Containers in NixOS
from [email protected] to [email protected] on 17 May 14:59
https://feddit.org/post/12610534

Hi everyone!

I’m in the process of finally doing containers right in my NixOS installation. This is my ‘wishlist’:

• podman containers should be run by users with minimal permissions
• separate user per container
• containers managed by systemd services for easier management

My current work-in-progress setup looks like this:

For each service (called $name), I have:

• a user and corresponding group (referred to as $uid in the following)
• a directory /srv/$name owned by $uid, in which mounted volumes are located

My containers are declared like this:

virtualisation.oci-containers.containers = {
    $name = {
        image = ...;
        ports = [ ... ];
        volumes = [
            "/srv/${name}/config:/config"
            ...
        ];
        user = $uid:$gid;
        extraOptions = [
            "--security-opt=no-new-privileges:true"
        ];
    };
};

Now for the parts I don’t fully understand yet:

• some images allow setting environment.PUID to specify a user. Does setting this option (and not setting user=$uid in the container declaration itself) mean that the container will be run as root, and the program inside will merely use PUID when e.g. creating files? This would still allow a malicious container to run commands as root on the host, right?

• virtualisation.oci-containers.containers creates a systemd service. Since this is not a user-service for my user $uid, I need sudo to start/stop the container. Does that mean that the systemd service is run with root permissions, but it executes the command to spawn the container as $uid? If whatever is running inside the container was malicious, is there a functional difference between the container being started ‘by root as $uid’ and it being started by me (after logging in as $uid)?

• Is it feasible to make these systemd services user-services owned by $uid instead?

• Are there further hardening steps I forgot about?

Thanks for your input!

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