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HomeCyber SecurityGoogle On-line Safety Weblog: Use-after-freedom: MiraclePtr

Google On-line Safety Weblog: Use-after-freedom: MiraclePtr

Reminiscence security bugs are essentially the most quite a few class of Chrome safety points and we’re persevering with to examine many options – each in C++ and in new programming languages. The commonest kind of reminiscence security bug is the “use-after-free”. We just lately posted about an thrilling collection of applied sciences designed to stop these. These applied sciences (collectively, *Scan, pronounced “star scan”) are very {powerful} however possible require {hardware} help for enough efficiency.

At the moment we’re going to speak a few totally different strategy to fixing the identical kind of bugs.

It’s exhausting, if not inconceivable, to keep away from use-after-frees in a non-trivial codebase. It’s not often a mistake by a single programmer. As a substitute, one programmer makes cheap assumptions about how a little bit of code will work, then a later change invalidates these assumptions. Abruptly, the info isn’t legitimate so long as the unique programmer anticipated, and an exploitable bug outcomes.

These bugs have actual penalties. For instance, based on Google Menace Evaluation Group, a use-after-free within the ChromeHTML engine was exploited this yr by North Korea.

Half of the recognized exploitable bugs in Chrome are use-after-frees:

Diving Deeper: Not All Use-After-Free Bugs Are Equal

Chrome has a multi-process structure, partly to make sure that net content material is remoted right into a sandboxed “renderer” course of the place little hurt can happen. An attacker subsequently often wants to seek out and exploit two vulnerabilities – one to attain code execution within the renderer course of, and one other bug to interrupt out of the sandbox.

The primary stage is usually the simpler one. The attacker has plenty of affect within the renderer course of. It’s straightforward to rearrange reminiscence in a particular approach, and the renderer course of acts upon many various sorts of net content material, giving a big “assault floor” that might doubtlessly be exploited.

The second stage, escaping the renderer sandbox, is trickier. Attackers have two choices how to do that:

  1. They’ll exploit a bug within the underlying working system (OS) by way of the restricted interfaces accessible inside Chrome’s sandbox.
  2. Or, they will exploit a bug in a extra {powerful}, privileged a part of Chrome – just like the “browser” course of. This course of coordinates all the opposite bits of Chrome, so essentially has to be omnipotent.

We think about the attackers squeezing by way of the slim a part of a funnel:

If we are able to scale back the scale of the slim a part of the funnel, we are going to make it as exhausting as potential for attackers to assemble a full exploit chain. We will scale back the scale of the orange slice by eradicating entry to extra OS interfaces inside the renderer course of sandbox, and we’re constantly engaged on that. The MiraclePtr venture goals to scale back the scale of the blue slice.

Right here’s a pattern of 100 current excessive severity Chrome safety bugs that made it to the secure channel, divided by root trigger and by the method they have an effect on.

You would possibly discover:

  • This doesn’t fairly add as much as 100 – that’s as a result of just a few bugs have been in different processes past the renderer or browser.
  • We claimed that the browser course of is the tougher half to use, but there are extra potentially-exploitable bugs! Which may be so, however we consider they’re usually tougher to use as a result of the attacker has much less management over reminiscence format.

As you may see, the most important class of bugs in every course of is: V8 within the renderer course of (JavaScript engine logic bugs – work in progress) and use-after-free bugs within the browser course of. If we are able to make that “skinny” bit thinner nonetheless by eradicating a few of these use-after-free bugs, we make the entire job of Chrome exploitation markedly tougher.

MiraclePtr: Stopping Exploitation of Use-After-Free Bugs

That is the place MiraclePtr is available in. It’s a expertise to stop exploitation of use-after-free bugs. In contrast to aforementioned *Scan applied sciences that supply a non-invasive strategy to this drawback, MiraclePtr depends on rewriting the codebase to make use of a brand new good pointer kind, raw_ptr<T>. There are a number of methods to implement MiraclePtr. We got here up with ~10 algorithms and in contrast the professionals and cons. After analyzing their efficiency overhead, reminiscence overhead, safety safety ensures, developer ergonomics, and many others., we concluded that BackupRefPtr was essentially the most promising answer.

