A new exploit for Intel processors?

Intel Coffee Lake

Whether or not this turns out to be a big deal is yet to be seen, but research from collaborators at UC Riverside, Binghampton University, Carnegie-Mellon Qatar, and the College of William and Mary, have discovered a new possible exploit in certain Intel processors that they have dubbed BranchScope – that link will take you straight to the research paper itself. The research is published in the Proceedings of the Twenty-Third International Conference on Architectural Support for Programming Languages and Operating Systems (ASPLOS).

This is a new form of predictive execution exploit, in which the CPU attempts to predict what will happen as the result of a particular instruction, then branch based off of that guess, then guess what will happen as a result of that particular instruction, and so on. It’s the same kind of manner in which a computerized chess game plays chess: Every time you make a move, the game will play out thousands of games as a result of that, trying to guess the moves you will make and the moves it can make in response, and decide on an appropriate next move as a result. It doesn’t make this guess blind, either: In the processor is a component of the Branch Prediction Unit, or BPU, known as a Pattern History Table (PHT) that stores the results of previous operations, and each time a particular branch is taken, the ‘value,’ for lack of a better term, of that branch is increased or decreased. That allows the BPU to be more accurate in its future predictions (Spectre attacked the other part of the BPU, the Branch Target Buffer, which focuses on the result of a branch as opposed to the choice it made).

The exploit, therefore, is to set up selected branches that will modify the PHT and run them so that the PHT will always select one branch over another in a given situation, as well as monitor that the changes to the PHT have actually taken place. These attacks can be used to ensure a particular path is taken when predictive execution happens, and that can be used to divulge information that is otherwise unaccessible, even to the OS, such as key segments, or even provide access to the Software Guard Extensions (SGX), an Intel feature that allows deft software developers to place sensitive or critical data off in protected areas of the CPU cache that should, ideally, be available to none besides the program itself. The whole purpose of the SGX is to prevent bad actors from accessing the data.

Both the Specter vulnerability and BranchScope as well remind me of the more widely-known and difficult to pull off NOP, or No-Operation, Slide (sometimes Sled or Ramp) type of attack, in which an attacker attempts to bypass a series of CPU instructions such that when an operation does happen, it ends up in a specific portion of memory where malicious code has already been installed and will then be run. These often fail, by the way, and in fact the section of memory that holds the malware will often be padded in front and behind so that the target memory location is bigger and the slide hits by luck; it’s a big shot in the dark. The hex value of the NOP is x90, and if you look at the image below which is tracing CPU instruction executions, you can actually see the slide happening as the series of ’90s’ at the top of the lower left and lower right window (source: samsclass).

NOP Slide (Lower left). Source: samsclass

NOP Slide (Lower left). Source: samsclass

This is called a side-channel attack because something is running that shouldn’t be, similar to how loading non-approved, outdated, or unkown-source OSs or apps on a smart phone is considered side-loading.

Predictive execution is complex, the exploits are complex, and as stated earlier, whether or not we see them in the wild is a completely separate issue; the payoff would really have to be worth the effort and I don’t see that being the case unless the exploit was adapted to pair with, say, a trojan horse-style attack, but even then the nature of the returned data would be useless without significant analysis. Knock on wood, but I don’t see this as a common attack vector in the near future.

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