Secure Desktops with Qubes: Extra Protection

USB VM

One of the major risks of compromise against a personal computer is the USB port. You can find a large number of stories on the internet about organizations (including governments) who were compromised because someone plugged in an untrusted USB key. There are even some fun hardware projects out there like the USB Rubber Ducky that provide what looks like an innocent USB thumbdrive but can act like a USB input device when you plug it in, and with its scripting language, you can program it to type whatever compromising keystrokes you want against your victim (including waiting some time later before unleashing your payload).

Given that just about anyone can create a malicious USB device now, you definitely want to be careful about what USB devices you plug in. Even Qubes installs may suffer the same risk, because by default, the dom0 VM is assigned the USB PCI controllers, so if you mistakenly plug in an infected USB key, it could potentially compromise your whole machine. Thankfully, Qubes provides a countermeasure for this with the option of creating a special USB VM that is assigned all of your USB PCI devices. With the USB VM in place, if an attacker plugs a malicious USB device in to your computer while you are away (or you plug it in yourself), the damage is contained to the USB VM.

Of course, if all of your USB devices are now assigned strictly to one VM, how can you use them on your other appVMs? For input devices like mice and keyboards, Qubes provides an input proxy service that will proxy input devices to the rest of the appVMs provided the user accept a prompt when the devices are plugged in. When you plug in a USB storage device, it shows up only in the USB VM for starters, and you then can assign it to other appVMs in the Qubes VM Manager by right-clicking on the appVM and selecting the device from the attach/detach block devices menu (be sure to detach it before you unplug it, otherwise Xen has been known to get confused about the state of the block device).

If you do want to enable the USB VM, the sys-usb USB VM shows up as an option during the install on the screen where you select which default appVMs to load. Otherwise, if you want to try it out post-install, you can run the following commands from the dom0 VM (Qubes 3.1 or newer):


$ qubesctl top.enable qvm.sys-usb
$ qubesctl state.highstate

These commands will run through an automated Salt script the Qubes team has put together that will configure the sys-usb VM appropriately. Of course, if you want to do this all by hand, you also could just create your own sysVM (I recommend not giving it a network card if you can help it), and in the Qubes VM Manager, go into that VM's settings and identify and assign your PCI USB controllers to it.

Now, there's a reason that sys-usb is disabled by default in the installer. Although desktop computers still offer PS/2 ports and many laptops use PS/2 as the interface for their main keyboard and mouse, some laptops (such as current MacBooks for instance) use a USB interface for the main keyboard. If that's the case, you can end up with a situation where you are locked out of your computer, because your USB keyboard will be assigned to your USB VM at boot, and you won't be able to log in. Another downside is that although there are services to share input devices and storage devices with other appVMs, any other USB devices (such as webcams or network cards) cannot be shared and can be used only from applications within the USB VM. Finally, the USB VM is unstable on some kinds of hardware, depending on how well it supports Qubes.

By default, only mice are allowed through the Qubes input proxy (and then only if you accept a prompt). Keyboards are not allowed through by default, because of the extra risk a malicious keyboard input device can pose to a system, including the fact that the USB VM can then read anything you type on that keyboard in other appVMs (such as passwords) or could enter its own keystrokes. If you are willing to accept this risk, you still can provide a level of protection by ensuring that you are prompted before an arbitrary USB keyboard is shared with the rest of the environment. In a dom0 terminal, add the following line to /etc/qubes-rpc/policy/qubes.InputKeyboard:


sys-usb dom0 ask

In this case, I specified sys-usb, but if you use a different appVM as your USB VM, use its name here instead.

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Kyle Rankin is SVP of Security and Infrastructure at Zero, the author of many books including Linux Hardening in Hostile Networks, DevOps Troubleshooting and The Official Ubuntu Server Book, and a columnist for Linux Journal. Follow him @kylerankin