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 sort 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 seemingly require {hardware} help for enough efficiency.
Immediately we’re going to speak a few totally different method to fixing the identical sort of bugs.
It’s arduous, if not not possible, to keep away from use-after-frees in a non-trivial codebase. It’s hardly ever 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. All of a sudden, the information isn’t legitimate so long as the unique programmer anticipated, and an exploitable bug outcomes.
These bugs have actual penalties. For instance, in keeping with Google Menace Evaluation Group, a use-after-free within the ChromeHTML engine was exploited this 12 months by North Korea.
Half of the identified 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 due to this fact normally wants to seek out and exploit two vulnerabilities – one to realize code execution within the renderer course of, and one other bug to interrupt out of the sandbox.
The primary stage is usually the better one. The attacker has a lot of affect within the renderer course of. It’s simple to rearrange reminiscence in a selected manner, 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:
- They’ll exploit a bug within the underlying working system (OS) by way of the restricted interfaces obtainable inside Chrome’s sandbox.
- 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 basically has to be omnipotent.
We think about the attackers squeezing by way of the slim a part of a funnel:
If we will cut back the dimensions of the slim a part of the funnel, we are going to make it as arduous as doable for attackers to assemble a full exploit chain. We will cut back the dimensions of the orange slice by eradicating entry to extra OS interfaces throughout the renderer course of sandbox, and we’re constantly engaged on that. The MiraclePtr mission goals to cut back the dimensions of the blue slice.
Right here’s 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 may discover:
- This doesn’t fairly add as much as 100 – that’s as a result of a couple of 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! That could be so, however we consider they’re sometimes tougher to use as a result of the attacker has much less management over reminiscence structure.
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 will 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 know-how to forestall exploitation of use-after-free bugs. In contrast to aforementioned *Scan applied sciences that supply a non-invasive method to this drawback, MiraclePtr depends on rewriting the codebase to make use of a brand new good pointer sort, 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 essentially the most promising answer.
The BackupRefPtr algorithm relies on reference counting. It makes use of help of Chrome’s personal heap allocator, PartitionAlloc, which carves out slightly additional house 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 bigger 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 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 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 towards exploitation. We are actually engaged on enabling BackupRefPtr within the community, utility and GPU processes, and for different platforms. Ultimately state, our aim 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, nonetheless. This safety safety comes at a price, which we’ve got fastidiously weighed in our resolution 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 effectively beneath their earlier ranges. Whereas we have been frightened about quarantined reminiscence, in observe this can be a tiny fraction (0.01%) of the browser course of utilization. By far the larger wrongdoer is the extra reminiscence wanted to retailer the reference depend. One may assume that including 4 bytes to every allocation wouldn’t be a giant deal. Nevertheless, there are lots of small allocations in Chrome, so even the 4B overhead 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 price. Including an atomic increment/decrement on widespread operations corresponding to pointer project has unavoidable overhead. Having excluded plenty of 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 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 of the principle thread rivalry (~7%). On 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 reduce the overhead. As future work, we’re exploring choices to broaden the pointer protection to on-stack pointers in order that we will shield towards extra use-after-free bugs.
Be aware that the first aim 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 plenty of bugs that have been beforehand undetected. We’ve got 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 must make MiraclePtr obtainable to all customers, acquire extra information on its impression by way of reported points, and additional refine our processes and tooling. Till that’s performed, we is not going to think about MiraclePtr when figuring out the severity of a bug or the reward quantity.
1 Measured in Chrome 99.
