nixos/sssd: Add secrets handling

Currently, it is not possible to supply sensitive credentials like
`ldap_default_authtok` without writing them to the nix store. This
This commit introduces a new option `environmentFile` where those
credentials can be supplied via environment substitution.

nixos/modules/services/misc/sssd.nix

@@ -3,6 +3,10 @@ with lib;
 let
   cfg = config.services.sssd;
   nscd = config.services.nscd;
+
+  dataDir = "/var/lib/sssd";
+  settingsFile = "${dataDir}/sssd.conf";
+  settingsFileUnsubstituted = pkgs.writeText "${dataDir}/sssd-unsubsituted.conf" cfg.config;
 in {
   options = {
     services.sssd = {
@@ -47,6 +51,30 @@ in {
           Kerberos will be configured to cache credentials in SSS.
         '';
       };
+      environmentFile = mkOption {
+        type = types.nullOr types.path;
+        default = null;
+        description = ''
+          Environment file as defined in <citerefentry>
+          <refentrytitle>systemd.exec</refentrytitle><manvolnum>5</manvolnum>
+          </citerefentry>.
+
+          Secrets may be passed to the service without adding them to the world-readable
+          Nix store, by specifying placeholder variables as the option value in Nix and
+          setting these variables accordingly in the environment file.
+
+          <programlisting>
+            # snippet of sssd-related config
+            [domain/LDAP]
+            ldap_default_authtok = $SSSD_LDAP_DEFAULT_AUTHTOK
+          </programlisting>
+
+          <programlisting>
+            # contents of the environment file
+            SSSD_LDAP_DEFAULT_AUTHTOK=verysecretpassword
+          </programlisting>
+        '';
+      };
     };
   };
   config = mkMerge [
@@ -60,22 +88,28 @@ in {
         wants = [ "nss-user-lookup.target" ];
         restartTriggers = [
           config.environment.etc."nscd.conf".source
-          config.environment.etc."sssd/sssd.conf".source
+          settingsFileUnsubstituted
         ];
         script = ''
           export LDB_MODULES_PATH+="''${LDB_MODULES_PATH+:}${pkgs.ldb}/modules/ldb:${pkgs.sssd}/modules/ldb"
           mkdir -p /var/lib/sss/{pubconf,db,mc,pipes,gpo_cache,secrets} /var/lib/sss/pipes/private /var/lib/sss/pubconf/krb5.include.d
-          ${pkgs.sssd}/bin/sssd -D
+          ${pkgs.sssd}/bin/sssd -D -c ${settingsFile}
         '';
         serviceConfig = {
           Type = "forking";
           PIDFile = "/run/sssd.pid";
+          StateDirectory = baseNameOf dataDir;
+          EnvironmentFile = lib.mkIf (cfg.environmentFile != null) cfg.environmentFile;
         };
-      };
-
-      environment.etc."sssd/sssd.conf" = {
-        text = cfg.config;
-        mode = "0400";
+        preStart = ''
+          [ -f ${settingsFile} ] && rm -f ${settingsFile}
+          old_umask=$(umask)
+          umask 0177
+          ${pkgs.envsubst}/bin/envsubst \
+            -o ${settingsFile} \
+            -i ${settingsFileUnsubstituted}
+          umask $old_umask
+        '';
       };
 
       system.nssModules = [ pkgs.sssd ];