ldap: do not write password to the Nix store

default.nix

@@ -240,11 +240,11 @@ in
           '';
         };
 
-        password = mkOption {
+        passwordFile = mkOption {
           type = types.str;
-          example = "not$4f3";
+          example = "/run/my-secret";
           description = ''
-            Password required to authenticate against the LDAP servers.
+            A file containing the password required to authenticate against the LDAP servers.
           '';
         };
       };

mail-server/common.nix

@@ -45,4 +45,25 @@ in
     if value.hashedPasswordFile == null then
       builtins.toString (mkHashFile name value.hashedPassword)
     else value.hashedPasswordFile) cfg.loginAccounts;
+
+  # Appends the LDAP bind password to files to avoid writing this
+  # password into the Nix store.
+  appendLdapBindPwd = {
+    name, file, prefix, passwordFile, destination
+  }: pkgs.writeScript "append-ldap-bind-pwd-in-${name}" ''
+    #!${pkgs.stdenv.shell}
+    set -euo pipefail
+
+    baseDir=$(dirname ${destination})
+    if (! test -d "$baseDir"); then
+      mkdir -p $baseDir
+      chmod 755 $baseDir
+    fi
+
+    cat ${file} > ${destination}
+    echo -n "${prefix}" >> ${destination}
+    cat ${passwordFile} >> ${destination}
+    chmod 600 ${destination}
+  '';
+
 }

mail-server/dovecot.nix

@@ -22,9 +22,10 @@ let
   cfg = config.mailserver;
 
   passwdDir = "/run/dovecot2";
-  passdbFile = "${passwdDir}/passdb";
+  passwdFile = "${passwdDir}/passwd";
   userdbFile = "${passwdDir}/userdb";
-
+  # This file contains the ldap bind password
+  ldapConfFile = "${passwdDir}/dovecot-ldap.conf.ext";
   bool2int = x: if x then "1" else "0";
 
   maildirLayoutAppendix = lib.optionalString cfg.useFsLayout ":LAYOUT=fs";
@@ -58,6 +59,41 @@ let
     '';
   };
 
+
+  ldapConfig = pkgs.writeTextFile {
+    name = "dovecot-ldap.conf.ext.template";
+    text = ''
+      ldap_version = 3
+      uris = ${lib.concatStringsSep " " cfg.ldap.uris}
+      ${lib.optionalString cfg.ldap.startTls ''
+      tls = yes
+      ''}
+      tls_require_cert = hard
+      tls_ca_cert_file = ${cfg.ldap.tlsCAFile}
+      dn = ${cfg.ldap.bind.dn}
+      sasl_bind = no
+      auth_bind = yes
+      base = ${cfg.ldap.searchBase}
+      scope = ${mkLdapSearchScope cfg.ldap.searchScope}
+      ${lib.optionalString (cfg.ldap.dovecot.userAttrs != "") ''
+      user_attrs = ${cfg.ldap.dovecot.user_attrs}
+      ''}
+      user_filter = ${cfg.ldap.dovecot.userFilter}
+      ${lib.optionalString (cfg.ldap.dovecot.passAttrs != "") ''
+      pass_attrs = ${cfg.ldap.dovecot.passAttrs}
+      ''}
+      pass_filter = ${cfg.ldap.dovecot.passFilter}
+    '';
+  };
+
+  setPwdInLdapConfFile = appendLdapBindPwd {
+    name = "ldap-conf-file";
+    file = ldapConfig;
+    prefix = "dnpass = ";
+    passwordFile = cfg.ldap.bind.passwordFile;
+    destination = ldapConfFile;
+  };
+
   genPasswdScript = pkgs.writeScript "generate-password-file" ''
     #!${pkgs.stdenv.shell}
 
@@ -75,7 +111,7 @@ let
       fi
     done
 
-    cat <<EOF > ${passdbFile}
+    cat <<EOF > ${passwdFile}
     ${lib.concatStringsSep "\n" (lib.mapAttrsToList (name: value:
       "${name}:${"$(head -n 1 ${passwordFiles."${name}"})"}::::::"
     ) cfg.loginAccounts)}
@@ -90,7 +126,7 @@ let
     ) cfg.loginAccounts)}
     EOF
 
-    chmod 600 ${passdbFile}
+    chmod 600 ${passwdFile}
     chmod 600 ${userdbFile}
   '';
 
