For the hardened profile disable symmetric multi threading. There seems to be no *proven* method of exploiting cache sharing between threads on the same CPU core, so this may be considered quite paranoid, considering the perf cost. SMT can be controlled at runtime, however. This is in keeping with OpenBSD defaults. TODO: since SMT is left to be controlled at runtime, changing the option definition should take effect on system activation. Write to /sys/devices/system/cpu/smt/control
126 lines
4.4 KiB
Nix
126 lines
4.4 KiB
Nix
{ config, lib, ... }:
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with lib;
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{
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meta = {
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maintainers = [ maintainers.joachifm ];
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};
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options = {
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security.allowUserNamespaces = mkOption {
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type = types.bool;
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default = true;
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description = ''
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Whether to allow creation of user namespaces. A recurring problem
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with user namespaces is the presence of code paths where the kernel's
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permission checking logic fails to account for namespacing, instead
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permitting a namespaced process to act outside the namespace with the
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same privileges as it would have inside it. This is particularly
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damaging in the common case of running as root within the namespace.
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When user namespace creation is disallowed, attempting to create
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a user namespace fails with "no space left on device" (ENOSPC).
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'';
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};
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security.protectKernelImage = mkOption {
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type = types.bool;
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default = false;
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description = ''
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Whether to prevent replacing the running kernel image.
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'';
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};
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security.allowSimultaneousMultithreading = mkOption {
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type = types.bool;
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default = true;
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description = ''
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Whether to allow SMT/hyperthreading. Disabling SMT means that only
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physical CPU cores will be usable at runtime, potentially at
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significant performance cost.
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</para>
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<para>
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The primary motivation for disabling SMT is to mitigate the risk of
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leaking data between threads running on the same CPU core (due to
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e.g., shared caches). This attack vector is unproven.
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</para>
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<para>
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Disabling SMT is a supplement to the L1 data cache flushing mitigation
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(see <xref linkend="opt-security.virtualization.flushL1DataCache"/>)
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versus malicious VM guests (SMT could "bring back" previously flushed
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data).
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</para>
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<para>
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'';
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};
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security.virtualization.flushL1DataCache = mkOption {
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type = types.nullOr (types.enum [ "never" "cond" "always" ]);
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default = null;
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description = ''
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Whether the hypervisor should flush the L1 data cache before
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entering guests.
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See also <xref linkend="opt-security.allowSimultaneousMultithreading"/>.
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</para>
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<para>
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<variablelist>
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<varlistentry>
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<term><literal>null</literal></term>
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<listitem><para>uses the kernel default</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>"never"</literal></term>
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<listitem><para>disables L1 data cache flushing entirely.
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May be appropriate if all guests are trusted.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>"cond"</literal></term>
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<listitem><para>flushes L1 data cache only for pre-determined
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code paths. May leak information about the host address space
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layout.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><literal>"always"</literal></term>
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<listitem><para>flushes L1 data cache every time the hypervisor
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enters the guest. May incur significant performance cost.
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</para></listitem>
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</varlistentry>
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</variablelist>
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'';
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};
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};
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config = mkMerge [
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(mkIf (!config.security.allowUserNamespaces) {
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# Setting the number of allowed user namespaces to 0 effectively disables
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# the feature at runtime. Note that root may raise the limit again
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# at any time.
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boot.kernel.sysctl."user.max_user_namespaces" = 0;
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assertions = [
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{ assertion = config.nix.useSandbox -> config.security.allowUserNamespaces;
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message = "`nix.useSandbox = true` conflicts with `!security.allowUserNamespaces`.";
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}
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];
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})
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(mkIf config.security.protectKernelImage {
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# Disable hibernation (allows replacing the running kernel)
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boot.kernelParams = [ "nohibernate" ];
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# Prevent replacing the running kernel image w/o reboot
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boot.kernel.sysctl."kernel.kexec_load_disabled" = mkDefault true;
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})
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(mkIf (!config.security.allowSimultaneousMultithreading) {
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boot.kernelParams = [ "nosmt" ];
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})
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(mkIf (config.security.virtualization.flushL1DataCache != null) {
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boot.kernelParams = [ "kvm-intel.vmentry_l1d_flush=${config.security.virtualization.flushL1DataCache}" ];
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})
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];
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}
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