BranchScope

TLDR

The directional component of the branch predictor stores the prediction on a given branch (taken or not‑taken) and is a different component from the branch target buffer (BTB) attacked by previous work. BranchScope is a new side‑channel attack that infers the direction of any conditional branch instruction in a victim program by manipulating the shared directional branch predictor. The attack targets complex hybrid branch predictors with unknown organization, forcing them to switch to a simple 1‑level mode to simplify direction recovery, and is demonstrated on recent Intel CPUs and an SGX enclave. BranchScope is the first fine‑grained attack on the directional branch predictor, expanding our understanding of its side‑channel vulnerability, and it successfully exploits recent Intel CPUs and an SGX enclave.

Abstract

We present BranchScope - a new side-channel attack where the attacker infers the direction of an arbitrary conditional branch instruction in a victim program by manipulating the shared directional branch predictor. The directional component of the branch predictor stores the prediction on a given branch (taken or not-taken) and is a different component from the branch target buffer (BTB) attacked by previous work. BranchScope is the first fine-grained attack on the directional branch predictor, expanding our understanding of the side channel vulnerability of the branch prediction unit. Our attack targets complex hybrid branch predictors with unknown organization. We demonstrate how an attacker can force these predictors to switch to a simple 1-level mode to simplify the direction recovery. We carry out BranchScope on several recent Intel CPUs and also demonstrate the attack against an SGX enclave.