In theory there’s no difference between theory and practice. In practice, there is. That’s why we prefer to try new ideas out in the real world and see which features actually helped users get where they want to go. In Chrome we call these tests Field Trials, and they’ve been part of every install steadily making Chrome better since day one. For example, Field Trials helped us improve our Omnibox pre-rendering by about 70 percent - making about 1 in 3 Omnibox navigations faster by a second or more.
If you’re contributing to Chrome, you should consider using Field Trials to help tune your features. Recently we’ve added infrastructure to help make designing and deploying these tests easier than ever. Field Trials will now have a regular update cycle similar to the one that delivers the newest version of Chrome to users, making it easier to rapidly release and retire new variations. We’ve also centralized the logic of which variations are active so that its easy for developers on M21 or higher to reset their variation state, using the --reset-variation-state command line flag. To learn more about variations of Chrome, check out Field Trials at the Chromium project.
Just over two months ago, Chrome sponsored the Pwnium browser hacking competition. We had two fantastic submissions, and successfully blocked both exploits within 24 hours of their unveiling. Today, we’d like to offer an inside look into the exploit submitted by Pinkie Pie.
So, how does one get full remote code execution in Chrome? In the case of Pinkie Pie’s exploit, it took a chain of six different bugs in order to successfully break out of the Chrome sandbox.
Pinkie’s first bug (117620) used Chrome’s prerendering feature to load a Native Client module on a web page. Prerendering is a performance optimization that lets a site provide hints for Chrome to fetch and render a page before the user navigates to it, making page loads seem instantaneous. To avoid sound and other nuisances from preloaded pages, the prerenderer blocks plug-ins from running until the user chooses to navigate to the page. Pinkie discovered that navigating to a pre-rendered page would inadvertently run all plug-ins—even Native Client plug-ins, which are otherwise permitted only for installed extensions and apps.
Of course, getting a Native Client plug-in to execute doesn’t buy much, because the Native Client process’ sandbox is even more restrictive than Chrome’s sandbox for HTML content. What Native Client does provide, however, is a low-level interface to the GPU command buffers, which are used to communicate accelerated graphics operations to the GPU process. This allowed Pinkie to craft a special command buffer to exploit the following integer underflow bug (117656) in the GPU command decoding:
The issue here is that if size_of_buffer is smaller than sizeof(uint32), the result would be a huge value, which was then used as input to the following function:
This calculation then overflowed and made the result of this function zero, instead of a value at least equal to sizeof(uint32). Using this, Pinkie was able to write eight bytes of his choice past the end of his buffer. The buffer in this case is one of the GPU transfer buffers, which are mapped in both processes’ address spaces and used to transfer data between the Native Client and GPU processes. The Windows allocator places the buffers at relatively predictable locations; and the Native Client process can directly control their size as well as certain object allocation ordering. So, this afforded quite a bit of control over exactly where an overwrite would occur in the GPU process.
The next thing Pinkie needed was a target that met two criteria: it had to be positioned within range of his overwrite, and the first eight bytes needed to be something worth changing. For this, he used the GPU buckets, which are another IPC primitive exposed from the GPU process to the Native Client process. The buckets are implemented as a tree structure, with the first eight bytes containing pointers to other nodes in the tree. By overwriting the first eight bytes of a bucket, Pinkie was able to point it to a fake tree structure he created in one of his transfer buffers. Using that fake tree, Pinkie could read and write arbitrary addresses in the GPU process. Combined with some predictable addresses in Windows, this allowed him to build a ROP chain and execute arbitrary code inside the GPU process.
The GPU process is still sandboxed well below a normal user, but it’s not as strongly sandboxed as the Native Client process or the HTML renderer. It has some rights, such as the ability to enumerate and connect to the named pipes used by Chrome’s IPC layer. Normally this wouldn’t be an issue, but Pinkie found that there’s a brief window after Chrome spawns a new renderer where the GPU process could see the renderer’s IPC channel and connect to it first, allowing the GPU process to impersonate the renderer (bug 117627).
Even though Chrome’s renderers execute inside a stricter sandbox than the GPU process, there is a special class of renderers that have IPC interfaces with elevated permissions. These renderers are not supposed to be navigable by web content, and are used for things like extensions and settings pages. However, Pinkie found another bug (117417) that allowed an unprivileged renderer to trigger a navigation to one of these privileged renderers, and used it to launch the extension manager. So, all he had to do was jump on the extension manager’s IPC channel before it had a chance to connect.
Once he was impersonating the extensions manager, Pinkie used two more bugs to finally break out of the sandbox. The first bug (117715) allowed him to specify a load path for an extension from the extension manager’s renderer, something only the browser should be allowed to do. The second bug (117736) was a failure to prompt for confirmation prior to installing an unpacked NPAPI plug-in extension. With these two bugs Pinkie was able to install and run his own NPAPI plug-in that executed outside the sandbox at full user privilege.
