The recent discovery of the “Pathfinder” attack marks a significant advancement in microarchitectural side-channel exploits, specifically targeting the intricate workings of modern processors. Developed by a collaborative team from top U.S. universities and Google, this technique leverages the shared state of processor components like caches, branch predictors, and translation buffers. Unlike previous attacks that primarily focused on manipulating the conditional branch predictor for coarse control flow disruptions, Pathfinder introduces a more nuanced approach. It utilizes the Pattern History Register (PHR) and Pattern History Tables (PHTs), allowing attackers not only to observe but also to modify these components to initiate controlled speculative execution similar to the Spectre attacks.
Pathfinder operates by reconstructing the control flow graph of a victim’s function at runtime based on observed values from the PHR. This register, which is inherently complex due to its combination of multiple branch outcomes with different addresses, offers a detailed snapshot of the control flow that goes beyond traditional capturing methods. This advanced capability allows Pathfinder to determine all potential control flow paths corresponding to the observed PHR values. The analysis is sophisticated due to the complex update function used by the PHR, but typically, it narrows down to a single likely path during execution.
The implications of such an attack are profound, as it not only allows for data exfiltration through established side-channel methods but also paves the way for new variations of Spectre attacks. By overwriting the PHR before executing the victim’s code, attackers can achieve a precise control flow manipulation that was not previously possible. This method abstracts the complex manipulation required in earlier attacks, making it more accessible and potentially more dangerous.
Moreover, the Pathfinder attack sheds new light on the need for robust Spectre mitigations that take into account non-deterministic speculative control flows. By exploiting the state of the Pattern History Register, Pathfinder can leak crucial information about global branch ordering and runtime control flow, exposing vulnerabilities in thousands of branch decisions across a program’s execution. This not only highlights the evolving nature of cybersecurity threats but also the continual need for advanced protective measures against such sophisticated exploits.
