Determine security requirements for automating kernel builds

[legoktm]

Splitting from https://github.com/freedomofpress/securedrop/issues/6514

On a technical level, to build packages one needs a machine (currently a clean Qubes VM) that has Docker installed, and then:

1. run make securedrop-core-5.15, wait 2+ hours
2. run make securedrop-workstation-5.15, wait 2+ hours
3. upload the build logs to the build-logs repo
4. sign and upload the source tarballs to S3
5. copy and upload the debs to apt-test, to kick off kernel testing

Currently these steps are done manually, on maintainer laptops. This seems ripe for automation, especially because it's a slow process.

One important note is that these builds are currently not reproducible (see #3).

So if we were to automate this process, what are the requirements for the build host? Would we be OK if:

1. it was entirely run on a CircleCI pipeline (or other cloud CI provider, e.g. CodeFresh)?
2. it was entirely run on a DO droplet we/infra controls?
3. it was entirely run on a physical machine under FPF control (e.g. in NYO)?
4. status quo, entirely run on a maintainer laptop

Pinging @l3th3 & @lsd-cat for security input

[thedeadliestcatch]

@legoktm Headbutting into this for a quick note:

• I would suggest leveraging a Qubes system that runs on a M/B compatible with coreboot, and a modern CPU (13th gen AFAIK should work). You will speed up the process.
• The sources and other sensitive directories should be read-only if containerized, and without external network access.
• This can be done with Docker. You could nest it inside a LXC container that you can snapshot, etc.

Unfortunately there is no easy way to protect non-repro builds, especially if using external infrastructure. We abhor CI for our kernel builds for that reason.

But there is a suggestion, which applies to repro builds:

• Have a completely offline system that has been sufficiently protected physically run scheduled compilations. Obtain checksums/signatures of the resulting binaries and packages.
• Verify manually on schedule that the signatures/checksums match with the publicly available packages.

If the builds are reproducible, then anybody can basically verify them in your team.

You could also deploy a data diode (if available to you or you trust someone to be capable enough to build one) to automate the retrieval of the signatures/checksums.

Secure CI is very much an unsolved problem, for now.

As for non repro builds: they would arguably be the most secure, but you give up the seed/structure randomization capabilities of the grsec patch. However, this is also the case for public/published kernel packages: you are already giving up the structures/ABI of the vmlinux and co. Any compile-time mitigations that affect system unpredictability are mutually exclusive with providing external access to the kernel binaries.