nixos/rspamd: add systemd service sandbox

Drop preStart script in favour of systemd StateDirectory parameter.

nixos/modules/services/mail/rspamd.nix

@@ -394,16 +394,43 @@ in
       restartTriggers = [ rspamdDir ];
 
       serviceConfig = {
-        ExecStart = "${pkgs.rspamd}/bin/rspamd ${optionalString cfg.debug "-d"} --user=${cfg.user} --group=${cfg.group} --pid=/run/rspamd.pid -c /etc/rspamd/rspamd.conf -f";
+        ExecStart = "${pkgs.rspamd}/bin/rspamd ${optionalString cfg.debug "-d"} -c /etc/rspamd/rspamd.conf -f";
         Restart = "always";
-        RuntimeDirectory = "rspamd";
-        PrivateTmp = true;
-      };
 
-      preStart = ''
+        User = "${cfg.user}";
-        ${pkgs.coreutils}/bin/mkdir -p /var/lib/rspamd
+        Group = "${cfg.group}";
-        ${pkgs.coreutils}/bin/chown ${cfg.user}:${cfg.group} /var/lib/rspamd
+
-      '';
+        RuntimeDirectory = "rspamd";
+        RuntimeDirectoryMode = "0755";
+        StateDirectory = "rspamd";
+        StateDirectoryMode = "0700";
+
+        AmbientCapabilities = "";
+        CapabilityBoundingSet = "";
+        DevicePolicy = "closed";
+        LockPersonality = true;
+        NoNewPrivileges = true;
+        PrivateDevices = true;
+        PrivateMounts = true;
+        PrivateTmp = true;
+        PrivateUsers = true;
+        ProtectClock = true;
+        ProtectControlGroups = true;
+        ProtectHome = true;
+        ProtectHostname = true;
+        ProtectKernelLogs = true;
+        ProtectKernelModules = true;
+        ProtectKernelTunables = true;
+        ProtectSystem = "strict";
+        RemoveIPC = true;
+        RestrictAddressFamilies = "AF_INET AF_INET6 AF_UNIX";
+        RestrictNamespaces = true;
+        RestrictRealtime = true;
+        RestrictSUIDSGID = true;
+        SystemCallArchitectures = "native";
+        SystemCallFilter = "@system-service";
+        UMask = "0077";
+      };
     };
   };
   imports = [