Author: Gernot Heiser, seL4 Foundation

In June 2020 we announced that the seL4 microkernel, the world’s first operating system (OS) kernel with a machine-checked proof of implementation correctness, has now also been verified for the RV64 architecture, making it the first formally verified OS for RISC-V. We are pleased to announce that this verification has now been extended to the executable binary, meaning that the machine code running on the processor is proved to be correct against the kernel’s specification. RISC-V is the first 64-bit architecture for which this has been achieved.

What does this mean?

The previously announced proof means that, according to the semantics of the C language in which seL4 is implemented, the kernel will always behave as specified. Among others this means that seL4 cannot be attacked with stack overflows, malformed inputs or other forms of code injection or control-flow hijacking – it is provably secure in a very strong sense. However, there is still the risk of security holes resulting from a buggy (or compromised) C compiler, or from the compiler and kernel developers interpreting the C semantics differently.

The newly completed binary verification completely removes these risks, it guarantees that properties we prove about the C code hold for the executable code, and thus that the executable kernel binary behaves as required by the kernel’s formal specification.

More than porting to a different ISA

While seL4’s implementation correctness proofs use interactive theorem proving, with hundreds of thousands of (mostly hand-written but machine-checked) lines of proof, the binary verification uses an automated tool chain (see the seL4 White Paper for details). The tool chain converts both the C code as well as the binary into an intermediate language that represents the control flow of the program. It then uses SMT solvers to prove equivalence of the two programs, one short code sequence at a time.

SMT solvers prove properties by a very efficient exploration of the state space, using state compression techniques to make the problem tractable. We had previously built the binary-verification toolchain for the 32-bit Armv7 architecture. As the state space grows exponentially with the word size, taking the step to a 64-bit architecture requires overcoming significant scalability challenges – which the ingenuity of our team around Matt Brecknell and Zoltan Kocsis could overcome in the end.

Further implications

This work represents a significant step for both the RISC-V and seL4 ecosystems. No 64-bit architecture other than RISC-V presently has an OS with such a comprehensive verification and security story. And seL4 has with RISC-V the ideal base for driving further innovation in computer system security, especially for our work on the systematic prevention of information leakage through timing channels, based on the approach we call time protection. Stay tuned for more exciting results to come!

We are now formalising the link between the two ecosystems by announcing that RISC-V International and the seL4 Foundation are joining each other as Associate Members.

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