(the following is partially excerpted from my next book)

Adam Shostack, author of Threat Modeling: Designing for Security has succinctly described a process of constructive critique within engineering:

“The back and forth of design and critique is not only a critical part of how an individual design gets better, but fields in which such criticism is the norm advance faster.”

The Spectre/Meltdown issues are the result of a design critique such as Shostack describes in his pithy quote given above.  In fact, one of the fundamental functions that I believe security research can play is providing designers with a constructive critique.

Using Spectre and Meltdown as an example of engineering critique, let’s look at some of the headlines from before the official issue announcement by the researchers:

“Kernel-memory-leaking Intel processor design flaw forces Linux, Windows redesign”, John Leyden and Chris Williams 2 Jan 2018 at 19:29, The Register, https://www.theregister.co.uk/2018/01/02/intel_cpu_design_flaw/

“A CRITICAL INTEL FLAW BREAKS BASIC SECURITY FOR MOST COMPUTERS”, Andy Greenberg, January 3, 2018, Wired Magazine, https://www.wired.com/story/critical-intel-flaw-breaks-basic-security-for-most-computers/

There were dozens of similar headlines (many merely repeating the first few, especially, The Register’s), all declaiming a “flaw” in CPUs. I want to draw the reader’s attention to the word, “flaw”. Are these issues “flaws”? Or, are they the result of something else?

The Register headline and that first article were based upon speculation that had been occurring amongst the open source community supporting the Linux Kernel. A couple of apparently odd changes that had been made to kernel code. But, the issues to which these changes responded were “embargoed”, that is, the reasoning behind the changes was known only to those making the changes.

Unlike typical open source changes, whose reasoning is public and often discussed by members of the community, these kernel changes had been made in opaquely without public comment, which of course, set concerned kernel community members wondering.

To observers not familiar with the reasoning behind the changes, it was clear that something was amiss and likely in relation to CPU functions; anxious observers were guessing what might be the motivation for those code changes.

Within the digital security universe, there exists an important dialog between security research aimed at discovering new attack techniques and the designers of the systems and protocols upon which that research is carried out. As Adam noted so very wryly, achieving solid designs, even great ones, and most importantly, resilient designs in the face of omnipresent attack requires an interchange of constructive critique. That is how Spectre and Meltdown were discovered and presented.

Neither of this collection of (at the time of announcement) new techniques involved exercising a flaw, that is, a design error – in other words, the headlines quoted just above were erroneous and rather misleading[2].

Speculative execution and the use of kernel mapped user memory pages by operating systems were intentional design choices that had been working as designed for more than 10 years. Taken together, at least some of the increases in CPU performance over that period can directly be tied to speculative execution design.

Furthermore, and quite importantly to this discussion, these design choices were made within the context of a rather different threat landscape. Some of today’s very active threat actors didn’t exist, or at least, were not nearly as active and certainly not as technically sophisticated circa 2005 as they are today, May 2018.

If I recall correctly (and I should be able to remember, since I was the technical lead for Cisco’s Web infrastructure and application security team at that time), in 2005, network attacks were being eclipsed by application focused attack methods, especially, web attack methods.

Today, web attacks are very “ho, hum”, very run of the ordinary, garden variety. But in 2005, when the first CPU speculative execution pipelines were being released, web applications were targets of choice at the cutting edge of digital security. Endpoint worms and gaining entrance through poor network ingress controls had been security’s focus up until the web application attack boom (if I may title it so?). At about that time, targeting web applications was fast displacing network periphery concerns. Attackers were in the process of shifting to targets that used a standard protocol (HTTP) which was guaranteed to pass through organizational firewalls. Many of the new targets’ were becoming always  available via the Public Internet.

Since the web application attack boom, attacks and targets have continued to evolve. The threat landscape changed dramatically over the years since the initial design of speculative execution CPUs. Alongside the changes in types of attackers as well as their targets, attacker and researcher sophistication has grown, as has an attackers’ toolbox. 2018 is a different security world than 2005. I see no end to this curve of technical growth in my crystal ball.

The problem is, when threat modeling, whether in 2005 or 2018, one considers the attacks of the past, those of moment, and then one must try one’s best to project from current understanding to attacks that might arise within the foreseeable future. Ten or twelve years seems an awfully long horizon of prescience, especially when considering the rate at which technical change continues to take place.

As new research begins to chew at the edges of any design, I believe that the wise and diligent practitioner revisits their existing threat models in light of developments.

If I were to fault the CPU and operating system makers whose products are subject to Spectre or Meltdown, it would be for a failure to anticipate where research might lead, as research has unfolded. CPU threat modelers could have taken into account advances in research indicating unexpected uses of cache memory.

Speculative execution leaves remnants of a speculated execution branch in cache memory when a branch has not been taken. It is those remnants that lie at the heart of this line of research.

A close examination of the unfolding research might very well have led those responsible for updating CPU threat models to consider the potential for something like Spectre and Meltdown. (Or, perhaps the threat models were updated, but other challenges prevented updates to CPU designs? CPU threat modelers, please tell us all what the real story is)

I’ve found a publication chain during the 3 years previous that, to me, points towards the new techniques. Spectre and Meltdown are not stand-alone discoveries, but lie on a body of CPU research that had been published regularly for several years.

As I wrote for McAfee’s Security Matters blog in January of 2018 (as a member of McAfee’s Advanced Threat Research Team),

“Meltdown and Spectre are new techniques that build upon previous work, such as “KASLR”  and other papers that discuss practical side-channel attacks. The current disclosures build upon such side-channels attacks through the innovative use of speculative execution….An earlier example of side-channel based upon memory caches was posted to Github in 2016 by one of the Spectre/Meltdown researchers, Daniel Gruss.” Daniel Gruss is one of the Spectre and Meltdown paper authors.

Reading these earlier papers, it appears to me that some of the parent techniques that would be used for the Spectre and Meltdown breakthroughs could have been read (should have been read?) by CPU security architects in order to re-evaluate the CPU’s threat model. That previously published research was most certainly available.

Of course, hindsight is always 20/20; I had the Spectre and Meltdown papers in hand as I reviewed previous research. Going the other way might be more difficult?

Spectre and Meltdown did not just spring miraculously from the head of Zeus, as it were. They are the results of a fairly long and concerted effort to discover problems with and thus, hopefully, improve the designs of modern processors. Indeed, the researchers engaged in responsible disclosure, not wishing to publish until fixes could be made available.

To complete our story, the driver that tipped the researchers to an early, zero-day disclosure (that is, disclosure without available mitigations or repairs) were the numerous speculative (if you’ve forgive the pun?) journalism (headlines quoted above) that gained traction based upon misleading (at best) or wrong conclusions. Claiming a major design “flaw” in millions of processors is certainly a reader catching headline. But, unfortunately, these claims were vastly off the mark since no flaw existed in the CPU or operating system designs.

While it may be more “interesting” to imagine a multi-year conspiracy to cover up known design issues by evil CPU makers, no such conspiracy appears to have taken place.

Rather, in the spirit of responsible disclosure, the researchers were waiting for mitigations to be made available to customers; CPU manufacturers and operating system coders were heads down at work figuring out what appropriate mitigations might be, and just how to implement these with the least amount of disruption. None of these parties was publicly discussing just why changes were being made, especially to the open source Linux kernel.

Which is precisely what one would expect in order to protect millions of CPU users: embargo the technical details to foil attackers. There is actually nothing unusual about such a process; it’s all very normal and typical, and unfortunately for news media, quite banal[3].

What we see through the foregoing example about Spectre and Meltdown is precisely the sort of rich dialog that should occur between designers and critics (researchers, in this case).

Designs are built against the backdrop and within the context of their security “moment”. Our designs cannot improve without collective critique amongst the designers; such dialog internal to an organization or at least, a development team is essential. I have spoken about this process repeatedly at conferences: “It takes a village to threat model,” (to misquote a famous USA politician.)

But, there’s another level, if you will, that can reach for a greater constructive critique.

Once a design is made available to independent critics, that is, security researchers, research discoveries can and I believe, should become part of an ongoing re-evaluation of the threat model, that is, the security of the design. In this way, we can, as an industry, reach for the constructive critique called for by Adam Shostack.

[1]In my humble experience, Adam is particularly good at expressing complex processes briefly and clearly. One of his many gifts as a technologist and leader in the security architecture space.

[2]Though salacious headlines apparently increase readership and thus advertising revenue. Hence, the misleading but emotion plucking headlines.

[3]Disclosure: I’ve been involved in numerous embargoed issues over the years.