Processor extensions for security

Better aligning hardware security with software security

Processor framework

The implementation of software security objectives can benefit substantially from hardware support. Processors commonly implement instructions or extensions specifically designed to support security. For instance, to implement isolation between processes, system software can rely on processor support for virtual memory. Or protection against buffer overflow attacks can be made more efficient thanks to dedicated instructions for bounds checking.

The research community has investigated a wide variety of such extensions, including support for taint tracking, fast encryption, countermeasures for memory corruption attacks, trusted computing, and so forth.

With the recent success of the open-source instruction set architecture RISC-V, it becomes a realistic option to build custom processors that include security extensions tailored for specific use-cases or applications.

In our research group, we are working on a RISC-V processor framework to support security research. Unlike many other open source RISC-V implementations, this processor is not designed with any efficiency criteria in mind. Instead, the goal of the design is to allow for easy experimentation with processor extensions and adaptations. To this end, the framework is implemented using SpinalHDL, a Scala-based HDL that allows to create interesting hardware abstraction in a high-level programming language. The processor framework itself is built around the concept of plugins: new logic can be added simply by implementing a plugin without having to touch any other implementation files.

Currently, this framework implements the following features:

  • RV32I(M)
  • Fully bypassed classic 5-stage RISC pipeline that is highly configurable:
    • Can be completely disabled (i.e., 5 CPI);
    • Easy to add and combine stages;
    • Bypassing could be replaced with stalling;
  • Supports instructions that take more than one cycle in a stage:
    • For example, the M-extension (MUL/DIV/REM);
    • Blocking memory accesses.
  • Machine-Level ISA (CSRs, traps);
  • SoC:
    • Integration with AXI4 and APB buses;
    • Simple peripherals (Machine Timer, UART,…).
  • Tested using riscv-tests and formally verified using riscv-formal.

Processor extensions

Some examples of extensions that we plan to implement and evaluate include:

  • Trusted Execution Environments (e.g., Sancus);
  • Support for capability based security (like CHERI) to enable experimenting with extensions of the basic capability model:
  • Extensions that enable the implementation of countermeasures against micro-architectural attacks, see for instance: