1 files changed, 155 insertions, 0 deletions
diff --git a/nixos/modules/security/misc.nix b/nixos/modules/security/misc.nix
new file mode 100644
index 00000000000..c20e067b8cc
--- /dev/null
+++ b/nixos/modules/security/misc.nix
@@ -0,0 +1,155 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+  meta = {
+    maintainers = [ maintainers.joachifm ];
+  };
+
+  imports = [
+    (lib.mkRenamedOptionModule [ "security" "virtualization" "flushL1DataCache" ] [ "security" "virtualisation" "flushL1DataCache" ])
+  ];
+
+  options = {
+    security.allowUserNamespaces = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether to allow creation of user namespaces.
+
+        The motivation for disabling user namespaces is the potential
+        presence of code paths where the kernel's permission checking
+        logic fails to account for namespacing, instead permitting a
+        namespaced process to act outside the namespace with the same
+        privileges as it would have inside it.  This is particularly
+        damaging in the common case of running as root within the namespace.
+
+        When user namespace creation is disallowed, attempting to create a
+        user namespace fails with "no space left on device" (ENOSPC).
+        root may re-enable user namespace creation at runtime.
+      '';
+    };
+
+    security.unprivilegedUsernsClone = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        When disabled, unprivileged users will not be able to create new namespaces.
+        By default unprivileged user namespaces are disabled.
+        This option only works in a hardened profile.
+      '';
+    };
+
+    security.protectKernelImage = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to prevent replacing the running kernel image.
+      '';
+    };
+
+    security.allowSimultaneousMultithreading = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether to allow SMT/hyperthreading.  Disabling SMT means that only
+        physical CPU cores will be usable at runtime, potentially at
+        significant performance cost.
+
+        The primary motivation for disabling SMT is to mitigate the risk of
+        leaking data between threads running on the same CPU core (due to
+        e.g., shared caches).  This attack vector is unproven.
+
+        Disabling SMT is a supplement to the L1 data cache flushing mitigation
+        (see <xref linkend="opt-security.virtualisation.flushL1DataCache"/>)
+        versus malicious VM guests (SMT could "bring back" previously flushed
+        data).
+      '';
+    };
+
+    security.forcePageTableIsolation = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to force-enable the Page Table Isolation (PTI) Linux kernel
+        feature even on CPU models that claim to be safe from Meltdown.
+
+        This hardening feature is most beneficial to systems that run untrusted
+        workloads that rely on address space isolation for security.
+      '';
+    };
+
+    security.virtualisation.flushL1DataCache = mkOption {
+      type = types.nullOr (types.enum [ "never" "cond" "always" ]);
+      default = null;
+      description = ''
+        Whether the hypervisor should flush the L1 data cache before
+        entering guests.
+        See also <xref linkend="opt-security.allowSimultaneousMultithreading"/>.
+
+        <variablelist>
+          <varlistentry>
+            <term><literal>null</literal></term>
+            <listitem><para>uses the kernel default</para></listitem>
+          </varlistentry>
+          <varlistentry>
+            <term><literal>"never"</literal></term>
+            <listitem><para>disables L1 data cache flushing entirely.
+            May be appropriate if all guests are trusted.</para></listitem>
+          </varlistentry>
+          <varlistentry>
+            <term><literal>"cond"</literal></term>
+            <listitem><para>flushes L1 data cache only for pre-determined
+            code paths.  May leak information about the host address space
+            layout.</para></listitem>
+          </varlistentry>
+          <varlistentry>
+            <term><literal>"always"</literal></term>
+            <listitem><para>flushes L1 data cache every time the hypervisor
+            enters the guest.  May incur significant performance cost.
+            </para></listitem>
+          </varlistentry>
+        </variablelist>
+      '';
+    };
+  };
+
+  config = mkMerge [
+    (mkIf (!config.security.allowUserNamespaces) {
+      # Setting the number of allowed user namespaces to 0 effectively disables
+      # the feature at runtime.  Note that root may raise the limit again
+      # at any time.
+      boot.kernel.sysctl."user.max_user_namespaces" = 0;
+
+      assertions = [
+        { assertion = config.nix.settings.sandbox -> config.security.allowUserNamespaces;
+          message = "`nix.settings.sandbox = true` conflicts with `!security.allowUserNamespaces`.";
+        }
+      ];
+    })
+
+    (mkIf config.security.unprivilegedUsernsClone {
+      boot.kernel.sysctl."kernel.unprivileged_userns_clone" = mkDefault true;
+    })
+
+    (mkIf config.security.protectKernelImage {
+      # Disable hibernation (allows replacing the running kernel)
+      boot.kernelParams = [ "nohibernate" ];
+      # Prevent replacing the running kernel image w/o reboot
+      boot.kernel.sysctl."kernel.kexec_load_disabled" = mkDefault true;
+    })
+
+    (mkIf (!config.security.allowSimultaneousMultithreading) {
+      boot.kernelParams = [ "nosmt" ];
+    })
+
+    (mkIf config.security.forcePageTableIsolation {
+      boot.kernelParams = [ "pti=on" ];
+    })
+
+    (mkIf (config.security.virtualisation.flushL1DataCache != null) {
+      boot.kernelParams = [ "kvm-intel.vmentry_l1d_flush=${config.security.virtualisation.flushL1DataCache}" ];
+    })
+  ];
+}