block access to child frames' contentWindow, contentDocument, place guard in blocking proxy to prevent an infinite loop

app/extensions/brave/content/scripts/blockCanvasFingerprinting.js

@@ -9,6 +9,7 @@
  */
 
 if (chrome.contentSettings.canvasFingerprinting == 'block') {
+
   Error.stackTraceLimit = Infinity // collect all frames
 
   // https://code.google.com/p/v8-wiki/wiki/JavaScriptStackTraceApi
@@ -46,12 +47,14 @@ if (chrome.contentSettings.canvasFingerprinting == 'block') {
   function getOriginatingScriptUrl () {
     var trace = getStackTrace(true)
 
-    if (trace.length < 3) {
+    if (trace.length < 4) {
       return ''
     }
 
-    // this script is at 0 and 1
-    var callSite = trace[2]
+    // This script is at positions 0 (this function), 1 (the reportBlock
+    // function) and 2 (the newGlobalFunction function).  Index 3 and beyond
+    // are on the client page.
+    var callSite = trace[3]
 
     if (callSite.isEval()) {
       // argh, getEvalOrigin returns a string ...
@@ -96,6 +99,18 @@ if (chrome.contentSettings.canvasFingerprinting == 'block') {
     return (possiblePropDesc && !possiblePropDesc.configurable)
   })
 
+  /**
+   * Returns a false-y default value, depending on how a proxy is being coerced.
+   *
+   * @param {String} hint
+   *   A string, describing the type of coercion being applied to the proxy.
+   *   Expected values are "string", "number" or "default".
+   *
+   * @return {String|Number|undefined}
+   *   A falsey value, determined by the type of coercion being applied.
+   *
+   * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/toPrimitive
+   */
   var valueOfCoercionFunc = function (hint) {
     if (hint === 'string') {
       return ''
@@ -106,70 +121,158 @@ if (chrome.contentSettings.canvasFingerprinting == 'block') {
     return undefined
   }
 
-  var allPurposeProxy = new Proxy(defaultFunc, {
-    get: function (target, property) {
+  // This object is used to map from names of blocking proxy objects,
+  // to the proxy object itself.  Its used to allow the handler of each
+  // proxy to refer to the proxy before the proxy is created (since the
+  // handler is needed to create the proxy, there would otherwise be a
+  // chicken and egg situation).
+  var proxyRegistery = {}
 
-      if (property === Symbol.toPrimitive) {
-        return valueOfCoercionFunc
+  /**
+   * Returns a handler object, for use in configuring a Proxy object.
+   *
+   * The returned handler will return a named proxy on get, set and apply
+   * operations up to `loopGuardMax` times (1000), and then returns undefined
+   * to avoid infinite loops.
+   *
+   * @param {String} proxyName
+   *   The name of the proxy ("registered" in the above "proxyRegistery"
+   *   object) to return when the proxy object is {get,set,apply}'ed.
+   * @param {?Function(String)} onTriggerCallback
+   *   An optional callback funciton that will be called whenever the
+   *   proxy object is get,'ed.  This callback is called with
+   *   a single argument, the name of the property being looked up.
+   *
+   * @return {Object}
+   *   A valid Proxy handler definition.
+   *
+   * @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/handler
+   */
+  var createProxyHandler = function (proxyName, onTriggerCallback) {
+
+    var loopGuardMax = 1000
+    var callCounter = 0
+
+    return {
+      get: function (target, property) {
+
+        if (onTriggerCallback !== undefined) {
+          onTriggerCallback(property)
+        }
+
+        // If the proxy has been called a large number of times on this page,
+        // it might be stuck in an loop.  To prevent locking up the page,
+        // return undefined to break the loop, and then resume the normal
+        // behavior on subsequent calls.
+        if (callCounter > loopGuardMax) {
+          callCounter = 0
+          return undefined
+        }
+
+        callCounter += 1
+
+        if (property === Symbol.toPrimitive) {
+          return valueOfCoercionFunc
+        }
+
+        if (property === 'toString') {
+          return ''
+        }
+
+        if (property === 'valueOf') {
+          return 0
+        }
+
+        return proxyRegistery[proxyName]
+      },
+      set: function () {
+        return proxyRegistery[proxyName]
+      },
+      apply: function () {
+        return proxyRegistery[proxyName]
+      },
+      ownKeys: function () {
+        return unconfigurablePropNames
+      },
+      has: function (target, property) {
+        return (unconfigurablePropNames.indexOf(property) > -1)
+      },
+      getOwnPropertyDescriptor: function (target, property) {
+        if (unconfigurablePropNames.indexOf(property) === -1) {
+          return undefined
+        }
+        return Object.getOwnPropertyDescriptor(defaultFunc, property)
       }
+    }
+  }
 
