"It's a Trap!"-How Speculation Invariance Can Be Abused with Forward Speculative Interference

TL;DR

This paper reveals how speculation invariant instructions, considered safe, can be manipulated through forward speculative interference to leak sensitive data via a new side-channel on actual hardware.

Contribution

It introduces the concept of forward speculative interference and demonstrates how it can be exploited to leak secrets, challenging existing assumptions about speculation invariance.

Findings

Forward speculative interference can turn invariant instructions into data transmitters.

The occupancy of the reorder buffer (ROB) can serve as a side-channel to leak secrets.

Hardware experiments confirm the feasibility of the attack.

Abstract

Speculative side-channel attacks access sensitive data and use transmitters to leak the data during wrong-path execution. Various defenses have been proposed to prevent such information leakage. However, not all speculatively executed instructions are unsafe: Recent work demonstrates that speculation invariant instructions are independent of speculative control-flow paths and are guaranteed to eventually commit, regardless of the speculation outcome. Compile-time information coupled with run-time mechanisms can then selectively lift defenses for speculation invariant instructions, reclaiming some of the lost performance. Unfortunately, speculation invariant instructions can easily be manipulated by a form of speculative interference to leak information via a new side-channel that we introduce in this paper. We show that forward speculative interference whereolder speculative instructions interfere with younger speculation invariant instructions effectively turns them into transmitters for secret data accessed during speculation. We demonstrate forward speculative interference on actual hardware, by selectively filling the reorder buffer (ROB) with instructions, pushing speculative invariant instructions in-or-out of the ROB on demand, based on a speculatively accessed secret. This reveals the speculatively accessed secret, as the occupancy of the ROB itself becomes a new speculative side-channel.