Reminiscence security bugs are probably 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 most typical kind of reminiscence security bug is the “use-after-free”. We lately posted about an thrilling sequence of applied sciences designed to forestall these. These applied sciences (collectively, *Scan, pronounced “star scan”) are very {powerful} however possible require {hardware} assist for enough efficiency.
As we speak we’re going to speak a few totally different strategy to fixing the identical kind of bugs.
It’s arduous, if not not possible, 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 affordable assumptions about how a little bit of code will work, then a later change invalidates these assumptions. Instantly, 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 12 months 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 internet content material is remoted right into a sandboxed “renderer” course of the place little hurt can happen. An attacker subsequently normally 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 commonly the better one. The attacker has a number of affect within the renderer course of. It’s straightforward to rearrange reminiscence in a selected method, and the renderer course of acts upon many alternative sorts of internet content material, giving a big “assault floor” that would doubtlessly be exploited.
The second stage, escaping the renderer sandbox, is trickier. Attackers have two choices how to do that:
- They will exploit a bug within the underlying working system (OS) via the restricted interfaces obtainable inside Chrome’s sandbox.
- 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 basically has to be omnipotent.
We think about the attackers squeezing via the slender a part of a funnel:
If we are able to cut back the scale of the slender a part of the funnel, we are going to make it as arduous as potential for attackers to assemble a full exploit chain. We are able to cut back the scale of the orange slice by eradicating entry to extra OS interfaces inside the renderer course of sandbox, and we’re repeatedly engaged on that. The MiraclePtr venture goals to cut back the scale of the blue slice.
Right here’s a pattern of 100 latest 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 doesn’t fairly add as much as 100 – that’s as a result of a number of bugs have been in different processes past the renderer or browser.
- We claimed that the browser course of is the harder half to take advantage of, but there are extra potentially-exploitable bugs! That could be so, however we imagine they’re sometimes tougher to take advantage of as a result of the attacker has much less management over reminiscence format.
As you’ll be able to 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 tougher.
MiraclePtr: Stopping Exploitation of Use-After-Free Bugs
That is the place MiraclePtr is available in. It’s a expertise to forestall exploitation of use-after-free bugs. In contrast to 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 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 so forth., we concluded that BackupRefPtr was probably the most promising answer.
The BackupRefPtr algorithm relies on reference counting. It makes use of assist of Chrome’s personal heap allocator, PartitionAlloc, which carves out somewhat further area for a hidden reference depend for every allocation. raw_ptr<T> increments or decrements the reference depend when it’s constructed, destroyed or modified. When the applying 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 obtainable for reuse as soon as the reference depend reaches 0. Quarantined reminiscence is poisoned to additional cut 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 at the moment are 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 all platforms as a result of that ensures {that a} given pointer is protected for all customers of Chrome.
Balancing Safety and Efficiency
There is no such thing as a free lunch, nonetheless. This safety safety comes at a price, which we’ve rigorously weighed in our determination making.
Unsurprisingly, the principle value 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 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 properly under their earlier ranges. Whereas we have been apprehensive about quarantined reminiscence, in observe this can be 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 wouldn’t be a giant deal. Nevertheless, there are numerous 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, e.g. 4096B->5120B.
We additionally thought of the efficiency value. Including an atomic increment/decrement on frequent operations corresponding to pointer project has unavoidable overhead. Having excluded quite a lot of performance-critical pointers, we drove this overhead down till we might achieve again the identical margin via 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 so forth. The one hostile change there1 is a rise of the principle thread competition (~7%). On Android1, along with an analogous enhance in the principle thread competition (~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 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 broaden 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 forestall exploitation of use-after-free bugs. Though it wasn’t designed for diagnosability, it already helped us discover and repair quite a lot of bugs that have been beforehand undetected. Now we have 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 via the Chrome Vulnerability Reward Program, even when these points are mitigated by MiraclePtr. We nonetheless must make MiraclePtr obtainable to all customers, accumulate extra knowledge 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.
