nixos: add option to lock kernel modules

Adds an option `security.lockKernelModules` that, when enabled, disables
kernel module loading once the system reaches its normal operating state.

The rationale for this over simply setting the sysctl knob is to allow
some legitmate kernel module loading to occur; the naive solution breaks
too much to be useful.

The benefit to the user is to help ensure the integrity of the kernel
runtime: only code loaded as part of normal system initialization will be
available in the kernel for the duration of the boot session.  This helps
prevent injection of malicious code or unexpected loading of legitimate
but normally unused modules that have exploitable bugs (e.g., DCCP use
after free CVE-2017-6074, n_hldc CVE-2017-2636, XFRM framework
CVE-2017-7184, L2TPv3 CVE-2016-10200).

From an aestethic point of view, enabling this option helps make the
configuration more "declarative".

Closes https://github.com/NixOS/nixpkgs/pull/24681

nixos/modules/module-list.nix

@@ -116,6 +116,7 @@
   ./security/duosec.nix
   ./security/grsecurity.nix
   ./security/hidepid.nix
+  ./security/lock-kernel-modules.nix
   ./security/oath.nix
   ./security/pam.nix
   ./security/pam_usb.nix

nixos/modules/security/lock-kernel-modules.nix

@@ -0,0 +1,36 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+  options = {
+    security.lockKernelModules = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Disable kernel module loading once the system is fully initialised.
+        Module loading is disabled until the next reboot.  Problems caused
+        by delayed module loading can be fixed by adding the module(s) in
+        question to <option>boot.kernelModules</option>.
+      '';
+    };
+  };
+
+  config = mkIf config.security.lockKernelModules {
+    systemd.services.disable-kernel-module-loading = rec {
+      description = "Disable kernel module loading";
+
+      wantedBy = [ config.systemd.defaultUnit ];
+      after = [ "systemd-udev-settle.service" "firewall.service" "systemd-modules-load.service" ] ++ wantedBy;
+
+      script = "echo -n 1 > /proc/sys/kernel/modules_disabled";
+
+      unitConfig.ConditionPathIsWritable = "/proc/sys/kernel";
+
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+      };
+    };
+  };
+}