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Much Ado About Type-2


VMware has finally lifted the lid on their long-promised mobile virtualization platform (MVP). And, surprise, it’s a Type-2 hypervisor! This is a bit of a let-down, and has some interesting implications on what MVP can (or rather cannot) do, which I’m going to explore in a few blogs.

First a bit of background. Observers of the mobile virtualization space will remember that about two years ago, VMware, better known for server and desktop virtualization products, bought our then competitor Trango. At the time they promised MVP-based products “should arrive in around 12 to 18 months“. That’s phones with MVP on it. Almost 24 months later, there isn’t even a product announcement for MVP. It’s been a bit like waiting for Godot…

In the meantime, the OKL4 Microvisor has been around for yonks. It’s available, it’s benchmarkable, it’s being deployed—it’s real. And, as befits something with “L4” in the name, it defines the state of the art of hypervisors for embedded systems.

Well, at last (least?) VMware presented their vision, accompanied by a demo, at a BOF at last week’s OSDI conference in Vancouver. Not exactly a high-profile announcement. And it’s a Type-2 hypervisor!

I’ve discussed Type-1 vs Type-2 in a blog a year ago, and another one a few months earlier, and will probably explore this topic a bit more in a future blog. For now I’ll focus on what VMware is trying to sell, and why it doesn’t actually doesn’t solve the problem they claim they are addressing. Further technical discussion will look at why they taking this particular stance. (Hint: If all you’ve got is a hammer, everything looks like a nail. Even an egg…)

Hypervisors (also called virtual machine monitors) are designed to provide multiple virtual machines which can each run an OS with all of its apps. The fundamental difference between a Type-1 hypervisor (such as OKL4) and a Type-2 is that the former runs on bare metal, between the hardware and the operating system(s). In contrast, a Type-2 hypervisor runs on top of an OS (which is why it’s also called a “hosted” hypervisor).

That difference is much more significant than it may seem. It implies a completely different relationship between the hypervisor and the various operating systems. With Type-1, the hypervisor is master, it controls the OSes (called “guests”). With Type-2, the master is an OS (the one which hosts the hypervisor), it controls the hypervisor, which can only control the other OSes. Keep this in mind.

So, what problems is VMware (pretending) to solve with their Type-2 hypervisor? The main use case they are highlighting is BYOD, “bring your own device”. (Yes, they adopted the terminology we introduced 18 Months ago—good on them!)

The motivation for BYOD is that smartphones have business as well as private use. People like to control their private phones: They want to decide on the type and model, and they want to install their choice of apps. In contrast, companies like control over the phones used for business: They want to decide the model (ideally a single one for everybody) and what software runs on them. This forces an increasing number of people to carry two phones, business and private.

The idea of BYOD is that a single phone can serve both purposes: a person buys a phone of their choice, takes it to their company’s IT dudes, and they install a virtual business phone on the BYOD handset. Sounds great, doesn’t it?

The devil is in the detail, and it’s those details which make MVP a non-solution.

Why do companies want control over the phone? There’s only one reason: security. The whole point of issuing smartphones to employees is to keep them linked into the enterprise IT infrastructure while they are on the move. Traditionally this is all about email, address books and calendars, but increasingly it is a much deeper integration, enabling the phone to access employee records, sales databases, engineering designs—anything you’d access from your computer screen in the office.

So, the bottom line is that companies are worried about the security and integrity of their data when accessed via the mobile device (phone, tablet or whatever it might be). They are worried that accessing this critical data from an uncontrolled phone puts the critical enterprise information at risk. And they are right: phones do get infected by malware, and with each application installed, the risk of infection increases. This is the core challenge BYOD must address.

Surely, VMware understands this? Maybe they do, but if so, why do they offer solution which doesn’t cut the mustard?

The reason I say this is that the BYOD model VMware is propagating does nothing to solve this fundamental security issue, while OKL4 does.

Type-1 (left), Type-2 (right)

Type-1 and Type-2 hypervisors and system structure

This is illustrated in the figures at the left. With OKL4, the (Type-1) hypervisor is in control of all hardware. It isolates the VMs and their OSes from each other. If the user gets their private OS infected, that’s tough for them, but the infection cannot spread across VMs to the business environment. In order to subvert this, the attacker either has to have already subverted some of the enterprise IT infrastructure (thus coming in from the business side into the business OS) or has to attack the hypervisor from the private VM. But the hypervisor has an extremely small attack surface! The hypervisor is very small (about 10,000 lines of code). Technically speaking, the business VM has a small trusted computing base (TCB).

In VMware’s Type-2 model, it’s quite different. The business environment is controlled by the hypervisor, which is controlled by the host OS (the one that comes with the BYOD phone). If this gets cracked, as it inevitably will be, then it’s trivial to crack the hypervisor, and then you control the business OS! The reason this is easy to crack is that in this setup, the business OS has a huge TCB. It includes the complete private OS, which likely comprises upwards of 1,000,000 lines of code—two orders of magnitude more than OKL4!

Now remember where we’re coming from. The original motivation for BYOD was that companies don’t trust people’s private phones with critical business data, because these phones get cracked, which would compromise the business data. The idea of BYOD, as promoted by OK Labs, is to provide a virtual business phone on the private handset which is just as secure as if it was a physically separate handset.

If you followed my argumentation above, you’ll see that VMware’s solution is no bit more secure than allowing people to access the business data through their normal private phones, without the detour via a hypervisor. In other words, MVP adds nothing to security. So why would you pay for it then? You might as well cut out the middle man and allow people to access the enterprise IT system from their unmodified private phones. Security-wise, there is no difference whatsoever.

At OK Labs, we believe that security isn’t something that’s solved with PR. It requires a technically-sound approach. It requires a minimal TCB. It requires OKL4.

Stay tuned for a more in-depth look at these issues.

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