-      if (property === 'toString') {
-        return ''
-      }
+  var mainFrameBlockingProxy = new Proxy(defaultFunc, createProxyHandler("mainFrameBlockingProxy"))
+  proxyRegistery["mainFrameBlockingProxy"] = mainFrameBlockingProxy
 
-      if (property === 'valueOf') {
-        return 0
-      }
+  /**
+   * Reports that a method related to fingerprinting was called by the page.
+   *
+   * @param {String} type
+   *   A category of the fingerprinting method, such as "SVG", "iFrame" or
+   *   "Canvas".
+   * @param {?String} scriptUrlToReport
+   *   Optional URL to report where the fingerprinting effort happened.
+   *   If this is undefined, then the script URL is determined from
+   *   a stack trace.
+   */
+  function reportBlock (type, scriptUrlToReport) {
 
-      return allPurposeProxy
-    },
-    set: function () {
-      return allPurposeProxy
-    },
-    apply: function () {
-      return allPurposeProxy
-    },
-    ownKeys: function () {
-      return unconfigurablePropNames
-    },
-    has: function (target, property) {
-      return (unconfigurablePropNames.indexOf(property) > -1)
-    },
-    getOwnPropertyDescriptor: function (target, property) {
-      if (unconfigurablePropNames.indexOf(property) === -1) {
-        return undefined
-      }
-      return Object.getOwnPropertyDescriptor(defaultFunc, property)
-    }
-  })
+    var scriptUrl
 
-  function reportBlock (type) {
-    var script_url = getOriginatingScriptUrl()
-    if (script_url) {
-      script_url = stripLineAndColumnNumbers(script_url)
+    if (scriptUrlToReport !== undefined) {
+      scriptUrl = scriptUrlToReport
     } else {
-      script_url = window.location.href
+      scriptUrl = getOriginatingScriptUrl()
+      if (scriptUrl) {
+        scriptUrl = stripLineAndColumnNumbers(scriptUrl)
+      } else {
+        scriptUrl = window.location.href
+      }
     }
+
     var msg = {
       type,
-      scriptUrl: stripLineAndColumnNumbers(script_url)
+      scriptUrl: stripLineAndColumnNumbers(scriptUrl)
     }
 
     // Block the read from occuring; send info to background page instead
     chrome.ipcRenderer.sendToHost('got-canvas-fingerprinting', msg)
-
-    return allPurposeProxy
   }
 
   /**
-   * Monitor the reads from a canvas instance
-   * @param item special item objects
+   * Replaces global method with one that reports the fingerprinting attempt.
+   *
+   * @param {Object} item
+   *   A definition for the method that should be replaced.
+   * @param {String} item.type
+   *   A category for the type of fingerprinting method being blocked, such
+   *   as "SVG" or "Canvas"
+   * @param {?String} item.propName
+   *   The name of the property / method to be modified.  If provided,
+   *   item.objName must also be provided.
+   * @param {?String} item.objName
+   *   The name of the global structure to modfiy.  If this is provided,
+   *   then the item.propName on the prototype of this object will be modified.
+   * @param {?String} item.methodName
+   *   A global, singleton method to overwrite.  If this is provided,
+   *   then item.propName and item.objName are ignored.
    */
   function trapInstanceMethod (item) {
+
+    var newGlobalFunction = function () {
+      reportBlock(item.type)
+      return mainFrameBlockingProxy
+    }
+
     if (!item.methodName) {
-      chrome.webFrame.setGlobal(item.objName + ".prototype." + item.propName, reportBlock.bind(null, item.type))
+      chrome.webFrame.setGlobal(item.objName + ".prototype." + item.propName, newGlobalFunction)
     } else {
-      chrome.webFrame.setGlobal(item.methodName, reportBlock.bind(null, item.type))
+      chrome.webFrame.setGlobal(item.methodName, newGlobalFunction)
     }
   }
 
