nix: harden Systemd service in NixOS module

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I086cdbfcd0a0d1d87173a2cf97f5b5416a6a6964

nix/modules/nixos.nix

@@ -25,59 +25,149 @@ in {
       '';
     };
 
-    user = mkOption {
-      type = str;
-      default = "troutbot";
-    };
-
-    group = mkOption {
-      type = str;
-      default = "troutbot";
-    };
-
-    port = mkOption {
-      type = port;
-      default = 3000;
-    };
-
     environmentFile = mkOption {
       type = nullOr str;
       default = null;
+      description = ''
+        Environment file for specifying additional settings such as secrets.
+      '';
     };
 
     configPath = mkOption {
       type = nullOr str;
       default = null;
+      description = ''
+        File path containing the Troutbot Typescript config.
+      '';
+    };
+
+    settings = {
+      port = mkOption {
+        type = port;
+        default = 3000;
+      };
+
+      user = mkOption {
+        type = str;
+        default = "troutbot";
+        description = "User to run Troutbot under";
+      };
+
+      group = mkOption {
+        type = str;
+        default = "troutbot";
+        description = "Group to run Troutbot under";
+      };
     };
   };
 
   config = mkIf cfg.enable {
-    users.users.${cfg.user} = {
-      isSystemUser = true;
-      group = cfg.group;
+    users = {
+      groups.${cfg.settings.group} = {};
+      users.${cfg.settings.user} = {
+        isSystemUser = true;
+        inherit (cfg.settings) group;
+      };
     };
 
-    users.groups.${cfg.group} = {};
-
     systemd.services.troutbot = {
-      description = "Troutbot";
-      after = ["network.target"];
+      description = "Troutbot - GitHub PR/Issue Analysis Bot";
+      documentation = ["https://github.com/notashelf/troutbot"];
+      after = ["network-online.target"];
+      wants = ["network-online.target"];
       wantedBy = ["multi-user.target"];
+
+      environment = {
+        NODE_ENV = "production";
+        CONFIG_PATH = cfg.configPath;
+        PORT = toString cfg.port;
+      };
+
       serviceConfig = {
         Type = "simple";
         User = cfg.user;
         Group = cfg.group;
         ExecStart = "${lib.getExe cfg.package}";
+
+        # Restart policy with rate limiting
         Restart = "on-failure";
-        EnvironmentFile = cfg.environmentFile;
-        NODE_ENV = "production";
-        CONFIG_PATH = cfg.configPath;
-        PORT = toString cfg.port;
+        RestartSec = "5s";
+        StartLimitInterval = "60s";
+        StartLimitBurst = 3;
+
+        # Timeouts
+        TimeoutStartSec = "30s";
+        TimeoutStopSec = "30s";
+
+        # Working directory and state
+        WorkingDirectory = "/var/lib/troutbot";
+        StateDirectory = "troutbot";
+        StateDirectoryMode = "0750";
+
+        # Environment file for secrets
+        EnvironmentFile = lib.mkIf (cfg.environmentFile != null) cfg.environmentFile;
+
+        # Security hardening
+        # Filesystem restrictions
         ProtectSystem = "strict";
         ProtectHome = true;
         PrivateTmp = true;
+        PrivateDevices = true;
+
+        # Process restrictions
         NoNewPrivileges = true;
+        ProtectKernelTunables = true;
+        ProtectKernelModules = true;
+        ProtectControlGroups = true;
+        ProtectClock = true;
+
+        # Memory and execution restrictions
+        MemoryDenyWriteExecute = true;
+        RestrictRealtime = true;
+        RestrictSUIDSGID = true;
+        LockPersonality = true;
+
+        # IPC cleanup
+        RemoveIPC = true;
+
+        # Capabilities
+        CapabilityBoundingSet = [""]; # no capabilities needed
+        AmbientCapabilities = []; # no ambient capabilities
+
+        # System call filtering
+        SystemCallFilter = [
+          "@system-service"
+          "~@privileged"
+          "~@resources"
+        ];
+        SystemCallErrorNumber = "EPERM";
+
+        # Address families, only IPv4/IPv6 needed
+        RestrictAddressFamilies = [
+          "AF_INET"
+          "AF_INET6"
+          "AF_UNIX"
+          "AF_NETLINK" # for DNS resolution
+        ];
+
+        UMask = "0027";
+        DeviceAllow = [];
       };
     };
+
+    serviceConfig = {
+      Type = "simple";
+      User = cfg.settings.user;
+      Group = cfg.settings.group;
+      ExecStart = "${lib.getExe cfg.package}";
+      Restart = "on-failure";
+      EnvironmentFile = mkIf (cfg.environmentFile != null) cfg.environmentFile;
+      WorkingDirectory = "/var/lib/troutbot";
+      StateDirectory = "troutbot";
+      ProtectSystem = "strict";
+      ProtectHome = true;
+      PrivateTmp = true;
+      NoNewPrivileges = true;
+    };
   };
 }