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Where is seL4 Heading?


Following CSIRO’s abandoning of Trustworthy Systems (TS) and the seL4 technology TS developed, the seL4 Foundation has seen a significant increase in Foundation membership, including a number of new Premium Members that get a seat on the Board. Among others, this has increased the Foundation’s budget ten-fold – a great shot in the arm for the Foundation and its role of growing the seL4 ecosystem.

Does that mean a change of control of seL4?

This is a concern I have come across, mostly from people who do not (yet) understand how an open-source community such as seL4 works. (And I have been informed that people behind some of the competing, yet inferior, technologies out there are actually feeding this concern; classical FUD as it has been used against open source for decades. If someone presents you with this FUD then please point them here.)

In short, there’s no basis to this concern.

The way the seL4 community operates has not changed since we set up the seL4 Foundation in April 2020 (other than having overcome much growing pain). We have a clearly-defined and open governance structure which consists of the seL4 Board (responsible mainly for Foundation finances and recruitment) and the Technical Steering Committee (TSC) for technical leadership. I’ll explain these below, after addressing some specific concerns people have raised.

Could some organisation undermine seL4’s security?

In a word: No.

To start with, like in any other functioning open-source project, contributions to seL4 are made openly, via pull requests on the public Github, and anyone subscribed sees each of them. Any pull request is carefully vetted and discussed publicly. It cannot be merged until it passes regression tests (both traditional testing and proofs) and is approved by at least one committer, a trusted expert blessed by the TSC. You can’t buy committer status, you can only earn it, by earning the trust of the TSC. And only after this approval, the PR can be merged by one of the committers. So far, so standard for open source.

However, seL4 is special, as we all know. And what makes it special is its formal (mathematical), machine-checked proofs. With other projects, regression tests are just that: tests. They exercise the software on carefully chosen inputs and the outputs are compared to expectations. As all testing, this is inherently incomplete and cannot fully protect against bugs (or malicious code) being introduced.

The seL4 proofs are different: They are complete in a strong sense; code that is proved correct against a specification cannot have bugs as far as the specification is concerned. And by re-running proofs before merging changes, we can be certain that the code is still bug-free. That’s the really unique thing about seL4: even if the reviewers and committers fail to spot a bug that’s been introduced, the proofs will catch it (by failing).

The proofs do not yet cover all aspects of seL4, for example the 64-bit Arm version is not yet verified, multicore operation is not yet verified, nor is the boot code, cache management, and the very small amount of assembler code. Hence there is still potential for bugs in those unverified bits (and occasionally we find some). But the unverified configurations share much code with verified ones, and overall the kernel is small, so that the risk is far reduced compared to almost all other software. Deliberately introducing a vulnerability even into those parts would be extremely hard. And it would be detected as soon as that code gets verified. But, as we always point out, when running an unverified configuration you have no guarantee. A good reason to work together to raise funds for verifying those missing parts!

Also, to repeat, the contribution process has not changed since we set up the Foundation. Members cannot buy committer status, and they certainly cannot buy changes to the code.

How about Board membership?

According to the seL4 Foundation Fund Charter, the seL4 Board manages the seL4 Foundation Fund, and the purpose of the seL4 Foundation Fund is “to raise, budget and spend funds in support of the seL4 technical project carried out by [the seL4 Foundation]”. In other words, the Board’s role is to raise money and decide how to spend it for the benefit of seL4. It has no say on technical decisions (other than by prioritising specific activities through directing money there).

The Charter also stipulates that the Board “will strive to make decisions based on consensus.” This is real: We are yet to experience a case where a Board decision was not unanimous (other than members abstaining for conflict-of-interest reasons). In fact, our experience is that Board members very much trust the seL4 Founders (June, Gerwin and myself, who have guaranteed Board seats for five years renewable). A diverse Board representing a breadth of stakeholders yet unified in trust is a great asset for the seL4 community!

What if that trust broke down?