@@ -267,4 +370,75 @@ if (chrome.contentSettings.canvasFingerprinting == 'block') {
     type: 'WebRTC',
     methodName: 'navigator.mediaDevices.enumerateDevices'
   })
+
+  var propertiesToReportInIframe = new Set(canvasMethods
+    .concat(canvasElementMethods)
+    .concat(webglMethods)
+    .concat(audioBufferMethods)
+    .concat(analyserMethods)
+    .concat(svgPathMethods)
+    .concat(svgTextContentMethods)
+    .concat(webrtcMethods)
+    .concat(['enumerateDevices']))
+
+  // Boolean guard used to make sure we don't report more than one iframe
+  // fingerprinting attempt per page.
+  var hasiFrameReported = false
+
+  /**
+   * Callback function called when the iframe proxy receives a "get".
+   *
+   * This callback is used to report fingerprinting attempts using methods
+   * extracted from an iframe.
+   *
+   * @param {String} property
+   *   The name of the propery being "get" in the iframe proxy.
+   *
+   * @return {Boolean}
+   *   true if a fingerprinting attempt was reported, otherwise false
+   */
+  var onIframeProxyCalled = function (property) {
+
+    if (hasiFrameReported === true) {
+      return false
+    }
+
+    if (propertiesToReportInIframe.has(property) === false) {
+      return false
+    }
+
+    hasiFrameReported = true
+    reportBlock("Iframe", window.location.href)
+    return true
+  }
+
+  var iframeBlockingProxy = new Proxy(defaultFunc, createProxyHandler("iframeBlockingProxy", onIframeProxyCalled))
+  proxyRegistery["iframeBlockingProxy"] = iframeBlockingProxy
+
+  chrome.webFrame.setGlobal("window.__braveIframeProxy", iframeBlockingProxy)
+
+  // Prevent access to frames' contentDocument / contentWindow
+  // properties, to prevent the parent frame from pulling unblocked
+  // references to blocked standards from injected frames.
+  // This may break some sites, but, fingers crossed, its not too much.
+  var pageScriptToInject = function () {
+    var frameTypesToModify = [window.HTMLIFrameElement, window.HTMLFrameElement]
+    var propertiesToModify = ['contentWindow', 'contentDocument']
+    var braveIframeProxy = window.__braveIframeProxy
+    delete window.__braveIframeProxy
+
+    frameTypesToModify.forEach(function (frameType) {
+      propertiesToModify.forEach(function (propertyToModify) {
+        Object.defineProperty(frameType.prototype, propertyToModify, {
+          get: () => {
+            // XXX: this breaks contentWindow.postMessage since the target window
+            // is now the parent window
+            return braveIframeProxy
+          }
+        })
+      })
+    })
+  }
+
+  chrome.webFrame.executeJavaScript(`(${pageScriptToInject.toString()})()`)
 }

test/bravery-components/braveryPanelTest.js

@@ -895,6 +895,20 @@ describe('Bravery Panel', function () {
         .tabByIndex(0)
         .waitUntil(verifyProxyBlocking)
     })
+    it('blocking access to fingerprinting methods on iframe.contentWindow', function * () {
+      const url = Brave.server.url('fingerprinting-blocking-from-child-frames.html')
+      yield this.app.client
+        .tabByIndex(0)
+        .loadUrl(url)
+        .waitForUrl(url)
+        .openBraveMenu(braveMenu, braveryPanel)
+      yield changeFpSetting(this.app.client, blockFpOption)
+      yield this.app.client
+        .waitForTextValue(fpStat, '1')
+        .keys(Brave.keys.ESCAPE)
+        .tabByIndex(0)
+        .waitUntil(verifyProxyBlocking)
+    })
     it('block device enumeration', function * () {
       const url = Brave.server.url('enumerate_devices.html')
       yield this.app.client

test/fixtures/fingerprinting-blocking-from-child-frames.html

@@ -0,0 +1,34 @@
+<html>
+<head>
+</head>
+<body>
+<div id="target">
+proxy blocking being tested
+</div>
+<script>
+(function () {
+
+    const targetElm = document.getElementById("target")
+    const iframe = document.createElement("iframe")
+    iframe.src = "https://example.com/"
+    document.body.appendChild(iframe)
+
+    // Accessing the `toDataUrl` property on the iframe-content-wrapping
+    // proxy should trigger a finger printing notification.
+    let iframeToDataUrl = iframe.contentWindow.HTMLCanvasElement.prototype.toDataURL
+
+    // If this is the proxied version of the toDataURL method, then
+    // we can look up any properties on it w/o throwing an exception.
+    // The below throwing means the proxy isn't injected correctly,
+    // and so the test should fail.
+
+    try {
+        iframeToDataUrl.nonexistant().properties['and'].methods()
+        targetElm.innerText = "proxy blocking works"
+    } catch (e) {
+        targetElm.innerText = "proxy blocking fail"
+    }
+}())
+</script>
+</body>
+</html>