nixos/kmonad: add new option enableHardening (#370437)

nixos/doc/manual/release-notes/rl-2505.section.md

@@ -126,6 +126,9 @@
   to review the new defaults and description of
   [](#opt-services.nextcloud.poolSettings).
 
+- `kmonad` is now hardened by default using common `systemd` settings.
+  If KMonad is used to execute shell commands, hardening may make some of them fail.  In that case, you can disable hardening using {option}`services.kmonad.keyboards.<name>.enableHardening` option.
+
 - `asusd` has been upgraded to version 6 which supports multiple aura devices. To account for this, the single `auraConfig` configuration option has been replaced with `auraConfigs` which is an attribute set of config options per each device. The config files may also be now specified as either source files or text strings; to account for this you will need to specify that `text` is used for your existing configs, e.g.:
   ```diff
   -services.asusd.asusdConfig = '''file contents'''

nixos/modules/services/hardware/kmonad.nix

@@ -41,6 +41,19 @@ let
           '';
         };
 
+        enableHardening = lib.mkOption {
+          type = lib.types.bool;
+          default = true;
+          example = false;
+          description = ''
+            Whether to enable systemd hardening.
+
+            ::: {.note}
+            If KMonad is used to execute shell commands, hardening may make some of them fail.
+            :::
+          '';
+        };
+
         defcfg = {
           enable = lib.mkEnableOption ''
             automatic generation of the defcfg block.
@@ -128,26 +141,60 @@ let
         StartLimitIntervalSec = 2;
         StartLimitBurst = 5;
       };
-      serviceConfig = {
-        ExecStart = ''
-          ${lib.getExe cfg.package} ${mkCfg keyboard} \
-            ${utils.escapeSystemdExecArgs cfg.extraArgs}
-        '';
-        Restart = "always";
-        # Restart at increasing intervals from 2s to 1m
-        RestartSec = 2;
-        RestartSteps = 30;
-        RestartMaxDelaySec = "1min";
-        Nice = -20;
-        DynamicUser = true;
-        User = "kmonad";
-        Group = "kmonad";
-        SupplementaryGroups = [
-          # These ensure that our dynamic user has access to the device node
-          config.users.groups.input.name
-          config.users.groups.uinput.name
-        ] ++ keyboard.extraGroups;
-      };
+      serviceConfig =
+        {
+          ExecStart = ''
+            ${lib.getExe cfg.package} ${mkCfg keyboard} \
+              ${utils.escapeSystemdExecArgs cfg.extraArgs}
+          '';
+          Restart = "always";
+          # Restart at increasing intervals from 2s to 1m
+          RestartSec = 2;
+          RestartSteps = 30;
+          RestartMaxDelaySec = "1min";
+          Nice = -20;
+          DynamicUser = true;
+          User = "kmonad";
+          Group = "kmonad";
+          SupplementaryGroups = [
+            # These ensure that our dynamic user has access to the device node
+            config.users.groups.input.name
+            config.users.groups.uinput.name
+          ] ++ keyboard.extraGroups;
+        }
+        // lib.optionalAttrs keyboard.enableHardening {
+          DeviceAllow = [
+            "/dev/uinput w"
+            "char-input r"
+          ];
+          CapabilityBoundingSet = [ "" ];
+          DevicePolicy = "closed";
+          IPAddressDeny = [ "any" ];
+          LockPersonality = true;
+          MemoryDenyWriteExecute = true;
+          PrivateNetwork = true;
+          PrivateUsers = true;
+          ProcSubset = "pid";
+          ProtectClock = true;
+          ProtectControlGroups = true;
+          ProtectHome = true;
+          ProtectHostname = true;
+          ProtectKernelLogs = true;
+          ProtectKernelModules = true;
+          ProtectKernelTunables = true;
+          ProtectProc = "invisible";
+          RestrictAddressFamilies = [ "none" ];
+          RestrictNamespaces = true;
+          RestrictRealtime = true;
+          SystemCallArchitectures = [ "native" ];
+          SystemCallErrorNumber = "EPERM";
+          SystemCallFilter = [
+            "@system-service"
+            "~@privileged"
+            "~@resources"
+          ];
+          UMask = "0077";
+        };
       # make sure the new config is used after nixos-rebuild switch
       # stopIfChanged controls[0] how a service is "restarted" during
       # nixos-rebuild switch.  By default, stopIfChanged is true, which stops