Libra: Architectural Support For Principled, Secure And Efficient Balanced Execution On High-End Processors (Extended Version)

TL;DR

Libra introduces a hardware-software co-design approach that securely balances secret-dependent branches on high-end processors, achieving security against control-flow leakage with minimal performance overhead and outperforming traditional linearization methods.

Contribution

This paper presents Libra, a novel hardware-software design that enables secure control-flow balancing on high-end processors, challenging the belief that linearization is the only effective countermeasure.

Findings

Libra achieves performance overhead comparable to insecure balanced code.

Libra outperforms state-of-the-art linearized code by 19.3%.

The design effectively prevents control-flow leakage without disabling critical hardware features.

Abstract

Control-flow leakage (CFL) attacks enable an attacker to expose control-flow decisions of a victim program via side-channel observations. Linearization (i.e., elimination) of secret-dependent control flow is the main countermeasure against these attacks, yet it comes at a non-negligible cost. Conversely, balancing secret-dependent branches often incurs a smaller overhead, but is notoriously insecure on high-end processors. Hence, linearization has been widely believed to be the only effective countermeasure against CFL attacks. In this paper, we challenge this belief and investigate an unexplored alternative: how to securely balance secret-dependent branches on higher-end processors? We propose Libra, a generic and principled hardware-software codesign to efficiently address CFL on high-end processors. We perform a systematic classification of hardware primitives leaking control flow from the literature, and provide guidelines to handle them with our design. Importantly, Libra enables secure control-flow balancing without the need to disable performance-critical hardware such as the instruction cache and the prefetcher. We formalize the semantics of Libra and propose a code transformation algorithm for securing programs, which we prove correct and secure. Finally, we implement and evaluate Libra on an out-of-order RISC-V processor, showing performance overhead on par with insecure balanced code, and outperforming state-of-the-art linearized code by 19.3%.