Just Another Geek

I am blogging about Information Security since 2003

08 Apr 2010

Document review of Qubes OS

Qubes OS

You must have heard about it, Invisible Things Lab released their own operating system, named Qubes OS (If you ask me, I would have refer to it as a Linux distribution instead). Their distribution focuses on security isolation and is based on their virtualization experience (for the record, Joanna and Rafal are the people behind most of the virtualization vulnerabilities found in the previous years).

Disclaimer: I do not had the occasion to test the system, this post is only based on my reading of their (great) QubesOs architecture paper (version 0.3). I did not read the source code or whatever so be careful with what is following :)

Target of the distribution

Maybe I am guessing wrong, but this distribution seems to be really dedicated for classified environments. Even if it is usable by anyone, some concepts make me believe this work will be sold to government or military people because it full-fills most of the requirements. Anyway, this is awesome to release it to the public, in an Open Source licence.

I am sure that this release will be really helpful for the people involved in “SEC&SI challenge” organized by the french government. This research project is dedicated to the construction of a secure Desktop platform (Linux based) usable by my grandma.

What is new?

There is nothing new by itself: only components already available in the community have been used (Linux kernel, Xen, Xorg, LUKS, device mapper, etc.), few code have been developed.

The beauty of their solution is that every techniques used are individually known but nobody had the idea to put them together like they did. Bravo!

The big picture

User activities can be identified in multiple security levels: web browsing, banking, corporate work, social networking, e-shopping, etc.

Each activity represents a security domain: banking activities are more critical than social networking (right?). Usually, on most of operating systems, every activities are in the same “space”: if you are compromised while browsing youtube.com, the attacker can access to your sensitive data (bank account or corporate email).

So QubesOS resolves this problem by running each domain into a virtual machine (called AppVM in QubesOS terminology), the virtualization solution they chose is Xen. They explain why they chose Xen instead of KVM in their architecture guide.

Thanks to the Xen hypervisor, each AppVM is isolated and cannot have access to other resources than its own.



  • Display\

    Until Qubes OS, every Linux distribution providing “multi level security” use one X server running on the Host (in Dom0) and each virtual machine uses the display thanks to:

    • VNC-over-ssh. Problem: VNC client and server have not really been audited and they are crippled of security issues.
    • X11 forwarding. Problem: There is basically no security control in X11, a X client can do anything on other windows like capturing or injecting keystrokes, snooping other applications, etc. This is problematic when windows do not have the same security level.

    A lesser used alternative is the use of one Xorg server per virtual machine: each security level is inside a virtual terminal. However, unless there is a video card per X session, every servers share the same hardware resource so a vulnerability in Xorg would impact other Xorg servers.

    The innovation of Qubes OS is to not use any of these methods. Each AppVM runs a Xorg instance (with a “dummy graphic driver”, which I guess is not tied to any hardware device) and a AppVM Window Manager.

    The dom0 domain runs the “real” Xorg server tied to the graphic card and multiple AppViewers, each AppViewer communicates with one AppVM Window Manager (which is inside a AppVM) via the Xen Ring buffer protocol. The task of each AppViewer is to proxify input devices to the right AppVM (depending on who has the focus). Each virtual machine uses a homemade input xorg-driver called xf86-input-mfndev (which gets its input from the ring buffer).

    Each AppVM Window Manager sends notifications to its associated AppViewer. The events monitored are: creation of new window, content refresh or change of window focus.

    When an AppViewer receives a content refresh notification, it requests to the AppVM Window Manager its composition buffer (the bitmap of the window content in other words). It receives these bytes from the ring buffer and displays it on the screen.

    The optimization, which is still being investigate, is to ask the address of the composition buffer instead of the sending the raw bitmap. This is possible because the dom0 has access to the address space of every AppVM so it can directly use the bytes to render it without involving a double-copy.

    However, I do not know if this optimization would be sufficient to handle video playback: the paper suggests that the user can watch video on youtube so it seems to work, but I don’t see how. Even on my “normal” desktop, the system goes slow if I simply disable Xvideo overlay.

  • Audio\

    Audio support is not yet implemented but will be certainly based on the same principle than the “composition buffer”: audio stream will be “written” in a buffer readable by AppViewer.

    That way, a dom0 daemon just has to mix every AppViewer audio streams and eventually sends the final stream to the sound card.

  • Clipboard\

    “Applicative clipboard” (in opposition to X11 clipboard mechanism) operations are supported between AppVM. The user has to press a special shortcut (S-C-v) which is intercepted by the dom0 and not passed to AppVM. At this point, the AppViewer triggers a command on the virtual machine, sending the content of the cursor selection through the Xen ring buffer. The bytes are then stored in a volatile file on dom0.

    When the special paste shortcut is pressed, the dom0 injects the stored result via the ring buffer again and emulates the paste action.

Storage architecture

AppsVMs share the same “base filesystem” in order to not waste disk space. For that matter, each domain mounts a read-only block device and mounts, on top of it, a copy-on-write block device (thanks to the kernel’s device-mapper) accessible only to the AppVM.

Each time an AppVM is started, the copy-on-write volume is deleted in order to have a clean environment. Persistent data (like user documents) are stored in another private volume which is restored at AppVM creation time.

Every block devices are exported by the Storage Domain. This abstraction layer is needed to make possible file-sharing between AppVMs (thanks to a homemade cryptographic protocol).

We can see that the Storage Domain has great powers. To counterbalance it, cryptography was used.

The “base read-only block device” is signed (on a per-block basis). The private key is available only to the TPM and the dom0.

Application specific volumes (the copy-on-write overlay and the persistent block device) are encrypted (with LUKS) with a key available only to AppVMs and the dom0.

Thanks to this design, a compromission of the Storage Domain would be worthless because any attempt to modify data would be detected and persistent files are encrypted so an attacker would be disappointed :)

Network architecture

Most of the remote vulnerabilities found in the Linux kernel have been discovered in device drivers like network adapters. Because any bug found in the kernel puts in danger the whole system, it would be great to find a way to isolate these drivers.

Thanks to recent CPU features, it is now possible to do such thing: Intel VT-d technology permits to safely give to a virtual machine access to a hardware device.

In other words, QubesOS now delegates the PCI wireless card to an AppVM, called Network domain. At this point, if a vulnerability is found in the wifi driver, only the virtual machine is compromised.

The Network domain is the border router: every AppVM routes its traffic through it. One of its task is also to enforce traffic policy: AppVMs are not allowed to communicate between each other, only HTTPS flows are allowed for the banking domain, only VPN traffic is allowed for corporate domain, etc.


On the paper, Qubes OS seems really well designed and robust from a security point of view. By glancing at the screenshots, the user experience seems good. I don’t know how good/bad are the performances: memory usage must be really high (because AFAIK, Xen does not implement the “Kernel Samepage Merging” feature available in KVM since 2.6.32).

But, anyway, congratulations to “Invisible Things Lab” for this great architecture!