installer: Don't run as root

There's many reason why it is and is going to
continue to be difficult to do this:

1. All display-managers (excluding slim) default PAM rules
disallow root auto login.

2. We can't use wayland

3. We have to use system-wide pulseaudio

4. It could break applications in the session.
This happened to dolphin in plasma5
in the past.

This is a growing technical debt, let's just use
passwordless sudo.

nixos/doc/manual/installation/installing.xml

···
   </para>
 
   <para>
-   You are logged-in automatically as <literal>root</literal>. (The
-   <literal>root</literal> user account has an empty password.)
+   You are logged-in automatically as <literal>nixos</literal>.
+   The <literal>nixos</literal> user account has an empty password so you
+   can use <command>sudo</command> without a password.
   </para>
 
   <para>
    If you downloaded the graphical ISO image, you can run <command>systemctl
-   start display-manager</command> to start KDE. If you want to continue on the
+   start display-manager</command> to start the desktop environment. If you want to continue on the
    terminal, you can use <command>loadkeys</command> to switch to your
    preferred keyboard layout. (We even provide neo2 via <command>loadkeys de
    neo</command>!)
···
 
    <para>
     If you would like to continue the installation from a different machine you
-    need to activate the SSH daemon via <literal>systemctl start
-    sshd</literal>. In order to be able to login you also need to set a
-    password for <literal>root</literal> using <literal>passwd</literal>.
+    need to activate the SSH daemon via <command>systemctl start
+    sshd</command>. You then must set a password for either <literal>root</literal> or
+    <literal>nixos</literal> with <command>passwd></command> to be able to login.
    </para>
   </section>
  </section>

nixos/doc/manual/release-notes/rl-1909.xml

···
        set up binfmt interpreters for each of those listed systems.
      </para>
    </listitem>
+   <listitem>
+     <para>
+     The installer now uses a less privileged <literal>nixos</literal> user whereas before we logged in as root.
+     To gain root privileges use <literal>sudo -i</literal> without a password.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 

nixos/modules/installer/cd-dvd/installation-cd-graphical-base.nix

···
 {
   imports = [ ./installation-cd-base.nix ];
 
+  # Whitelist wheel users to do anything
+  # This is useful for things like pkexec
+  #
+  # WARNING: this is dangerous for systems
+  # outside the installation-cd and shouldn't
+  # be used anywhere else.
+  security.polkit.extraConfig = ''
+    polkit.addRule(function(action, subject) {
+      if (subject.isInGroup("wheel")) {
+        return polkit.Result.YES;
+      }
+    });
+  '';
+
   services.xserver = {
     enable = true;
 
     # Don't start the X server by default.
     autorun = mkForce false;
 
-    # Automatically login as root.
+    # Automatically login as nixos.
     displayManager.slim = {
       enable = true;
-      defaultUser = "root";
+      defaultUser = "nixos";
       autoLogin = true;
     };
 
···
 
   # Enable sound in graphical iso's.
   hardware.pulseaudio.enable = true;
-  hardware.pulseaudio.systemWide = true; # Needed since we run plasma as root.
 
   environment.systemPackages = [
     # Include gparted for partitioning disks.

nixos/modules/installer/cd-dvd/installation-cd-graphical-kde.nix

···
 # This module defines a NixOS installation CD that contains X11 and
-# Plasma5.
+# Plasma 5.
 
 { config, lib, pkgs, ... }:
 
···
       Version=1.0
       Type=Application
       Name=NixOS Manual
-      Exec=firefox ${config.system.build.manual.manualHTMLIndex}
+      Exec=firefox ${config.system.build.manual.manual}/share/doc/nixos/index.html
       Icon=text-html
     '';
 
+    homeDir = "/home/nixos/";
+    desktopDir = homeDir + "Desktop/";
+
   in ''
-    mkdir -p /root/Desktop
-    ln -sfT ${manualDesktopFile} /root/Desktop/nixos-manual.desktop
-    ln -sfT ${pkgs.konsole}/share/applications/org.kde.konsole.desktop /root/Desktop/org.kde.konsole.desktop
-    ln -sfT ${pkgs.gparted}/share/applications/gparted.desktop /root/Desktop/gparted.desktop
+    mkdir -p ${desktopDir}
+    chown nixos ${homeDir} ${desktopDir}
+
+    ln -sfT ${manualDesktopFile} ${desktopDir + "nixos-manual.desktop"}
+    ln -sfT ${pkgs.gparted}/share/applications/gparted.desktop ${desktopDir + "gparted.desktop"}
+    ln -sfT ${pkgs.konsole}/share/applications/org.kde.konsole.desktop ${desktopDir + "org.kde.konsole.desktop"}
   '';
 
 }

nixos/modules/profiles/installation-device.nix

···
     #services.rogue.enable = true;
 
     # Disable some other stuff we don't need.
-    security.sudo.enable = mkDefault false;
     services.udisks2.enable = mkDefault false;
 
+    # Use less privileged nixos user
+    users.users.nixos = {
+      isNormalUser = true;
+      extraGroups = [ "wheel" "networkmanager" "video" ];
+      # Allow the graphical user to login without password
+      initialHashedPassword = "";
+    };
+
+    # Allow the user to log in as root without a password.
+    users.users.root.initialHashedPassword = "";
+
+    # Allow passwordless sudo from nixos user
+    security.sudo = {
+      enable = mkDefault true;
+      wheelNeedsPassword = mkForce false;
+    };
+
     # Automatically log in at the virtual consoles.
-    services.mingetty.autologinUser = "root";
+    services.mingetty.autologinUser = "nixos";
 
     # Some more help text.
     services.mingetty.helpLine =
       ''
 
-        The "root" account has an empty password.  ${
+        The "nixos" and "root" account have empty passwords.  ${
           optionalString config.services.xserver.enable
-            "Type `systemctl start display-manager' to\nstart the graphical user interface."}
+            "Type `sudo systemctl start display-manager' to\nstart the graphical user interface."}
       '';
 
     # Allow sshd to be started manually through "systemctl start sshd".
···
     # because we have the firewall enabled. This makes installs from the
     # console less cumbersome if the machine has a public IP.
     networking.firewall.logRefusedConnections = mkDefault false;
-
-    # Allow the user to log in as root without a password.
-    users.users.root.initialHashedPassword = "";
   };
 }