ProcessorFuzz: Guiding Processor Fuzzing using Control and Status Registers

Abstract

ProcessorFuzz is a novel hardware fuzzer designed for efficient verification of complex processor Register-Transfer Level (RTL) designs. It introduces a CSR-transition coverage metric that guides the fuzzing process by monitoring Control and Status Register (CSR) changes to explore new processor states, overcoming limitations of prior coverage signals. Evaluated on major RISC-V cores like Rocket, BOOM, and BlackParrot, ProcessorFuzz accelerated bug discovery by 1.23x compared to DIFUZZRTL and successfully uncovered nine new, confirmed bugs.

Report

Key Highlights

• Novel Fuzzer: ProcessorFuzz is presented as an effective hardware fuzzing solution for verifying complex processor RTL designs.
• CSR Guidance: The core innovation is the use of a CSR-transition coverage metric to guide fuzzing, identifying "interesting" inputs that lead to new processor states.
• Efficiency: The tool triggered a set of ground-truth bugs 1.23\times faster (on average) than the existing tool DIFUZZRTL.
• Bug Discovery: Exposed 8 new bugs across the evaluated RISC-V cores and 1 new bug in a reference model, all confirmed by the respective project developers.
• Agnostic Design: ProcessorFuzz is agnostic to the Hardware Description Language (HDL) used and does not require any instrumentation of the processor design.

Technical Details

• Target Architecture: The verification technique focuses on Register-Transfer Level (RTL) designs of processors.
• Guiding Mechanism: CSRs (Control and Status Registers) are monitored. Since CSRs control and hold the critical state of the processor, observing transitions (changes) in their values serves as feedback to guide the fuzzer toward unique operational states.
• Implementation Benefits: By avoiding instrumentation, ProcessorFuzz supports a wide array of RTL designs, overcoming a limitation common in earlier hardware fuzzing approaches.
• Evaluation Subjects: Real-world, open-source RISC-V processors were used for evaluation:
  • Rocket
  • BOOM
  • BlackParrot

Implications

• Increased RISC-V Security: By identifying subtle micro-architectural bugs—which can manifest as security vulnerabilities like side channels or functional errors—ProcessorFuzz significantly aids in hardening critical open-source RISC-V implementations.
• Enhanced Verification Utility: The HDL-agnostic nature of ProcessorFuzz makes it highly versatile, allowing verification engineers to apply advanced fuzzing techniques across diverse design flows (e.g., Chisel, Verilog) without costly integration steps.
• Raising Verification Standards: The demonstrated ability to consistently and quickly find new bugs in mature, heavily scrutinized projects (like BOOM and Rocket) establishes a new, higher standard for robust processor verification tools, directly contributing to the trustworthiness of the broader hardware ecosystem.