@@ -226,7 +262,7 @@ in
 
         passdb {
           driver = passwd-file
-          args = ${passdbFile}
+          args = ${passwdFile}
         }
 
         userdb {
@@ -238,12 +274,12 @@ in
         ${lib.optionalString cfg.ldap.enable ''
         passdb {
           driver = ldap
-          args = /etc/dovecot/dovecot-ldap.conf.ext
+          args = ${ldapConfFile}
         }
 
         userdb {
           driver = ldap
-          args = /etc/dovecot/dovecot-ldap.conf.ext
+          args = ${ldapConfFile}
           default_fields = home=/var/vmail/ldap/%u uid=${toString cfg.vmailUID} gid=${toString cfg.vmailUID}
         }
         ''}
@@ -310,37 +346,6 @@ in
       '';
     };
 
-    environment.etc = lib.optionalAttrs (cfg.ldap.enable) {
-      "dovecot/dovecot-ldap.conf.ext" = {
-        mode = "0600";
-        uid = config.ids.uids.dovecot2;
-        gid = config.ids.gids.dovecot2;
-        text = ''
-          ldap_version = 3
-          uris = ${lib.concatStringsSep " " cfg.ldap.uris}
-          ${lib.optionalString cfg.ldap.startTls ''
-          tls = yes
-          ''}
-          tls_require_cert = hard
-          tls_ca_cert_file = ${cfg.ldap.tlsCAFile}
-          dn = ${cfg.ldap.bind.dn}
-          dnpass = ${cfg.ldap.bind.password}
-          sasl_bind = no
-          auth_bind = yes
-          base = ${cfg.ldap.searchBase}
-          scope = ${mkLdapSearchScope cfg.ldap.searchScope}
-          ${lib.optionalString (cfg.ldap.dovecot.userAttrs != "") ''
-          user_attrs = ${cfg.ldap.dovecot.user_attrs}
-          ''}
-          user_filter = ${cfg.ldap.dovecot.userFilter}
-          ${lib.optionalString (cfg.ldap.dovecot.passAttrs != "") ''
-          pass_attrs = ${cfg.ldap.dovecot.passAttrs}
-          ''}
-          pass_filter = ${cfg.ldap.dovecot.passFilter}
-        '';
-      };
-    };
-
     systemd.services.dovecot2 = {
       preStart = ''
         ${genPasswdScript}
@@ -351,10 +356,10 @@ in
           ${pkgs.dovecot_pigeonhole}/bin/sievec "$k"
         done
         chown -R '${dovecot2Cfg.mailUser}:${dovecot2Cfg.mailGroup}' '${stateDir}/imap_sieve'
-      '';
+      '' + (lib.optionalString cfg.ldap.enable setPwdInLdapConfFile);
     };
 
-    systemd.services.postfix.restartTriggers = [ genPasswdScript ];
+    systemd.services.postfix.restartTriggers = [ genPasswdScript ] ++ (lib.optional cfg.ldap.enable [setPwdInLdapConfFile]);
 
     systemd.services.dovecot-fts-xapian-optimize = lib.mkIf (cfg.fullTextSearch.enable && cfg.fullTextSearch.maintenance.enable) {
       description = "Optimize dovecot indices for fts_xapian";

mail-server/postfix.nix

@@ -133,13 +133,13 @@ let
       smtpd_sasl_security_options = "noanonymous";
       smtpd_sasl_local_domain = "$myhostname";
       smtpd_client_restrictions = "permit_sasl_authenticated,reject";
-      smtpd_sender_login_maps = "hash:/etc/postfix/vaccounts${lib.optionalString cfg.ldap.enable ",ldap:${ldapSenderLoginMap}"}";
+      smtpd_sender_login_maps = "hash:/etc/postfix/vaccounts${lib.optionalString cfg.ldap.enable ",ldap:${ldapSenderLoginMapFile}"}";
       smtpd_sender_restrictions = "reject_sender_login_mismatch";
       smtpd_recipient_restrictions = "reject_non_fqdn_recipient,reject_unknown_recipient_domain,permit_sasl_authenticated,reject";
       cleanup_service_name = "submission-header-cleanup";
     };
 
-  commonLdapConfig = lib.optionalString (cfg.ldap.enable) ''
+  commonLdapConfig = ''
     server_host = ${lib.concatStringsSep " " cfg.ldap.uris}
     start_tls = ${if cfg.ldap.startTls then "yes" else "no"}
     version = 3
@@ -151,26 +151,47 @@ let
 
     bind = yes
     bind_dn = ${cfg.ldap.bind.dn}
-    bind_pw = ${cfg.ldap.bind.password}
   '';
 
