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Using and Managing USB Devices
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==============================
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Creating and Using a USB qube
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-----------------------------
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**Warning:** This has the potential to prevent you from connecting a keyboard to Qubes via USB. There are problems with doing this with a encrypted install (LUKS). If you find yourself in this situation, see this [issue][2270-comm23].
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The connection of an untrusted USB device to dom0 is a security risk since dom0,
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like almost every OS, reads partition tables automatically and since the whole
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USB stack is put to work to parse the data presented by the USB device in order
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to determine if it is a USB mass storage device, to read its configuration, etc.
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This happens even if the drive is then assigned and mounted in another qube.
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To avoid this risk, it is possible to prepare and utilize a USB qube.
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A USB qube acts as a secure handler for potentially malicious USB devices,
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preventing them from coming into contact with dom0 (which could otherwise be
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fatal to the security of the whole system). With a USB qube, every time you
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connect an untrusted USB drive to a USB port managed by that USB controller, you
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will have to attach it to the qube in which you wish to use it (if different
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from the USB qube itself), either by using Qubes VM Manager or the command line
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(see instructions above).
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You can create a USB qube using the management stack by performing the following
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steps as root in dom0:
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1. Enable `sys-usb`:
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qubesctl top.enable qvm.sys-usb
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2. Apply the configuration:
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qubesctl state.highstate
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Alternatively, you can create a USB qube manually as follows:
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1. Read the [Assigning Devices] page to learn how to list and identify your
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USB controllers. Carefully check whether you have a USB controller that
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would be appropriate to assign to a USB qube. Note that it should be free
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of input devices, programmable devices, and any other devices that must be
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directly available to dom0. If you find a free controller, note its name
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and proceed to step 2.
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2. Create a new qube. Give it an appropriate name and color label
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(recommended: `sys-usb`, red). If you need to attach a networking device,
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it might make sense to create a NetVM. If not, an AppVM might make more
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sense. (The default `sys-usb` is a NetVM.)
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3. In the qube's settings, go to the "Devices" tab. Find the USB controller
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that you identified in step 1 in the "Available" list. Move it to the
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"Selected" list.
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**Caution:** By assigning a USB controller to a USB qube, it will no longer
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be available to dom0. This can make your system unusable if, for example,
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you have only one USB controller, and you are running Qubes off of a USB
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drive.
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4. Click "OK." Restart the qube.
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5. Recommended: Check the box on the "Basic" tab which says "Start VM
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automatically on boot." (This will help to mitigate attacks in which
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someone forces your system to reboot, then plugs in a malicious USB
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device.)
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If the USB qube will not start, see [here][faq-usbvm].
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How to hide all USB controllers from dom0
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-----------------------------------------
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If you create a USB qube manually, there will be a brief period of time during the
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boot process during which dom0 will be exposed to your USB controllers (and any
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attached devices). This is a potential security risk, since even brief exposure
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to a malicious USB device could result in dom0 being compromised. There are two
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approaches to this problem:
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1. Physically disconnect all USB devices whenever you reboot the host.
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2. Hide (i.e., blacklist) all USB controllers from dom0.
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**Warning:** If you use a USB [AEM] device, do not use the second option. Using
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a USB AEM device requires dom0 to have access to the USB controller to which
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your USB AEM device is attached. If dom0 cannot read your USB AEM device, AEM
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will hang.
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The procedure to hide all USB controllers from dom0 is as follows:
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1. Open the file `/etc/default/grub` in dom0.
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2. Find the line that begins with `GRUB_CMDLINE_LINUX`.
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3. Add `rd.qubes.hide_all_usb` to that line.
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4. Save and close the file.
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5. Run the command `grub2-mkconfig -o /boot/grub2/grub.cfg` in dom0.
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6. Reboot.
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(Note: Beginning with R3.2, `rd.qubes.hide_all_usb` is set automatically if you
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opt to create a USB qube during installation. This also occurs automatically if
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you choose to [create a USB qube] using the `qubesctl` method, which is the
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first pair of steps in the linked section.)
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**Warning:** A USB keyboard cannot be used to type the disk passphrase
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if USB controllers were hidden from dom0. Before hiding USB controllers
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make sure your laptop keyboard is not internally connected via USB
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(by checking output of `lsusb` command) or that you have a PS/2 keyboard at hand
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(if using a desktop PC). Failure to do so will render your system unusable.
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Removing a USB qube
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-------------------
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**Warning:** This procedure will result in your USB controller(s) being attached
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directly to dom0.
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1. Shut down the USB qube.
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2. In Qubes Manager, right-click on the USB qube and select "Remove VM."
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3. Open the file `/etc/default/grub` in dom0.
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4. Find the line(s) that begins with `GRUB_CMDLINE_LINUX`.
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5. If `rd.qubes.hide_all_usb` appears anywhere in those lines, remove it.
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6. Save and close the file.
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7. Run the command `grub2-mkconfig -o /boot/grub2/grub.cfg` in dom0.
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8. Reboot.
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Security Warning about USB Input Devices
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----------------------------------------
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**Important security warning. Please read this section carefully!**
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If you connect USB input devices (keyboard and mouse) to a VM, that VM will effectively have control over your system.
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Because of this, the benefits of using a USB qube are much smaller than using a fully untrusted USB qube.
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In addition to having control over your system, such VM can also sniff all the input you enter there (for example, passwords in the case of a USB keyboard).
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There is no simple way to protect against sniffing, but you can make it harder to exploit control over input devices.
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If you have only a USB mouse connected to a USB qube, but the keyboard is connected directly to dom0 (using a PS/2 connector, for example), you simply need to lock the screen when you are away from your computer.
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You must do this every time you leave your computer unattended, even if there no risk of anyone else having direct physical access to your computer.
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This is because you are guarding the system not only against anyone with local access, but also against possible actions from a potentially compromised USB qube.
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How to attach USB drives
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----------
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(**Note:** In the present context, the term "USB drive" denotes any
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[USB mass storage device][mass-storage]. In addition to smaller flash memory
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