nixos/nginx: update hardening settings

- Set an explicit umask that allows u+rwx and g+r.
- Adds `ProtectControlGroups` and `ProtectKernelLogs`, there should be
  no need to access either.
- Adds `ProtectClock` to prevent write-access to the system clock.
- `ProtectProc` hides processes from other users within the /proc
  filesystem and `ProcSubSet` hides all files/directories unrelated to
  the process management of the units process.
- Sets `RemoveIPC`, as there is no SysV or POSIX IPC within nginx that I
  know of.
- Restricts the creation of arbitrary namespaces
- Adds a reasonable `SystemCallFilter` preventing calls to @privileged,
  @obsolete and others.

And finally applies some sorting based on the order these options appear
in systemd.exec(5).

nixos/modules/services/web-servers/nginx/default.nix

@@ -819,28 +819,38 @@ in
         # Logs directory and mode
         LogsDirectory = "nginx";
         LogsDirectoryMode = "0750";
+        # Proc filesystem
+        ProcSubset = "pid";
+        ProtectProc = "invisible";
+        # New file permissions
+        UMask = "0027"; # 0640 / 0750
         # Capabilities
         AmbientCapabilities = [ "CAP_NET_BIND_SERVICE" "CAP_SYS_RESOURCE" ];
         CapabilityBoundingSet = [ "CAP_NET_BIND_SERVICE" "CAP_SYS_RESOURCE" ];
         # Security
         NoNewPrivileges = true;
-        # Sandboxing
+        # Sandboxing (sorted by occurrence in https://www.freedesktop.org/software/systemd/man/systemd.exec.html)
         ProtectSystem = "strict";
         ProtectHome = mkDefault true;
         PrivateTmp = true;
         PrivateDevices = true;
         ProtectHostname = true;
+        ProtectClock = true;
         ProtectKernelTunables = true;
         ProtectKernelModules = true;
+        ProtectKernelLogs = true;
         ProtectControlGroups = true;
         RestrictAddressFamilies = [ "AF_UNIX" "AF_INET" "AF_INET6" ];
+        RestrictNamespaces = true;
         LockPersonality = true;
         MemoryDenyWriteExecute = !(builtins.any (mod: (mod.allowMemoryWriteExecute or false)) cfg.package.modules);
         RestrictRealtime = true;
         RestrictSUIDSGID = true;
+        RemoveIPC = true;
         PrivateMounts = true;
         # System Call Filtering
         SystemCallArchitectures = "native";
+        SystemCallFilter = "~@chown @cpu-emulation @debug @keyring @ipc @module @mount @obsolete @privileged @raw-io @reboot @setuid @swap";
       };
     };