Welcome back to our latest update on MiraclePtr, our project to protect against use-after-free vulnerabilities in Google Chrome. If you need a refresher, you can read our previous blog post detailing MiraclePtr and its objectives.

More platforms

We are thrilled to announce that since our last update, we have successfully enabled MiraclePtr for more platforms and processes:

• In June 2022, we enabled MiraclePtr for the browser process on Windows and Android.
• In September 2022, we expanded its coverage to include all processes except renderer processes.
• In June 2023, we enabled MiraclePtr for ChromeOS, macOS, and Linux.

Furthermore, we have changed security guidelines to downgrade MiraclePtr-protected issues by one severity level!

Evaluating Security Impact

First let’s focus on its security impact. Our analysis is based on two primary information sources: incoming vulnerability reports and crash reports from user devices. Let's take a closer look at each of these sources and how they inform our understanding of MiraclePtr's effectiveness.

Bug reports

Chrome vulnerability reports come from various sources, such as:

• Chrome Vulnerability Reward Program participants,
• our fuzzing infrastructure,
• internal and external teams investigating security incidents.

For the purposes of this analysis, we focus on vulnerabilities that affect platforms where MiraclePtr was enabled at the time the issues were reported. We also exclude bugs that occur inside a sandboxed renderer process. Since the initial launch of MiraclePtr in 2022, we have received 168 use-after-free reports matching our criteria.

What does the data tell us? MiraclePtr effectively mitigated 57% of these use-after-free vulnerabilities in privileged processes, exceeding our initial estimate of 50%. Reaching this level of effectiveness, however, required additional work. For instance, we not only rewrote class fields to use MiraclePtr, as discussed in the previous post, but also added MiraclePtr support for bound function arguments, such as Unretained pointers. These pointers have been a significant source of use-after-frees in Chrome, and the additional protection allowed us to mitigate 39 more issues.

Moreover, these vulnerability reports enable us to pinpoint areas needing improvement. We're actively working on adding support for select third-party libraries that have been a source of use-after-free bugs, as well as developing a more advanced rewriter tool that can handle transformations like converting std::vector<T*> into std::vector<raw_ptr<T>>. We've also made several smaller fixes, such as extending the lifetime of the task state object to cover several issues in the “this pointer” category.

Crash reports

Crash reports offer a different perspective on MiraclePtr's effectiveness. As explained in the previous blog post, when an allocation is quarantined, its contents are overwritten with a special bit pattern. If the allocation is used later, the pattern will often be interpreted as an invalid memory address, causing a crash when the process attempts to access memory at that address. Since the dereferenced address remains within a small, predictable memory range, we can distinguish MiraclePtr crashes from other crashes.

Although this approach has its limitations — such as not being able to obtain stack traces from allocation and deallocation times like AddressSanitizer does — it has enabled us to detect and fix vulnerabilities. Last year, six critical severity vulnerabilities were identified in the default setup of Chrome Stable, the version most people use. Impressively, five of the six were discovered while investigating MiraclePtr crash reports! One particularly interesting example is CVE-2022-3038. The issue was discovered through MiraclePtr crash reports and fixed in Chrome 105. Several months later, Google's Threat Analysis Group discovered an exploit for that vulnerability used in the wild against clients of a different Chromium-based browser that hadn’t shipped the fix yet.

To further enhance our crash analysis capabilities, we've recently launched an experimental feature that allows us to collect additional information for MiraclePtr crashes, including stack traces. This effectively shortens the average crash report investigation time.

Performance

MiraclePtr enables us to have robust protection against use-after-free bug exploits, but there is a performance cost associated with it. Therefore, we have conducted experiments on each platform where we have shipped MiraclePtr, which we used in our decision-making process.

The main cost of MiraclePtr is memory. Specifically, the memory usage of the browser process increased by 5.5-8% on desktop platforms and approximately 2% on Android. Yet, when examining the holistic memory usage across all processes, the impact remains within a moderate 1-3% range to lower percentiles only.

The main cause of the additional memory usage is the extra size to allocate the reference count. One might think that adding 4 bytes to each allocation wouldn’t be a big deal. However, there are many small allocations in Chrome, so even the 4B overhead is not negligible. Moreover, PartitionAlloc also uses pre-defined allocation bucket sizes, so this extra 4B pushes certain allocations (particularly power-of-2 sized) into a larger bucket, e.g. 4096B → 5120B.

We also considered the performance cost. We verified that there were no regressions to the majority of our top-level performance metrics, including all of the page load metrics, like Largest Contentful Paint, First Contentful Paint and Cumulative Layout Shift. We did find a few regressions, such as a 10% increase in the 99th percentile of the browser process main thread contention metric, a 1.5% regression in First Input Delay on ChromeOS, and a 1.5% regression in tab startup time on Android. The main thread contention metric tries to estimate how often a user input can be delayed and so for example on Windows this was a change from 1.6% to 1.7% at the 99th percentile only. These are all minor regressions. There has been zero change in daily active usage, and we do not anticipate these regressions to have any noticeable impact on users.

Conclusion

In summary, MiraclePtr has proven to be effective in mitigating use-after-free vulnerabilities and enhancing the overall security of the Chrome browser. While there are performance costs associated with the implementation of MiraclePtr, our analysis suggests that the benefits in terms of security improvements far outweigh these. We are committed to continually refining and expanding the feature to cover more areas. For example we are working to add coverage to third-party libraries used by the GPU process, and we plan to enable BRP on the renderer process. By sharing our findings and experiences, we hope to contribute to the broader conversation surrounding browser security and inspire further innovation in this crucial area.