Add support for TrustedTypes (Polymer#5692)

* Add support for TrustedTypes to html-tag.js

This avoids setting innerHTML to a string.

* Comment on why .slice() the  XMLSerializer output

* Handle data binding Trusted Types into attributes

Fixes Polymer#5648

* Lint clean

.eslintrc.json

@@ -40,6 +40,7 @@
     "Polymer": true,
     "ShadyDOM": true,
     "ShadyCSS": true,
-    "JSCompiler_renameProperty": true
+    "JSCompiler_renameProperty": true,
+    "trustedTypes": true
   }
 }

lib/mixins/property-accessors.js

@@ -26,6 +26,14 @@ while (proto) {
   proto = Object.getPrototypeOf(proto);
 }
 
+const isTrustedType = (() => {
+  if (!window.trustedTypes) {
+    return () => false;
+  }
+  return (val) => trustedTypes.isHTML(val) ||
+        trustedTypes.isScript(val) || trustedTypes.isScriptURL(val);
+})();
+
 /**
  * Used to save the value of a property that will be overridden with
  * an accessor. If the `model` is a prototype, the values will be saved
@@ -215,6 +223,14 @@ export const PropertyAccessors = dedupingMixin(superClass => {
           if (value instanceof Date) {
             return value.toString();
           } else if (value) {
+            if (isTrustedType(value)) {
+              /**
+               * Here `value` isn't actually a string, but it should be
+               * passed into APIs that normally expect a string, like
+               * elem.setAttribute.
+               */
+              return /** @type {?} */ (value);
+            }
             try {
               return JSON.stringify(value);
             } catch(x) {

lib/utils/html-tag.js

@@ -9,13 +9,33 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
 */
 import './boot.js';
 
+/**
+ * Our TrustedTypePolicy for HTML which is declared using the Polymer html
+ * template tag function.
+ *
+ * That HTML is a developer-authored constant, and is parsed with innerHTML
+ * before any untrusted expressions have been mixed in. Therefor it is
+ * considered safe by construction.
+ *
+ * @type {!TrustedTypePolicy|undefined}
+ */
+const policy = window.trustedTypes &&
+    trustedTypes.createPolicy('polymer-html-literal', {createHTML: (s) => s});
+
 /**
  * Class representing a static string value which can be used to filter
  * strings by asseting that they have been created via this class. The
  * `value` property returns the string passed to the constructor.
  */
 class LiteralString {
-  constructor(string) {
+  /**
+   * @param {!ITemplateArray} strings Constant parts of tagged template literal
+   * @param {!Array<*>} values Variable parts of tagged template literal
+   */
+  constructor(strings, values) {
+    assertValidTemplateStringParameters(strings, values);
+    const string = values.reduce(
+        (acc, v, idx) => acc + literalValue(v) + strings[idx + 1], strings[0]);
     /** @type {string} */
     this.value = string.toString();
   }
@@ -48,7 +68,15 @@ function literalValue(value) {
  */
 function htmlValue(value) {
   if (value instanceof HTMLTemplateElement) {
-    return /** @type {!HTMLTemplateElement } */(value).innerHTML;
+    // Use the XML serializer to avoid xMSS attacks from browsers' sometimes
+    // unexpected formatting / cleanup of innerHTML.
+    const serializedNewTree = new XMLSerializer().serializeToString(
+        /** @type {!HTMLTemplateElement } */ (value));
+    // The XMLSerializer is similar to .outerHTML, so slice off the leading
+    // and trailing parts of the <template> wrapper tag.
+    return serializedNewTree.slice(
+        serializedNewTree.indexOf('>') + 1,
+        serializedNewTree.lastIndexOf('</'));
   } else if (value instanceof LiteralString) {
     return literalValue(value);
   } else {
@@ -92,12 +120,35 @@ function htmlValue(value) {
  * @return {!HTMLTemplateElement} Constructed HTMLTemplateElement
  */
 export const html = function html(strings, ...values) {
-  const template = /** @type {!HTMLTemplateElement} */(document.createElement('template'));
-  template.innerHTML = values.reduce((acc, v, idx) =>
-      acc + htmlValue(v) + strings[idx + 1], strings[0]);
+  assertValidTemplateStringParameters(strings, values);
+  const template =
+      /** @type {!HTMLTemplateElement} */ (document.createElement('template'));
+  let value = values.reduce(
+      (acc, v, idx) => acc + htmlValue(v) + strings[idx + 1], strings[0]);
+  if (policy) {
+    value = policy.createHTML(value);
+  }
+  template.innerHTML = value;
   return template;
 };
 
+/**
+ * @param {!ITemplateArray} strings Constant parts of tagged template literal
+ * @param {!Array<*>} values Array of values from quasis
+ */
+const assertValidTemplateStringParameters = (strings, values) => {
+  // Note: if/when https://github.com/tc39/proposal-array-is-template-object
+  // is standardized, use that instead when available, as it can perform an
+  // unforgable check (though of course, the function itself can be forged).
+  if (!Array.isArray(strings) || !Array.isArray(strings.raw) ||
+      (values.length !== strings.length - 1)) {
+    // This is either caused by a browser bug, a compiler bug, or someone
+    // calling the html template tag function as a regular function.
+    //
+    throw new TypeError('Invalid call to the html template tag');
+  }
+};
+
 /**
  * An html literal tag that can be used with `html` to compose.
  * a literal string.
@@ -123,6 +174,5 @@ export const html = function html(strings, ...values) {
  * @return {!LiteralString} Constructed literal string
  */
 export const htmlLiteral = function(strings, ...values) {
-  return new LiteralString(values.reduce((acc, v, idx) =>
-      acc + literalValue(v) + strings[idx + 1], strings[0]));
+  return new LiteralString(strings, values);
 };