A single rogue Board member cannot do much damage (other than making the Board’s life harder), and the fixed representation of the founders acts as a further stabiliser. If a significant number of Board members tried to pull into a different direction, then that would be a problem, but would be a reflection of serious issues in the seL4 community as a whole. I do not have any concerns in this respect. Our Board members, whether elected by members or representing Premium Members share a common interest: they are there because they are vested into the technology; their companies are developing their products around seL4. Their interest is to see seL4 flourish, not undermined.

How about the TSC?

The role of the Technical Steering Committee is defined by the seL4 Foundation Technical Charter. Its role is defining the rules governing development of seL4 and the core open-source technologies around it, enable and foster technical collaboration and provide technical oversight.

Who is on the TSC?

Its members are the original committers, plus further trusted contributors the TSC decides to invite. So it’s really the trusted technical leadership – you can’t buy your way in there. (Note that this is different from, e.g., RISC-V International; Premium Members get representation on the RISC-V TSC.)

The seL4 TSC also blesses new reviewers and committers, ensuring that only people who have earned the trust of the community can authorise changes to the mainline code (but never on their own).

Will CSIRO’s abandonment slow seL4 development?

This is an obvious concern, and there was a very serious risk that this could happen.

Financially, the impact is minimal. In CSIRO we had some base funding that helped us maintain the technology. While this has disappeared, it was quickly picked up by the community. Community contributions have increased massively since we moved out of CSIRO, and the Foundation membership has grown (increasing its budget ten-fold). Those two developments together are easily making up for the loss. And all major developments have been externally funded ever since TS became part of CSIRO. CSIRO’s commitment to seL4 was never more than superficial.

The biggest risk right after CSIRO’s move was to lose most of TS’s people. Mountains of money are no use if you don’t have the skilled people to spend it on. And that risk was very real: After word of CSIRO’s actions got out, many people had job offers within days. Had not UNSW, within a day(!), stepped up to support the team for half a year, the result might well have been catastrophic for seL4.

Thanks to UNSW, that has been avoided. We still lost many people, but most stayed in the community and keep contributing as part of their new job. This is healthy, as it broadens the community and helps it mature to the point where it is no longer dependent on a single organisation. And UNSW gave us the buffer we needed to line up new funding to continue the TS team and grow it back. We’re in the middle of this, and it’s going well – you’ll hear some announcements soon. For now I can just say how extremely grateful we all are to Aaron Quigley, Head of School of Computer Science & Engineering at UNSW, for moving quickly and decisively to protect TS and seL4.

How will we fund major projects?

There are a number of major projects we’re looking to fund, mostly the “big ticket” verification projects (AArch64 and multicore), some medium-sized verification projects (completing the RISC-V verification story and completing verification of the MCS kernel), as well as a number of systems projects (such as multicore VMM, device virtualisation, an actual seL4-based OS). We need external funding for those, and that would not have been different with CSIRO.

But we’re in a much stronger position now. The community has realised that these things won’t happen on their own, they need to contribute. And the community, and especially the Foundation, has grown, which helps. And we have a much stronger (and growing) ecosystem of companies providing services around seL4, which itself strengthens the ecosystem and will lead to further investment. One of these companies is, of course, Proofcraft, the provider of verification services founded by the leaders of the seL4 verification.

The future

Personally I’m much more bullish about seL4’s future than I was a year ago (and those who heard me talk about seL4 then will understand that this means something!) CSIRO was, in many ways, a weight that slowed us down. When I bring money into UNSW, I remain in control of the funds, and can hire as needed. At CSIRO, in contrast, we were always at the mercy of management, much of which are people who have never achieved anything of scientific significance in their life, but think they know better than those who have – definitely a poor fit.

Since the divorce, things have been moving faster in the right direction than I anticipated, and by its anniversary we will be in a far stronger position than before. You’ll see some indications of that in the near future.

Stay tuned, stay calm, and trust your kernel (and the team behind it!)

One Comment
  1. Sounds like a great plan. Formal verification mixed with a carefully conceived governance makes for an explosive combination in our quest for ultra-high levels of assurance. Go Sel4!

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