Entropy seed regresses in quality and does not actually init RNG

[zx2c4]

This report is the analog of https://bugs.gentoo.org/835926.

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There are two security problems with OpenRC's RNG initialization shell script.

1. Writing into /dev/{u}random doesn't actually initialize the RNG. It writes into the entropy pool, but no entropy is credited. It is impossible to actually credit from a shell script.

2. Reading a seed back immediately after writing into /dev/{u}random will regress in entropy at early boot, because /dev/{u}random doesn't actually start using the written seed until the RNG is initialized. This is bad news bears, as it means if you start with a good seed, you'll immediately get a bad, potentially deterministic, one.

There are two solutions:

a) To fix problem (2) with a shellscript, you can adopt this approach from buildroot: https://git.buildroot.net/buildroot/commit/?id=f0986de551f46e72268857fd817986e9be697cd0 which is also used in systemd: systemd/systemd@da2862e

b) To fix problem (1) and to more robustly address problem (2), you should use my seedrng.c code:
https://git.zx2c4.com/seedrng/about/
https://git.zx2c4.com/seedrng/tree/seedrng.c

Since OpenRC is a rather nimble project, I'd recommend going straight to solution (b) and skipping over the intermediate solution (a).

I'm looking now into bringing seedrng into busybox for use in buildroot. It would also make sense to bring this into openrc. seedrng.c is C that's meant to be copied and pasted into OpenRC somewhere, and I don't mind if it gets reformatted/renamed/re-whatevered. It's meant to be mangled as needed. Maybe it's a thing for libexec; maybe it's a thing for sbin. I'm okay with whatever in that regard. Just off hand, looking closer at openrc, it'd probably make sense to call this openrc-seedrng, akin to openrc-shutdown:

  executable('openrc-shutdown',
    ['openrc-shutdown.c', 'broadcast.c', 'rc-sysvinit.c', rc_misc_c,
                usage_c, rc_wtmp_c, version_h],
    c_args : cc_branding_flags,
    include_directories: [incdir, einfo_incdir, rc_incdir],
    link_with: [libeinfo, librc],
    install: true,
    install_dir: sbindir)

And then you can adjust seedrng.c's printing to use libeinfo.

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CC @vapier @williamh