Google Online Security Blog: Use-after-freedom: MiraclePtr

Reminiscence security bugs are probably the most quite a few class of Chrome safety points and we’re persevering with to investigate many solutions – each in C++ and in new programming languages. The most typical kind of reminiscence security bug is the “use-after-free”. We recently 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 this time we’ll speak about a special strategy to fixing the identical kind of bugs.

It is onerous, if not inconceivable, to keep away from use-after-frees in a non-trivial codebase. It is not often a mistake by a single programmer. As an alternative, one programmer makes cheap assumptions about how a little bit of code will work, then a later change invalidates these assumptions. Immediately, the info is not 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 in the ChromeHTML engine was exploited this year 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 to multi process architecture, partly to make sure that internet content material is remoted right into a sandboxed “renderer” course of the place little hurt can happen. An attacker due to this fact 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 simplest one. The attacker has plenty of affect within the renderer course of. It is easy to rearrange reminiscence in a particular means, and the renderer course of acts upon many various sorts of internet content material, giving a big “assault floor” that might probably be exploited.

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

  1. They will exploit a bug within the underlying working system (OS) via the restricted interfaces accessible inside Chrome’s sandbox.
  2. Or, they’ll 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 have you ever to be omnipotent.

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

If we are able to cut back the dimensions of the slim a part of the funnel, we are going to make it as onerous as doable for attackers to assemble a full exploit chain. We are able to cut back the dimensions 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 dimensions of the blue slice.

This is a pattern of 100 current excessive severity Chrome safety bugs that made it to the steady channel, divided by root trigger and by the method they have an effect on.

You would possibly discover:

  • This does not fairly add as much as 100 – that is 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 probably the most troublesome half to use, but there are extra potentially-exploitable bugs! That could be so, however we consider they’re usually more durable to use as a result of the attacker has much less management over reminiscence format.

As you may see, the largest 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 more durable.

MiraclePtr: Stopping Exploitation of Use-After-Free Bugs

that is the place MiraclePtr you eat in. It’s a expertise to stop exploitation of use-after-free bugs. Not like aforementioned *Scan applied sciences that provide a non-invasive strategy to this drawback, MiraclePtr depends on rewriting the codebase to make use of a brand new sensible pointer kind, raw_ptr. 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 so forth., we concluded that BackupRefPtr was probably 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 somewhat further area for a hidden reference depend for every allocation. raw_ptr increments or decrements the reference depend when it is constructed, destroyed or modified. When the appliance calls free/delete and the reference depend is larger than 0, PartitionAlloc quarantines that reminiscence area as an alternative of instantly releasing it. The reminiscence area is then solely made accessible for reuse as soon as the reference depend reaches 0. Quarantined reminiscence is poisoned to additional cut back the chance that use-after-free accesses will lead to 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 remains to be pointing to the item.
b->doSomething();  // Use-after-free is neutralized.

We efficiently rewrote more than 15,000 raw pointers within the Chrome codebase into raw_ptr, 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 towards exploitation. We are actually engaged on enabling BackupRefPtr within the community, utility and GPU processes, and for different platforms. In the long run state, our objective is to allow BackupRefPtr on there platforms as a result of that ensures {that a} given pointer is protected for there customers of Chrome.

Balancing Safety and Efficiency

There isn’t a free lunch, nonetheless. This safety safety comes at a value, which we’ve got fastidiously weighed in our choice making.

Unsurprisingly, the principle price is reminiscence. Fortunately, associated investments into PartitionAlloc over the previous 12 months led to 10-25% whole reminiscence financial savings, relying on utilization patterns and platforms. So we have been capable of 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 nicely under their earlier ranges. Whereas we have been fearful about quarantined reminiscence, in observe 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 depend. One would possibly assume that including 4 bytes to every allocation would not be a giant deal. Nonetheless, there are a lot of small allocations in Chrome, so even the 4B overhead shouldn’t be negligible. PartitionAlloc additionally makes use of pre-defined bucket sizes, so this further 4B pushes sure allocations (notably power-of-2 sized) into a bigger bucket, eg 4096B->5120B.

We additionally thought-about the efficiency price. Including an atomic increment/decrement on frequent operations akin to pointer project has unavoidable overhead. Having excluded numerous performance-critical pointers, we drove this overhead down till we may acquire again the identical margin via different efficiency optimizations. On Home windows, no statistically vital efficiency regressions have been noticed on most of our top-level efficiency metrics like Largest Contentful Paint, First Enter Delay, and so forth. The one hostile change there1 is a rise in the principle thread rivalry (~7%). Android1, along with an identical enhance in the principle 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 impression on consumer 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 attenuate the overhead. As future work, we’re exploring choices to increase the pointer protection to on-stack pointers in order that we are able to defend towards extra use-after-free bugs.

Notice 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 numerous bugs that have been beforehand undetected. We’ve ongoing efforts to make MiraclePtr crash studies much more informative and actionable.

Proceed to Present Us Suggestions

Final however not least, we would wish to encourage safety researchers to proceed to report points via the Chrome Vulnerability Reward Program, even when these points are mitigated by MiraclePtr. We nonetheless have to make MiraclePtr accessible to all customers, acquire extra information on its impression via reported points, and additional refine our processes and tooling. Till that’s carried out, we is not going to 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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