The BackupRefPtr algorithm is predicated on reference counting. It makes use of help of Chrome’s personal heap allocator, PartitionAlloc, which carves out a bit of additional house for a hidden reference rely for every allocation. raw_ptr<T> increments or decrements the reference rely when it’s constructed, destroyed or modified. When the appliance calls free/delete and the reference rely is larger than 0, PartitionAlloc quarantines that reminiscence area as a substitute of instantly releasing it. The reminiscence area is then solely made accessible for reuse as soon as the reference rely reaches 0. Quarantined reminiscence is poisoned to additional scale back the probability that use-after-free accesses will end in exploitable circumstances, and in hope that future accesses result in an easy-to-debug crash, turning these safety points into less-dangerous ones.

class A { ... };
class B {
  B(A* a) : a_(a) {}
  void doSomething() { a_->doSomething(); }
  raw_ptr<A> a_;  // MiraclePtr

std::unique_ptr<A> a = std::make_unique<A>();
std::unique_ptr<B> b = std::make_unique<B>(a.get());
a = nullptr;  // The free is delayed as a result of the MiraclePtr continues to be pointing to the article.
b->doSomething();  // Use-after-free is neutralized.

We efficiently rewrote greater than 15,000 uncooked pointers within the Chrome codebase into raw_ptr<T>, then enabled BackupRefPtr for the browser course of on Home windows and Android (each 64 bit and 32 bit) in Chrome 102 Steady. We anticipate that MiraclePtr meaningfully reduces the browser course of assault floor of Chrome by defending ~50% of use-after-free points in opposition to exploitation. We are actually engaged on enabling BackupRefPtr within the community, utility and GPU processes, and for different platforms. Ultimately state, our objective is to allow BackupRefPtr on all platforms as a result of that ensures {that a} given pointer is protected for all customers of Chrome.

Balancing Safety and Efficiency

There isn’t a free lunch, nevertheless. This safety safety comes at a value, which we’ve got rigorously weighed in our determination making.

Unsurprisingly, the primary value is reminiscence. Fortunately, associated investments into PartitionAlloc over the previous yr led to 10-25% whole reminiscence financial savings, relying on utilization patterns and platforms. So we have been in a position to spend a few of these financial savings on safety: MiraclePtr elevated the reminiscence utilization of the browser course of 4.5-6.5% on Home windows and three.5-5% on Android1, nonetheless effectively beneath their earlier ranges. Whereas we have been apprehensive about quarantined reminiscence, in follow it is a tiny fraction (0.01%) of the browser course of utilization. By far the larger perpetrator is the extra reminiscence wanted to retailer the reference rely. One would possibly suppose that including 4 bytes to every allocation wouldn’t be a giant deal. Nevertheless, there are a lot of small allocations in Chrome, so even the 4B overhead just isn’t negligible. PartitionAlloc additionally makes use of pre-defined bucket sizes, so this additional 4B pushes sure allocations (notably power-of-2 sized) into a bigger bucket, e.g. 4096B->5120B.

We additionally thought of the efficiency value. Including an atomic increment/decrement on frequent operations similar to pointer task has unavoidable overhead. Having excluded various performance-critical pointers, we drove this overhead down till we may achieve again the identical margin by way of different efficiency optimizations. On Home windows, no statistically important efficiency regressions have been noticed on most of our top-level efficiency metrics like Largest Contentful Paint, First Enter Delay, and many others. The one hostile change there1 is a rise of the primary thread rivalry (~7%). On Android1, along with the same enhance in the primary thread rivalry (~6%), there have been small regressions in First Enter Delay (~1%), Enter Delay (~3%) and First Contentful Paint (~0.5%). We do not anticipate these regressions to have a noticeable influence on person expertise, and are assured that they’re strongly outweighed by the extra security for our customers.

We must always emphasize that MiraclePtr presently protects solely class/struct pointer fields, to reduce the overhead. As future work, we’re exploring choices to develop the pointer protection to on-stack pointers in order that we are able to defend in opposition to extra use-after-free bugs.

Word that the first objective of MiraclePtr is to stop exploitation of use-after-free bugs. Though it wasn’t designed for diagnosability, it already helped us discover and repair various bugs that have been beforehand undetected. We’ve ongoing efforts to make MiraclePtr crash stories much more informative and actionable.

Proceed to Present Us Suggestions

Final however not least, we’d wish to encourage safety researchers to proceed to report points by way of the Chrome Vulnerability Reward Program, even when these points are mitigated by MiraclePtr. We nonetheless have to make MiraclePtr accessible to all customers, gather extra knowledge on its influence by way of reported points, and additional refine our processes and tooling. Till that’s completed, we won’t take into account MiraclePtr when figuring out the severity of a bug or the reward quantity.

1 Measured in Chrome 99.



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