nixos/lldap: bootstrap jwt_secret if not provided

If not provided, lldap defaults to `secretjwtsecret` as value which is
hardcoded in the code base.

See https://github.com/lldap/lldap/blob/v0.5.0/server/src/infra/configuration.rs#L76-L77

This is really bad, because it is trivially easy to generate an admin
access token/cookie as attacker, if a `jwt_secret` is known.

nixos/modules/services/databases/lldap.nix

@@ -107,8 +107,21 @@ in
       wants = [ "network-online.target" ];
       after = [ "network-online.target" ];
       wantedBy = [ "multi-user.target" ];
+      # lldap defaults to a hardcoded `jwt_secret` value if none is provided, which is bad, because
+      # an attacker could create a valid admin jwt access token fairly trivially.
+      # Because there are 3 different ways `jwt_secret` can be provided, we check if any one of them is present,
+      # and if not, bootstrap a secret in `/var/lib/lldap/jwt_secret_file` and give that to lldap.
+      script = lib.optionalString (!cfg.settings ? jwt_secret) ''
+        if [[ -z "$LLDAP_JWT_SECRET_FILE" ]] && [[ -z "$LLDAP_JWT_SECRET" ]]; then
+          if [[ ! -e "./jwt_secret_file" ]]; then
+            ${lib.getExe pkgs.openssl} rand -base64 -out ./jwt_secret_file 32
+          fi
+          export LLDAP_JWT_SECRET_FILE="./jwt_secret_file"
+        fi
+      '' + ''
+         ${lib.getExe cfg.package} run --config-file ${format.generate "lldap_config.toml" cfg.settings}
+      '';
       serviceConfig = {
-        ExecStart = "${lib.getExe cfg.package} run --config-file ${format.generate "lldap_config.toml" cfg.settings}";
         StateDirectory = "lldap";
         WorkingDirectory = "%S/lldap";
         User = "lldap";