nixos/mosquitto: harden systemd unit

It can still network, it can only access the ssl related files if ssl is
enabled.

✗ PrivateNetwork=                                             Service has access to the host's network                                            0.5
✗ RestrictAddressFamilies=~AF_(INET|INET6)                    Service may allocate Internet sockets                                               0.3
✗ DeviceAllow=                                                Service has a device ACL with some special devices                                  0.1
✗ IPAddressDeny=                                              Service does not define an IP address allow list                                    0.2
✗ RootDirectory=/RootImage=                                   Service runs within the host's root directory                                       0.1
✗ RestrictAddressFamilies=~AF_UNIX                            Service may allocate local sockets                                                  0.1

→ Overall exposure level for mosquitto.service: 1.1 OK 🙂

nixos/modules/services/networking/mosquitto.nix

@@ -233,15 +233,50 @@ in
         ExecStart = "${pkgs.mosquitto}/bin/mosquitto -c ${mosquittoConf}";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
 
-        ProtectSystem = "strict";
+        # Hardening
-        ProtectHome = true;
+        CapabilityBoundingSet = "";
+        DevicePolicy = "closed";
+        LockPersonality = true;
+        MemoryDenyWriteExecute = true;
+        NoNewPrivileges = true;
         PrivateDevices = true;
         PrivateTmp = true;
-        ReadWritePaths = "${cfg.dataDir}";
+        PrivateUsers = true;
+        ProtectClock = true;
         ProtectControlGroups = true;
+        ProtectHome = true;
+        ProtectHostname = true;
+        ProtectKernelLogs = true;
         ProtectKernelModules = true;
         ProtectKernelTunables = true;
-        NoNewPrivileges = true;
+        ProtectProc = "invisible";
+        ProcSubset = "pid";
+        ProtectSystem = "strict";
+        ReadWritePaths = [
+          cfg.dataDir
+          "/tmp"  # mosquitto_passwd creates files in /tmp before moving them
+        ];
+        ReadOnlyPaths = with cfg.ssl; lib.optionals (enable) [
+          certfile
+          keyfile
+          cafile
+        ];
+        RemoveIPC = true;
+        RestrictAddressFamilies = [
+          "AF_UNIX"  # for sd_notify() call
+          "AF_INET"
+          "AF_INET6"
+        ];
+        RestrictNamespaces = true;
+        RestrictRealtime = true;
+        RestrictSUIDSGID = true;
+        SystemCallArchitectures = "native";
+        SystemCallFilter = [
+          "@system-service"
+          "~@privileged"
+          "~@resources"
+        ];
+        UMask = "0077";
       };
       preStart = ''
         rm -f ${cfg.dataDir}/passwd

nixos/tests/mosquitto.nix

@@ -1,4 +1,4 @@
-import ./make-test-python.nix ({ pkgs, ... }:
+import ./make-test-python.nix ({ pkgs, lib, ... }:
 
 let
   port = 1888;
@@ -30,6 +30,9 @@ in {
           ];
         };
       };
+
+      # disable private /tmp for this test
+      systemd.services.mosquitto.serviceConfig.PrivateTmp = lib.mkForce false;
     };
 
     client1 = client;