-  ldapSenderLoginMap = lib.optionalString (cfg.ldap.enable)
+  ldapSenderLoginMap = pkgs.writeText "ldap-sender-login-map.cf" ''
-    (pkgs.writeText "ldap-sender-login-map.cf" ''
+    ${commonLdapConfig}
-      ${commonLdapConfig}
+    query_filter = ${cfg.ldap.postfix.filter}
-      query_filter = ${cfg.ldap.postfix.filter}
+    result_attribute = ${cfg.ldap.postfix.mailAttribute}
-      result_attribute = ${cfg.ldap.postfix.mailAttribute}
+  '';
-    '');
+  ldapSenderLoginMapFile = "/run/postfix/ldap-sender-login-map.cf";
+  appendPwdInSenderLoginMap = appendLdapBindPwd {
+    name = "ldap-sender-login-map";
+    file = ldapSenderLoginMap;
+    prefix = "bind_pw = ";
+    passwordFile = cfg.ldap.bind.passwordFile;
+    destination = ldapSenderLoginMapFile;
+  };
 
-  ldapVirtualMailboxMap = lib.optionalString (cfg.ldap.enable)
+  ldapVirtualMailboxMap = pkgs.writeText "ldap-virtual-mailbox-map.cf" ''
-    (pkgs.writeText "ldap-virtual-mailbox-map.cf" ''
+    ${commonLdapConfig}
-       ${commonLdapConfig}
+    query_filter = ${cfg.ldap.postfix.filter}
-       query_filter = ${cfg.ldap.postfix.filter}
+    result_attribute = ${cfg.ldap.postfix.uidAttribute}
-       result_attribute = ${cfg.ldap.postfix.uidAttribute}
+  '';
-    '');
+  ldapVirtualMailboxMapFile = "/run/postfix/ldap-virtual-mailbox-map.cf";
+  appendPwdInVirtualMailboxMap = appendLdapBindPwd {
+    name = "ldap-virtual-mailbox-map";
+    file = ldapVirtualMailboxMap;
+    prefix = "bind_pw = ";
+    passwordFile = cfg.ldap.bind.passwordFile;
+    destination = ldapVirtualMailboxMapFile;
+  };
 in
 {
   config = with cfg; lib.mkIf enable {
 
+    systemd.services.postfix-setup = lib.mkIf cfg.ldap.enable {
+      preStart = ''
+        ${appendPwdInVirtualMailboxMap}
+        ${appendPwdInSenderLoginMap}
+      '';
+      restartTriggers = [ appendPwdInVirtualMailboxMap appendPwdInSenderLoginMap ];
+    };
+
     services.postfix = {
       enable = true;
       hostname = "${sendingFqdn}";
@@ -202,7 +223,7 @@ in
         virtual_mailbox_maps = [
           (mappedFile "valias")
         ] ++ lib.optionals (cfg.ldap.enable) [
-          "ldap:${ldapVirtualMailboxMap}"
+          "ldap:${ldapVirtualMailboxMapFile}"
         ];
         virtual_transport = "lmtp:unix:/run/dovecot2/dovecot-lmtp";
         # Avoid leakage of X-Original-To, X-Delivered-To headers between recipients

tests/ldap.nix

@@ -28,6 +28,8 @@ pkgs.nixosTest {
           ${pkgs.python3}/bin/python ${../scripts/mail-check.py} $@
         '')];
 
+      environment.etc.bind-password.text = bindPassword;
+
       services.openldap = {
         enable = true;
         settings = {
@@ -45,7 +47,7 @@ pkgs.nixosTest {
                   "olcMdbConfig"
                 ];
                 olcDatabase = "{1}mdb";
-                olcDbDirectory = "/var/lib/openldap";
+                olcDbDirectory = "/var/lib/openldap/example";
                 olcSuffix = "dc=example";
               };
             };
@@ -96,7 +98,7 @@ pkgs.nixosTest {
           ];
           bind = {
             dn = "cn=mail,dc=example";
-            password = bindPassword;
+            passwordFile = "/etc/bind-password";
           };
           searchBase = "ou=users,dc=example";
           searchScope = "sub";
@@ -141,6 +143,10 @@ pkgs.nixosTest {
       machine.succeed("doveadm user -u alice@example.com")
       machine.succeed("doveadm user -u bob@example.com")
 
+    with subtest("Files containing secrets are only readable by root"):
+      machine.succeed("ls -l /run/postfix/*.cf | grep -e '-rw------- 1 root root'")
+      machine.succeed("ls -l /run/dovecot2/dovecot-ldap.conf.ext | grep -e '-rw------- 1 root root'")
+
     with subtest("Test account/mail address binding"):
       machine.fail(" ".join([
         "mail-check send-and-read",