path/authorized
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done
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Now, device_is_my_type() is where the juice for a lockdown is. Just
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checking if the class, type and protocol match something is the worse
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security verification you can make (or the best, for someone willing
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to break it). If you need something secure, use crypto and Certificate
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Authentication or stuff like that. Something simple for an storage key
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could be::
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function device_is_my_type()
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{
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echo 1 > authorized # temporarily authorize it
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# FIXME: make sure none can mount it
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mount DEVICENODE /mntpoint
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sum=$(md5sum /mntpoint/.signature)
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if [ $sum = $(cat /etc/lockdown/keysum) ]
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then
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echo "We are good, connected"
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umount /mntpoint
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# Other stuff so others can use it
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else
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echo 0 > authorized
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fi
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}
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Of course, this is lame, you'd want to do a real certificate
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verification stuff with PKI, so you don't depend on a shared secret,
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etc, but you get the idea. Anybody with access to a device gadget kit
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can fake descriptors and device info. Don't trust that. You are
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welcome.
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Interface authorization
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-----------------------
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There is a similar approach to allow or deny specific USB interfaces.
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That allows to block only a subset of an USB device.
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Authorize an interface::
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$ echo 1 > /sys/bus/usb/devices/INTERFACE/authorized
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Deauthorize an interface::
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$ echo 0 > /sys/bus/usb/devices/INTERFACE/authorized
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The default value for new interfaces
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on a particular USB bus can be changed, too.
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Allow interfaces per default::
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$ echo 1 > /sys/bus/usb/devices/usbX/interface_authorized_default
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Deny interfaces per default::
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$ echo 0 > /sys/bus/usb/devices/usbX/interface_authorized_default
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Per default the interface_authorized_default bit is 1.
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So all interfaces would authorized per default.
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Note:
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If a deauthorized interface will be authorized so the driver probing must
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be triggered manually by writing INTERFACE to /sys/bus/usb/drivers_probe
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For drivers that need multiple interfaces all needed interfaces should be
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authorized first. After that the drivers should be probed.
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This avoids side effects.
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