Spectre Returns! Speculation Attacks using the Return Stack Buffer

TL;DR

SpectreRSB is a new class of speculative execution attack exploiting the return stack buffer in modern CPUs, capable of bypassing existing defenses and affecting local, SGX, kernel, and cross-address space scenarios.

Contribution

This paper introduces SpectreRSB, a novel Spectre-class attack exploiting the return stack buffer, demonstrating its effectiveness and limitations of current defenses.

Findings

SpectreRSB can attack local processes and SGX enclaves.

Existing defenses like Retpoline and microcode patches do not fully prevent SpectreRSB.

RSB refilling mitigates some SpectreRSB attacks on certain Intel CPUs.

Abstract

The recent Spectre attacks exploit speculative execution, a pervasively used feature of modern microprocessors, to allow the exfiltration of sensitive data across protection boundaries. In this paper, we introduce a new Spectre-class attack that we call SpectreRSB. In particular, rather than exploiting the branch predictor unit, SpectreRSB exploits the return stack buffer (RSB), a common predictor structure in modern CPUs used to predict return addresses. We show that both local attacks (within the same process such as Spectre 1) and attacks on SGX are possible by constructing proof of concept attacks. We also analyze additional types of the attack on the kernel or across address spaces and show that under some practical and widely used conditions they are possible. Importantly, none of the known defenses including Retpoline and Intel's microcode patches stop all SpectreRSB attacks. We believe that future system developers should be aware of this vulnerability and consider it in developing defenses against speculation attacks. In particular, on Core-i7 Skylake and newer processors (but not on Intel's Xeon processor line), a patch called RSB refilling is used to address a vulnerability when the RSB underfills; this defense interferes with SpectreRSB's ability to launch attacks that switch into the kernel. We recommend that this patch should be used on all machines to protect against SpectreRSB.