qubes-doc/common-tasks/usb.md

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doc	Using and Managing USB Devices	/doc/usb/	/doc/stick-mounting/ /en/doc/stick-mounting/ /doc/StickMounting/ /wiki/StickMounting/ /doc/usbvm/ /en/doc/usbvm/ /doc/USBVM/ /wiki/USBVM/ /doc/sys-usb/

Using and Managing USB Devices

How to attach USB drives

(Note: In the present context, the term "USB drive" denotes any USB mass storage device. In addition to smaller flash memory sticks, this includes things like USB external hard drives.)

Qubes OS supports the ability to attach a USB drive (or just one or more of its partitions) to any qube easily, no matter which qube actually handles the USB controller. (The USB controller may be assigned on the Devices tab of a qube's settings page in Qubes VM Manager or by using the qvm-pci command. For guidance on finding the correct USB controller, see here.)

USB drive mounting is integrated into the Qubes VM Manager GUI. Simply insert your USB drive, right-click on the desired qube in the Qubes VM Manager list, click Attach/detach block devices, and select your desired action and device. This, however, only works for the whole device. If you would like to attach individual partitions, you must use the command-line tool.

The command-line tool you may use to mount whole USB drives or their partitions is qvm-block. This tool can be used to assign a USB drive to a qube as follows:

1. Insert your USB drive.

2. In a dom0 console (running as a normal user), list all available block devices:

   qvm-block -l
   

   This will list all available block devices connected to any USB controller in your system, no matter which qube hosts the controller. The name of the qube hosting the USB controller is displayed before the colon in the device name. The string after the colon is the name of the device used within the qube, like so:

   dom0:sdb1     Cruzer () 4GiB
   
   usbVM:sdb1    Disk () 2GiB
   

   Note: If your device is not listed here, you may refresh the list by calling (from the qube to which the device is connected):

   sudo udevadm trigger --action=change
   

3. Assuming your USB drive is attached to dom0 and is sdb, we attach the device to a qube like so:

   qvm-block -a personal dom0:sdb

   This will attach the device to the qube as /dev/xvdi if that name is not already taken by another attached device, or /dev/xvdj, etc.

   You may also mount one partition at a time by using the same command with the partition number after sdb.

   Warning: when working with single partitions, it is possible to assign the same partition to multiple qubes. For example, you could attach sdb1 to qube1 and then sdb to qube2. It is up to the user not to make this mistake. The Xen block device framework currently does not provide an easy way around this. Point 2 of this comment on issue 1072 gives details about this.

4. The USB drive is now attached to the qube. If using a default qube, you may open the Nautilus file manager in the qube, and your drive should be visible in the Devices panel on the left.

5. When you finish using your USB drive, click the eject button or right-click and select Unmount.

6. In a dom0 console, detach the stick:

   qvm-block -d

7. You may now remove the device.

Warning: Do not remove the device before detaching it from the VM! Otherwise, you will not be able to attach it anywhere later. See issue 1082 for details.

There have been reports that when attaching a single partition, the Nautilus file manager would not see it and automatically mount it (see issue 623). This problem seems to be resolved (see this comment on issue 1072).

If, however, the device does not appear in Nautilus, you will need to mount it manually. The device will show up as /dev/xvdi (or /dev/xvdj if there is already one device attached -- if two, /dev/xvdk, and so on).

What if I removed the device before detaching it from the VM?

Currently (until issue 1082 gets implemented), if you remove the device before detaching it from the qube, Qubes OS (more precisely, libvirtd) will think that the device is still attached to the qube and will not allow attaching further devices under the same name. The easiest way to recover from such a situation is to reboot the qube to which the device was attached, but if this isn't an option, you can manually recover from the situation by following these steps:

1. Physically connect the device back. You can use any device as long as it will be detected under the same name (for example, sdb).

