The need to drop privileges at all is a natural consequence of a process spawning API with inherit-by-default capability semantics. You'd never build a new VM or OS this way if you didn't require compatibility with existing software. The secure solution has always been default-nothing semantics, with whitelisted capabilities granted via explicit arguments in the process spawning API.

The closest you can get to that model on Linux is the strict mode in seccomp, which disallows every syscall except read(), write(), exit() and sigreturn(). It's more or less a way to restrict a process to being "pure compute/memory". If the process then wants to poke and prod at the outside world, it can only do so by reading/writing the file descriptors it inherited prior to the seccomp call. You can build a RPC on top of that to emulate the "whitelist", with access control and restrictions/policies enforced by whatever is listening on the other end.

Justine Tunney wrote a port of OpenBSD pledge to Linux, in the form of a wrapper for seccomp-bpf.

https://archive.is/3FSWy

Not sure why her TLS cert expired months ago. Ted Unangst (OpenBSD dev) also seems to have disappeared, which is concerning.

Interesting no mentioned about OpenBSD pledge(2), unveil(2). By far the easiest sandboxing out there. An example:

    #ifdef OpenBSD
      if(pledge("stdio rpath wpath cpath",NULL) == -1)
        err(1,"pledge\n");
    #endif

Very easy, all the other examples seem rather complex to me. unveil(2) is just as easy.