Design patterns for privilege separating systemd services?

["Colin Walters" <walters@xxxxxxxxxx>]

I'm having a debate with the SSSD team over here around multiple systemd units and privilege separation: https://github.com/SSSD/sssd/issues/3412

And we also had a related topic come up in Fedora CoreOS where we have a privileged service (rpm-ostreed.service) and we want a separate unprivileged management service (zincati.service) to be able to perform privileged operations; right now we install a polkit rule to authorize the `zincati` user, but I think that's quite hacky.  I'd actually like to use DynamicUser=yes for zincati.

I think what I want is a way to tell systemd to create something like a private socketpair() between the two services dynamically.  So the "privileged" part of zincati.service is just a systemd drop-in like:

/usr/lib/systemd/system/rpm-ostreed.service.d/
```
[Unit]
DynamicSocketPair=zincati.service
```

Then when either service starts for the first time, systemd creates a socketpair() and hands off one end to the activating service.  And there'd be an extension of sd_listen_fds() that gives me the tuple (fd, "zincati.service") or so?

Perhaps a pattern that can be implemented today for this is for the privileged helper to create an "inaccessible directory" e.g. /run/rpm-ostree/privapi (last component mode 0700) with a .socket unit there, and then in zincati.service uses WorkingDirectory=/run/rpm-ostree/privapi to traverse it and access /run/rpm-ostree/privapi/rpmostree.socket or so.

The main goal here is to make this communication channel truly private to the participating services - as opposed to e.g. exposing a DBus service or globally-accessible socket, since that brings in the problems of authorization and increases attack surface generally.

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