Toward transient-execution attack mitigations on CHERI

Abstract

This thesis explores how to protect Capability Hardware Enhanced RISC Instructions (CHERI) systems from transient-execution attacks. Transient-execution attacks shocked the computing world as they allow security mechanisms to be circumvented via seemingly safe performance-enhancing mechanisms. These attacks use misguided speculation to access secrets and transmit them via a side channel. Since the initial discovery of this attack class, every year saw fresh attacks being discovered with a lack of mitigation mechanisms. CHERI defines architectural capabilities that help to tackle spatial and temporal memory safety issues. However, the CHERI ISA has not been designed with transient-execution vulnerabilities in mind. In order to satisfy performance requirements, CHERI implementations employ out-of-order and speculative execution mechanisms. The lack of ISA-level guarantees leads to multiple attack scenarios on conventional and CHERI systems. This thesis demonstrates a full-scale attack on CHERI-Toooba that manages to break CHERI's security guarantees in speculation and reliably leaks a secret value. Motivated by these findings, I developed ISA-level contracts that restrict speculative execution for both conventional and CHERI systems. As a major contribution, these contracts close a gap in architectures by giving guarantees about speculation mechanisms, which allows secure software to be built atop these contracts. I evaluated my contracts on CHERI-Toooba, which is an out-of-order, superscalar implementation of CHERI-RISC-V. My contracts offer substantial security guarantees and can surprisingly lead to improvements in both cycle performance as well as area usage on FPGAs. Making fine-grained compartmentalisation robust against transient-execution attacks is critical for the overall security of CHERI systems. In this work, I compare multiple different solutions and introduce Thread ID Capability (TIDC) registers as a fast and compact solution to compartmentalisation on CHERI-RISC-V. In order to facilitate fast and secure transitions between compartments, this work finds that microarchitectures must not allow microarchitectural state to leak. This thesis suggests multiple approaches to separating microarchitectural state and evaluates their performance on multiple compartmentalisation models. Last, this thesis introduces the notion of Compartment ID (CID) sealing. In this work, I implement a necessary subset of this approach that allows out-of-order microarchitectures to keep track of current software-defined CIDs. I find this approach to have significant performance advantages over conventional speculation fences.