Ramifications of Rewriting a Browser Element in Rust

The previous posts in this Fearless Security collection examine memory safety and line safety in Corrosion. This closing post uses the particular Quantum CSS project as an example to explore the real world impact of spinning code in Rust.

The style component could be the part of a browser that can be applied CSS rules to a page. It is a top-down process on the DOM shrub: given the parent style, the particular styles of children can be calculated independently— a perfect use-case for parallel calculation. By 2017, Mozilla had produced two previous attempts to parallelize the style system using C++. Both had failed.

Quantum CSS resulted from a need to improve web page performance. Improving security is a content byproduct.

Rewrites code to make it faster; furthermore makes it more secure

There’ s i9000 a large overlap between memory protection violations and security-related bugs, and we expected this rewrite to reduce the particular attack surface in Firefox. On this page, I will summarize the potential security vulnerabilities that have appeared in the styling program code since Firefox’ s initial discharge in 2002 . Then I’ ll look at what could and may not have been prevented by using Corrosion.

Over the course of its life time, there have been 69 security bugs within Firefox’ s style component. In the event that we’ d had a time device and could have written this element in Rust from the start, 51 (73. 9%) of these bugs would not are actually possible. While Rust makes it simpler to write better code, it’ h not foolproof.


Rust is a contemporary systems programming language that is type- and memory-safe. As a side effect of those safety guarantees, Rust programs may also be known to be thread-safe at compile period. Thus, Rust can be a particularly wise decision when:

✅ digesting untrusted input safely.
✅ introducing parallelism to improve performance.
✅ integrating isolated components in to an existing codebase.

Nevertheless , there are classes of bugs that will Rust explicitly does not address— especially correctness bugs. In fact , during the Mess CSS rewrite, engineers accidentally reintroduced a critical security bug that acquired previously been patched in the C++ code, regressing the fix just for bug 641731 . This allowed global background leakage via SVG image files, resulting in frustrate 1420001 . As a trivial history-stealing bug, this is rated security-high. The initial fix was an additional check to see when the SVG document was being used being an image. Unfortunately, this check has been overlooked during the rewrite.

While there were automated tests designed to catch : visited rule violations like this, used, they didn’ t detect this particular bug. To speed up our automatic tests, we temporarily turned off the particular mechanism that tested this feature— tests aren’ t particularly helpful if they aren’ t run. The chance of re-implementing logic errors can be mitigated by good test coverage (and really running the tests). There’ ersus still a danger of introducing brand new logic errors.

Because developer familiarity with the Rust vocabulary increases, best practices will improve. Code composed in Rust will become even more safe. While it may not prevent all probable vulnerabilities, Rust eliminates an entire course of the most severe bugs.

Quantum CSS Security Bugs

Overall, bugs related to storage, bounds, null/uninitialized variables, or integer overflow would be prevented by default within Rust. The miscellaneous bug We referenced above would not have been prevented— it was a crash due to a failed portion.

Security bugs simply by category

All of the insects in this analysis are related to protection, but only 43 received recognized security classifications. (These are designated by Mozilla’ s security technicians based on educated “ exploitability” guesses. ) Normal bugs might show missing features or problems such as crashes. While undesirable, crashes don’ t result in data leakage or even behavior modification. Official security insects can range from low severity (highly limited in scope) to crucial vulnerability (might allow an opponent to run arbitrary code on the user’ s platform).

There’ s a significant overlap between storage vulnerabilities and severe security complications. Of the 34 critical/high bugs, thirty-two were memory-related.

Protection rated bug breakdown

Comparing Rust and C++ program code

Annoy 955914 is a pile buffer overflow in the GetCustomPropertyNameAt function. The particular code used the wrong variable regarding indexing, which resulted in interpreting storage past the end of the array. This might either crash while accessing a negative pointer or copy memory to some string that is passed to another element.

The ordering of CSS properties (both longhand and custom) is kept in an array, mOrder . Each element is either displayed by its CSS property worth or, in the case of custom properties, with a value that starts at eCSSProperty_COUNT (the total number of non-custom CSS properties). To retrieve the name of a customized property, first, you have to retrieve the particular custom property value from mOrder , then accessibility the name at the corresponding index from the mVariableOrder array, which stores the custom made property names in order.

Vulnerable C++ code:

  void GetCustomPropertyNameAt(uint32_t aIndex, nsAString& aResult) const {
MOZ_ASSERT(mOrder[aIndex] > = eCSSProperty_COUNT);

aResult. Truncate();
aResult. AppendLiteral("var-");
aResult. Append(mVariableOrder[aIndex]);

The problem occurs at {collection|range|series} 6 when using aIndex to access an element of the particular mVariableOrder array. aIndex is intended for use with the mOrder {variety|range|assortment} not the mVariableOrder array. The related element for the custom property {displayed|symbolized} by aIndex in mOrder is actually mOrder[aIndex] - eCSSProperty_COUNT .

Fixed C++ code:

  void Get CustomPropertyNameAt(uint32_t aIndex, nsAString& aResult) const {
MOZ_ASSERT(mOrder[aIndex] > = eCSSProperty_COUNT);

uint32_t variableIndex = mOrder[aIndex] - eCSSProperty_COUNT;
aResult. Truncate();
aResult. AppendLiteral("var-");
aResult. Append(mVariableOrder[variableIndex]);

Comparative Rust code

Whilst Rust is similar to C++ in some ways, idiomatic Rust uses different abstractions plus data structures. Rust code will appear very different from C++ (see beneath for details). First, let’ {h|t|s i9000|ersus} consider what would happen if we converted the vulnerable code as actually as possible:

  fn GetCustomPropertyNameAt(& self, aIndex: usize) -> String {
assert! (self. mOrder[aIndex] > sama dengan self. eCSSProperty_COUNT);

let mut outcome = "var-". to_string();
result += & self. mVariableOrder[aIndex];

The particular Rust compiler would accept the particular code, since there is no way to determine the duration of vectors before runtime. Unlike arrays, whose length must be known, the particular Vec type in Rust is dynamically size. However , the standard library vector execution has built-in bounds checking. For the invalid index is used, the program instantly terminates in a controlled fashion, {avoiding|stopping} any illegal access.

The real code in {Portion|Mess} CSS uses very different data {constructions|buildings}, so there’ s no {precise|specific} equivalent. For example , we use Rust’ s powerful built-in data {constructions|buildings} to unify the ordering plus property name data. This allows all of us to avoid having to maintain two {impartial|self-employed|3rd party|indie} arrays. Rust data structures furthermore improve data encapsulation and reduce the possibilities of these kinds of logic errors. Because the program code needs to interact with C++ code consist of parts of the browser engine, {the brand new|the newest} GetCustomPropertyNameAt function doesn’ t look like idiomatic Rust code. It still provides all of the safety guarantees while offering a more understandable abstraction of the {fundamental|root} data.

tl; doctor;

Due to the overlap {among|in between} memory safety violations and security-related bugs, we can say that Rust program code should result in fewer critical CVEs (Common Vulnerabilities plus Exposures). However , even Rust {is just not|is not really} foolproof. Developers still need to be conscious of correctness bugs and data seapage attacks. Code review, testing, plus fuzzing still remain essential for {keeping|sustaining|preserving} secure libraries.

Compilers can’ t catch every error that programmers can make. However , Corrosion has been designed to remove the burden associated with memory safety from our shoulders, {permitting|enabling} us to focus on logical correctness plus soundness instead.

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