Comment by lrvick | Hacker News Reader

Comment by lrvick | Hacker News Reader

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I for one do not want any code in the TCB that is not formally verified, tiny, and easy to audit with every line of code justified.

SeL4 is the only open, mature, and formally verified L4 solution I am aware of that can run on modern servers, but IIRC given vCPUs are not first class objects and any low level CPU virtualization features are handled in user space, it comes at a real performance cost making it only suitable for single-core or non-performance-critical applications.

Nova by contrast was built to handle the expensive and trust critical TPM and vCPU handling directly for high performance remotely attestable multi-core applications without having to reach for something as heavy as Xen, though formal verification is admittedly not as complete as SeL4 yet AFAICT, though lots of recent progress here.

Those are pretty big tradeoffs, but if you or anyone else knows other viable xen alternatives this general shape with a tiny easily-auditable footprint, please chime in!

Like some of what I'm saying is just that if you tell a systems security person "we're using seL4 because of its formal verification", you're going to get eye rolls. Whatever else you want to say about it, the formally-verified configuration of seL4 doesn't actually do much, meaning whatever system you're building on top of it is going to be where all the security drama is anyways.

I'm certainly not telling you to use Xen. I wouldn't use Xen for anything untrusted.

> the formally-verified configuration of seL4 doesn't actually do much

As a systems security engineer myself I would disagree and know several others that would as well. All depends on your circles, applications, and threat model I suppose.

Xen, Hyper-V, and KVM have all had major security flaws in the form of logic violations, spacial memory corruptions, use-after-free, and pointer corruptions that allowed hypervisor trust boundary violations. I expect most if not all of those classes of bugs could have been prevented by formal verification.

Xen likely has a bad reputation with both of us because of a history of preventable flaws like these.

In my applications every single daemon gets its own sandbox, which is a VM whenever possible. Some with network, some without network, different filesystem and vsock access for maximum damage control and minimal trust between daemons. The hypervisor is the most important security boundary to me especially when isolating user-controlled workloads. The more confidence I can get that it is correct the better.

You're responding to a comment where I wrote that I would never use Xen with an argument not to use Xen. All I'm saying is that "formal verification", at least as it exists in current systems, is not why I wouldn't use Xen.

> an argument not to use Xen

Not my intent but perhaps I communicated poorly. It was an argument that the -reason- Xen has had so many bugs is because it was not built for correctness.

Formal verification should be a bare minimum for a TCB component to know many classes of bugs that have plagued the most popular hypervisors cannot happen.

We both clearly agree not to use Xen.

Anyway, I appreciate the engagement on this all the same.

Few even know alternatives to xen exist!

Yeah, all I'm saying is that I agree with you about Xen, but that I don't agree about the centrality of formal methods --- at least on current projects with current verifications, things could get different now that AI lets us be more ambitious with verification --- in determining attack surface.

Generally, the less your OS can do, apart from strictly hypervising guest operating systems, the safer you probably are. Hence why people do things like seL4.

> the less your OS can do, apart from strictly hypervising guest operating systems, the safer you probably are

100% agreement here and I -would- take something tiny I can audit myself over something huge and formally verified if forced to choose. Thankfully I do not have to choose.

> AI lets us be more ambitious with verification

Exactly. For TCB code formal verification is a minimum, not sufficient, IMO. Too cheap these days to not have very complete test coverage, formal verification, risc0 execution proofs, seccomp tracing. For security critical code we should demand all of it now.

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