2. Attach the device manually to the same VM using the xl block-attach command. It is important to use the same "frontend" device name (by default, xvdi). You can get it from the qvm-block listing:

   [user@dom0 ~]$ qvm-block
   sys-usb:sda DataTraveler_2.0 () 246 MiB (attached to 'testvm' as 'xvdi')
   [user@dom0 ~]$ xl block-attach testvm phy:/dev/sda backend=sys-usb xvdi
   

   In above example, all xl block-attach parameters can be deduced from the output of qvm-block. In order:

   • testvm - name of target qube to which device was attached - listed in brackets by qvm-block command
   • phy:/dev/sda - physical path at which device appears in source qube (just after source qube name in qvm-block output)
   • backend=sys-usb - name of source qube, can be omitted in case of dom0
   • xvdi - "frontend" device name (listed at the end of line in qvm-block output)

3. Now properly detach the device, either using Qubes VM Manager or the qvm-block -d command.

Creating and Using a USB qube

The connection of an untrusted USB device to dom0 is a security risk since dom0, like almost every OS, reads partition tables automatically and since the whole USB stack is put to work to parse the data presented by the USB device in order to determine if it is a USB mass storage device, to read its configuration, etc. This happens even if the drive is then assigned and mounted in another qube.

To avoid this risk, it is possible to prepare and utilize a USB qube. However, Xen does not yet provide working PVUSB functionality, so only USB mass storage devices can be passed to individual qubes.

For this reason, you may wish to avoid using a USB qube if you do not have a USB controller free of input devices and programmable devices, although Qubes R3.1 introduced support for USB mice and keyboards (see below).

A USB qube acts as a secure handler for potentially malicious USB devices, preventing them from coming into contact with dom0 (which could otherwise be fatal to the security of the whole system). With a USB qube, every time you connect an untrusted USB drive to a USB port managed by that USB controller, you will have to attach it to the qube in which you wish to use it (if different from the USB qube itself), either by using Qubes VM Manager or the command line (see instructions above). Again, this works only for USB mass storage devices. Other devices cannot currently be virtualized.

You can create a USB qube using the management stack by performing the following steps as root in dom0:

1. Enable sys-usb:

   qubesctl top.enable qvm.sys-usb
   

2. Apply the configuration:

   qubesctl state.highstate
   

Alternatively, you can create a USB qube manually as follows:

1. Read the Assigning Devices page to learn how to list and identify your USB controllers. Carefully check whether you have a USB controller that would be appropriate to assign to a USB qube. Note that it should be free of input devices, programmable devices, and any other devices that must be directly available to dom0. If you find a free controller, note its name and proceed to step 2.

2. Create a new qube. Give it an appropriate name and color label (recommended: sys-usb, red). If you need to attach a networking device, it might make sense to create a NetVM. If not, an AppVM might make more sense. (The default sys-usb is a NetVM.)

3. In the qube's settings, go to the "Devices" tab. Find the USB controller that you identified in step 1 in the "Available" list. Move it to the "Selected" list.

   Caution: By assigning a USB controller to a USB qube, it will no longer be available to dom0. This can make your system unusable if, for example, you have only one USB controller, and you are running Qubes off of a USB drive.

4. Click "OK." Restart the qube.

5. Recommended: Check the box on the "Basic" tab which says "Start VM automatically on boot." (This will help to mitigate attacks in which someone forces your system to reboot, then plugs in a malicious USB device.)

If the USB qube will not start, see here.

Supported USB device types

As of Qubes R3.1, it is possible to attach:

• USB mice
• USB keyboards (see below)
• USB block devices (such as USB mass storage devices)
  • When attaching one of these, you should get a notification about the new device, then you should be able to attach it to a qube in Qubes VM Manager.

Other devices, such as USB webcams, will also work, but they will be accessible only from the USB qube itself, as explained above.

How to use a USB keyboard

In order to use a USB keyboard, you must first attach it to a USB qube, then give that qube permission to pass keyboard input to dom0. Note that allowing keyboard access from a USB qube gives it great power. In short:

• It will see whatever you type on that keyboard (including passwords).
• It will be able to inject keystrokes, which basically means that it will be able to enter any command. For example, if some malware catches your screenlocker password, it will be able to unlock the screen when you are not present. (For more details, see here.)

If you are sure you wish to proceed, then you must edit the qubes.InputKeyboard policy file in dom0, which is located here:

/etc/qubes-rpc/policy/qubes.InputKeyboard

Add a line like this one to the top of the file:

sys-usb dom0 ask

(Change sys-usb to your desired USB qube.)

You can now use your USB keyboard.