So, that’s the long and impressive path Pinkie Pie took to crack Chrome. All the referenced bugs were fixed some time ago, but some are still restricted to ensure our users and Chromium embedders have a chance to update. However, we’ve included links so when we do make the bugs public, anyone can investigate in more detail.
In an upcoming post, we’ll explain the details of Sergey Glazunov’s exploit, which relied on roughly 10 distinct bugs. While these issues are already fixed in Chrome, some of them impact a much broader array of products from a range of companies. So, we won’t be posting that part until we’re comfortable that all affected products have had an adequate time to push fixes to their users.
Last year we proposed the Web Intents API to help web applications integrate with one another with minimal effort. We've now enabled an experimental version of the API in the most recent stable version of Chrome, to gather feedback from the web community and shape the future of the Web Intents API.
This prototype version of Web Intents makes it easier for developers to try out the API and experience its benefits first hand:
- Developers who build client apps will be able to easily include functionality from other web services (e.g., photo editing).
- Developers creating those services will no longer need to invest time and resources to negotiate and build hardcoded integrations - they can just focus on offering a great quality product with the integration facilitated by the API.
In addition, this implementation of Web Intents can help the design discussions in the W3C web intents open standards list. After all, it's impossible to build a complex API—especially one that requires an ecosystem of apps—without feedback from web developers using it in the wild.
We expect that Web Intents will evolve significantly, potentially in backwards-incompatible ways, as feedback from real world usage trickles in. Because of its experimental status, the current live version is prefixed and only allows applications to register as services in their Chrome Web Store app manifest.
Once the API is stable, we plan to remove this restriction.
To learn more on how to use the experimental Web Intents API check out the Web Developers' Guide to Web Intents in Chrome. If you choose to experiment with Web Intents, be sure to follow our discussion group, where we'll announce any impending breaking changes in Chrome's implementation.
Like with other multithreaded applications, debugging Web Workers may be a tricky task and having good instruments makes this process much easier. Chrome Developer Tools provides full debugging support for scripts running in both dedicated and shared workers.
You can now use the powerful Scripts, Timeline, Profiles and Console panels to develop Web Workers:
All dedicated workers running in the inspected page are listed in the Scripts panel, under the Workers section. Clicking on a worker URL will open a new Developer Tools window attached to the worker. If you need to debug a dedicated worker’s initialization, there is also an option to pause workers on start. This will suspend the execution in all starting dedicated workers at the very first statement.
Shared workers are more independent than dedicated ones as they can be used concurrently by several pages. All running shared workers can be discovered through the chrome://inspect page. Each shared worker listed there has inspect and terminate links next to its URL. These links allow you to launch a Developer Tools window attached to that worker or terminate the worker respectively.
When you need to debug a shared worker initialization, it is enough to terminate the current instance of the worker leaving the Developer Tools window open and reload one of the worker clients. The worker will restart and Developer Tools window will automatically re-attach to the new instance.
Web browsers are big, complicated pieces of software that are extremely difficult to secure. In the case of Chrome, it’s an even more interesting challenge as we contend with a codebase that evolves at a blisteringly fast pace. All of this means that we need to move very quickly to keep up, and one of the ways we do so is with a scaled out fuzzing infrastructure.
Chrome’s fuzzing infrastructure (affectionately named "ClusterFuzz") is built on top of a cluster of several hundred virtual machines running approximately six-thousand simultaneous Chrome instances. ClusterFuzz automatically grabs the most current Chrome LKGR (Last Known Good Revision), and hammers away at it to the tune of around fifty-million test cases a day. That capacity has roughly quadrupled since the system’s inception, and we plan to quadruple it again over the next few weeks.
With that kind of volume, we’d be overloaded if we just automated the test case generation and crash detection. That’s why we’ve automated the entire fuzzing pipeline, including the following processes:
- Managing test cases and infrastructure - To run at maximum capacity we need to generate a constant stream of test cases, distribute them across thousands of Chrome instances running on hundreds of virtual machines, and track the results.
- Analyzing crashes - The only crashes we care about for security purposes are the exploitable ones. So we use Address Sanitizer to instrument our Chrome binaries and provide detailed reports on potentially exploitable crashes.
- Minimizing test cases - Fuzzer test cases are often very large files—usually as much as several hundred kilobytes each. So we take the generated test cases and distill them down to the few, essential pieces that actually trigger the crash.
- Identifying regressions - The first step in getting a crash fixed is figuring out where it is and who should fix it. So this phase tracks the crash down to the range of changes that introduced it.
- Verifying fixes - In order to verify when a crash is actually fixed, which we run the open crash cases against each new LKGR build.
