Switchpoline: A Software Mitigation for Spectre-BTB and Spectre-BHB on ARMv

Spectre-BTB, also known as Spectre Variant 2, is often considered the most dangerous Spectre variant. While there are widely-deployed software workarounds on x86, such as Retpoline, there are no automated software workarounds for protecting generic userspace applications on ARMv8. Moreover, hardware solutions do not consider in-place mistraining or variants such as branch-history injection (Spectre-BHI), also known as Spectre-BHB. In this paper, we introduce Switchpoline, the first automated Spectre-BTB and Spectre-BHB software workaround protecting C and C++ userspace applications on ARMv8 against all variants of Spectre-BTB and Spectre-BHB. The main security of Switchpoline is that eliminating indirect branches eliminates attacks on indirect branches. Switchpoline is based on a static compiler pass and a dynamic just-in-time (JIT) compiler component that rewrite indirect control-flow transfers into direct control-flow transfers. Switchpoline successfully prevents Spectre-BTB and Spectre-BHB in userspace applications with a negligible mean performance overhead of 1.8 % measured in the SPEC CPU 2017 benchmark. Moreover, unlike many x86-specific mitigations, Switchpoline is compatible with existing orthogonal defenses, such as (hardware) CFI or Spectre-PHT mitigations. Hence, Switchpoline is a practical generic software mitigation on ARMv8.