Sure, the inter-process isolation provided by a monolithic kernel such as Windows or Linux could never be compared to the inter-VM isolation offered even by the most lousy hypervisors. This is simply because the sizes of the interfaces exposed to untrusted entities (processes in case of a monolithic kernel; VMs in case of a hypervisor) are just incomparable. Just think about all those Windows system calls and GDI calls which any process can call and which contains probably thousands of bugs still waiting to be discovered by some kid with IDA. And think about those tens of thousands of drivers, which also expose (often unsecured) IOCTLs, as well as parsing the incoming packets, USB devices infos, filesystem metadata, etc. And then think about various additional services exposed by system processes, which are not part of the kernel, but which are still trusted and privileged. And now think about the typical interface that needs to be exposed to a typical VM: it's “just” the virtualized CPU, some emulated devices (some old-fashined Pentium-era chipset, SVGA graphics adapter, etc) and virtualized memory.
Anyway, knowing all this, I still believed that Qubes WNI would make a whole lot of sense. This is because Qubes WNI would still offer a significant boost over the “Just Windows” default security, which is (still) essentially equivalent to the MS-DOS security model. And this is a real pity, because Windows OS has long implemented very sophisticated security mechanisms, such as complex ACLs applicable to nearly any object, as well as recent mechanisms such as UIPI/UAC, etc. So, why not use all those sophisticated security to bring some real-world security to Windows desktops!
And, best of all, once people start using Qubes WNI, and they liked it, they could then pretty seamlessly upgrade to Xen-based Qubes OS, or perhaps Hyper-V-based Qubes OS (when we implement it) and their system would look and behave very similarly. Albeit with orders of magnitude stronger security. Finally, if we could get our Odyssey Framework to be flexible enough to support both Qubes WNI, as well as Xen-based Qubes OS, we should then be able to support any hypervisor or other isolation mechanism in the future.
And so we decided to build the Qubes WNI. Lots of work we invested in building Qubes WNI was actually WNI-independent, because it e.g. covered adjusting the core Odyssey framework to be more flexible (after all “WNI” is quite a non-standard hypervisor) as well as some components that were Windows-specific, but not WNI-specific (e.g. could very well be used on Hyper-V based Qubes OS in the future). But we also invested lots of time into evaluating all those Windows security mechanisms in order to achieve our specific goals (e.g. proper GUI isolation, networking isolation, kernel object spaces isolation, etc)...
Sadly this all has turned out to be a story without a happy end, as we have finally came to the conclusion that consumer Windows OS, with all those one-would-think sophisticated security mechanisms, is just not usable for any real-world domain isolation.
And today we publish a technical paper about our findings on Windows security model and mechanisms and why we concluded they are inadequate in practice. The paper has been written by Rafał Wojdyła who joined ITL a few months ago with the main task of implementing Qubes WNI. I think most people will be able to learn a thing or two about Windows security model by reading this paper.
Also, we still do have this little hope that somebody will read the paper and then write to us: “Oh, you're guys so dumb, you could just use this and that mechanism, to solve all your problems with WNI!” :)
The paper can be downloaded from here.