In addition to manageability, this level of scale and automation provides a very important additional benefit. By aggressively tracking the Chrome LKGR builds, ClusterFuzz is evolving into a real-time security regression detection capability. To appreciate just what that means, consider that ClusterFuzz has detected 95 unique vulnerabilities since we brought it fully online at the end of last year. In that time, 44 of those vulnerabilities were identified and fixed before they ever had a chance to make it out to a stable release. As we further refine our process and increase our scale, we expect potential security regressions in stable releases to become increasingly less common.
Just like Chrome itself, our fuzzing work is constantly evolving and pushing the state of the art in both scale and techniques. In keeping with Chrome’s security principles, we’re helping to make the web safer by upstreaming the security fixes into projects we rely upon, like WebKit and FFmpeg. As we expand and improve ClusterFuzz, users of those upstream projects will continue to benefit.
The Dart team invites you to the first global Dart hackathon, a collaboration between the Dart team and the developer community. Sign up and have fun hacking on Dart to build modern client and server side web apps and libraries. Current hackathon locations include:
- North America:
- Silicon Valley, California, USA
- South America:
- São Paulo, Brazil
- Europe and Middle East:
- London, England
- Prague, Czech Republic
- Tel Aviv, Israel
- Bacolod City, Philippines
- Chandigarh, India
- Goa, India
- Karnataka, India
- Manipal, India
- New Delhi, India
- Seoul, Korea
- Tokyo, Japan
Hackathon dates vary by location. Check out the full list for the schedule.The Dart project is still in technology preview, which means you’ll be hacking on early access code, but that’s all part of the fun. We’re eager to see what you build, and we hope you can make it. Register today!
Last January, Chrome was the first major browser to preview WebRTC, HTML5's new real time audio and video stack. Since then, we've been hard at work keeping up with the evolving specification, fixing bugs and listening to the web community’s feedback.
The main parts of the WebRTC specification are now stable and are coming soon to all 200M+ Chrome users. With this blog post, we want to help developers plan for what will be introduced in this first stable release later this year.
Our implementation will support multiple independent PeerConnections, each capable of sending and receiving multiple independent media sources.
ICE / STUN / TURN
ICE and STUN are standardized methods for establishing a peer-to-peer connection on the Internet, even if the two end points are behind private network addresses (NAT). Chrome’s current stack deviates from the official current standards. We are working to fix this.
We will also support TURN servers to allow connections through tougher firewalls, where relaying and encapsulation are needed. Exactly what type of TURN will be supported is TBD.
Encryption will be mandatory for all usage of WebRTC in Chrome. For our first stable release, we will implement DTLS-SRTP.
VP8, iSAC, iLBC, G.711
The video codec support by Chrome will be VP8. We've made several major improvements inside and around VP8 to ensure it can deliver a great real time experience. On the audio side, we will initially support iSAC, iLBC, G.711, and DTMF, with iSAC being the default. It is a royalty free wideband codec optimized for speech, open sourced at webrtc.org.
More functionality and features will appear in future versions of Chrome. We’ll work on prioritizing them once we get the basics right:
- Data API. Implementation will start once the network stack is ready.
- Screen sharing.
- PeerConnection proxying. The ability to relay a stream to a third party will not make our first version.
- Recording. MediaRecorder specification work has not been completed yet.
We’ve enabled GPU-accelerated Canvas2D on capable Windows and Mac computers, which should make web applications like games perform even better than a pure software implementation. GPU-accelerated Canvas2D has previously been enabled in the Beta channel for quite some time, so hopefully developers have had a chance to try it out. We’re continuing to make improvements and tweaks to our Canvas2D implementation, so please file a bug in our public issue tracker if you encounter problems.
Over the past several months, the number of daily app and extensions downloads from the Chrome Web Store has more than doubled. We are now seeing millions of downloads per day. Some apps and extensions have grown even faster thanks to inline installation, a feature we launched a few months ago.
With inline installation, you can allow Chrome users who visit your web site to install your apps and extensions directly without requiring them to visit the Chrome Web Store. This creates a smoother experience for your users as it eliminates an extra step where potential users could drop off.
Here are a few examples of the impact of inline installation:
- Chrome extensions Evernote Clearly and Evernote Web Clipper derive 15% and 25% of their Chrome installations (respectively) from their inline installation implementation
- Rovio implemented inline installation for their Angry Birds Chrome game and saw their install rate jump by almost 10%)
- Equire, a CRM extension that integrates with Gmail, saw a 66% increase in Chrome user retention after they implemented inline installation.
Implementing inline installation is very easy:
- Provide a link to your Chrome Web Store item.
- Write some script to check for whatever client-side capabilities your app requires (support for WebGL, the Web Audio API, etc). Modernizr is a great library to use for this.
The user sees the same Add To Chrome dialog prompt that they would on the store, confirm the install, and they're done – all without leaving your site.