nixos/cage: init

Add a cage module to nixos. This can be used to make kiosk-style
systems that boot directly to a single application. The user (demo by
default) is automatically logged in by this service and the
program (xterm by default) is automatically started.

This is useful for some embedded, single-user systems where we want
automatic booting. To keep the system secure, the user should have
limited privileges.

Based on the service provided in the Cage wiki here:

https://github.com/Hjdskes/cage/wiki/Starting-Cage-on-boot-with-systemd

Co-Authored-By: Florian Klink <flokli@flokli.de>

nixos/modules/module-list.nix

@@ -807,6 +807,7 @@
   ./services/ttys/agetty.nix
   ./services/ttys/gpm.nix
   ./services/ttys/kmscon.nix
+  ./services/wayland/cage.nix
   ./services/web-apps/atlassian/confluence.nix
   ./services/web-apps/atlassian/crowd.nix
   ./services/web-apps/atlassian/jira.nix

nixos/modules/services/wayland/cage.nix

@@ -0,0 +1,99 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.services.cage;
+in {
+  options.services.cage.enable = mkEnableOption "cage kiosk service";
+
+  options.services.cage.user = mkOption {
+    type = types.str;
+    default = "demo";
+    description = ''
+      User to log-in as.
+    '';
+  };
+
+  options.services.cage.extraArguments = mkOption {
+    type = types.listOf types.str;
+    default = [];
+    defaultText = "[]";
+    description = "Additional command line arguments to pass to Cage.";
+    example = ["-d"];
+  };
+
+  options.services.cage.program = mkOption {
+    type = types.path;
+    default = "${pkgs.xterm}/bin/xterm";
+    description = ''
+      Program to run in cage.
+    '';
+  };
+
+  config = mkIf cfg.enable {
+
+    # The service is partially based off of the one provided in the
+    # cage wiki at
+    # https://github.com/Hjdskes/cage/wiki/Starting-Cage-on-boot-with-systemd.
+    systemd.services."cage-tty1" = {
+      enable = true;
+      after = [
+        "systemd-user-sessions.service"
+        "plymouth-start.service"
+        "plymouth-quit.service"
+        "systemd-logind.service"
+        "getty@tty1.service"
+      ];
+      before = [ "graphical.target" ];
+      wants = [ "dbus.socket" "systemd-logind.service" "plymouth-quit.service"];
+      wantedBy = [ "graphical.target" ];
+      conflicts = [ "getty@tty1.service" ];
+
+      restartIfChanged = false;
+      serviceConfig = {
+        ExecStart = ''
+          ${pkgs.cage}/bin/cage \
+            ${escapeShellArgs cfg.extraArguments} \
+            -- ${cfg.program}
+        '';
+        User = cfg.user;
+
+        ConditionPathExists = "/dev/tty1";
+        IgnoreSIGPIPE = "no";
+
+        # Log this user with utmp, letting it show up with commands 'w' and
+        # 'who'. This is needed since we replace (a)getty.
+        UtmpIdentifier = "%n";
+        UtmpMode = "user";
+        # A virtual terminal is needed.
+        TTYPath = "/dev/tty1";
+        TTYReset = "yes";
+        TTYVHangup = "yes";
+        TTYVTDisallocate = "yes";
+        # Fail to start if not controlling the virtual terminal.
+        StandardInput = "tty-fail";
+        StandardOutput = "syslog";
+        StandardError = "syslog";
+        # Set up a full (custom) user session for the user, required by Cage.
+        PAMName = "cage";
+      };
+    };
+
+    security.pam.services.cage.text = ''
+      auth    required pam_unix.so nullok
+      account required pam_unix.so
+      session required pam_unix.so
+      session required ${pkgs.systemd}/lib/security/pam_systemd.so
+    '';
+
+    hardware.opengl.enable = mkDefault true;
+
+    systemd.targets.graphical.wants = [ "cage-tty1.service" ];
+
+    systemd.defaultUnit = "graphical.target";
+  };
+
+  meta.maintainers = with lib.maintainers; [ matthewbauer flokli ];
+
+}

nixos/tests/all-tests.nix

@@ -39,6 +39,7 @@ in
   buildbot = handleTest ./buildbot.nix {};
   caddy = handleTest ./caddy.nix {};
   cadvisor = handleTestOn ["x86_64-linux"] ./cadvisor.nix {};
+  cage = handleTest ./cage.nix {};
   cassandra = handleTest ./cassandra.nix {};
   ceph-single-node = handleTestOn ["x86_64-linux"] ./ceph-single-node.nix {};
   ceph-multi-node = handleTestOn ["x86_64-linux"] ./ceph-multi-node.nix {};

nixos/tests/cage.nix

@@ -0,0 +1,43 @@
+import ./make-test-python.nix ({ pkgs, ...} :
+
+{
+  name = "cage";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ matthewbauer flokli ];
+  };
+
+  machine = { ... }:
+
+  {
+    imports = [ ./common/user-account.nix ];
+    services.cage = {
+      enable = true;
+      user = "alice";
+      program = "${pkgs.xterm}/bin/xterm -cm -pc"; # disable color and bold to make OCR easier
+    };
+
+    # this needs a fairly recent kernel, otherwise:
+    #   [backend/drm/util.c:215] Unable to add DRM framebuffer: No such file or directory
+    #   [backend/drm/legacy.c:15] Virtual-1: Failed to set CRTC: No such file or directory
+    #   [backend/drm/util.c:215] Unable to add DRM framebuffer: No such file or directory
+    #   [backend/drm/legacy.c:15] Virtual-1: Failed to set CRTC: No such file or directory
+    #   [backend/drm/drm.c:618] Failed to initialize renderer on connector 'Virtual-1': initial page-flip failed
+    #   [backend/drm/drm.c:701] Failed to initialize renderer for plane
+    boot.kernelPackages = pkgs.linuxPackages_latest;
+
+    virtualisation.memorySize = 1024;
+  };
+
+  enableOCR = true;
+
+  testScript = { nodes, ... }: let
+    user = nodes.machine.config.users.users.alice;
+  in ''
+    with subtest("Wait for cage to boot up"):
+        start_all()
+        machine.wait_for_file("/run/user/${toString user.uid}/wayland-0.lock")
+        machine.wait_until_succeeds("pgrep xterm")
+        machine.wait_for_text("alice@machine")
+        machine.screenshot("screen")
+  '';
+})