Threat
Known example
How SLSA could have helped
A
Submit bad code to the source repository
Linux hypocrite commits: Researcher attempted to intentionally introduce vulnerabilities into the Linux kernel via patches on the mailing list.
Two-person review caught most, but not all, of the vulnerabilities.
B
Compromise source control platform
PHP: Attacker compromised PHP’s self-hosted git server and injected two malicious commits.
A better-protected source code platform would have been a much harder target for the attackers.
C
Build with official process but from code not matching source control
Webmin: Attacker modified the build infrastructure to use source files not matching source control.
A SLSA-compliant build server would have produced provenance identifying the actual sources used, allowing consumers to detect such tampering.
D
Compromise build platform
SolarWinds: Attacker compromised the build platform and installed an implant that injected malicious behavior during each build.
Higher SLSA levels require stronger security controls for the build platform, making it more difficult to compromise and gain persistence.
E
Use bad dependency (i.e. A-H, recursively)
event-stream: Attacker added an innocuous dependency and then updated the dependency to add malicious behavior. The update did not match the code submitted to GitHub (i.e. attack F).
Applying SLSA recursively to all dependencies would have prevented this particular vector, because the provenance would have indicated that it either wasn’t built from a proper builder or that the source did not come from GitHub.
F
Upload an artifact that was not built by the CI/CD system
CodeCov: Attacker used leaked credentials to upload a malicious artifact to a GCS bucket, from which users download directly.
Provenance of the artifact in the GCS bucket would have shown that the artifact was not built in the expected manner from the expected source repo.
G
Compromise package repository
Attacks on Package Mirrors: Researcher ran mirrors for several popular package repositories, which could have been used to serve malicious packages.
Similar to above (F), provenance of the malicious artifacts would have shown that they were not built as expected or from the expected source repo.
H
Trick consumer into using bad package
Browserify typosquatting: Attacker uploaded a malicious package with a similar name as the original.
SLSA does not directly address this threat, but provenance linking back to source control can enable and enhance other solutions.
- name: Generate provenance
uses: slsa-framework/github-actions-demo@v0.1
with:
artifact_path: <path-to-artifact/directory>
One of the main challenges of evaluating Rust for use within the Android platform was ensuring we could provide sufficient interoperability with our existing codebase. If Rust is to meet its goals of improving security, stability, and quality Android-wide, we need to be able to use Rust anywhere in the codebase that native code is required. To accomplish this, we need to provide the majority of functionality platform developers use. As we discussed previously, we have too much C++ to consider ignoring it, rewriting all of it is infeasible, and rewriting older code would likely be counterproductive as the bugs in that code have largely been fixed. This means interoperability is the most practical way forward.
Before introducing Rust into the Android Open Source Project (AOSP), we needed to demonstrate that Rust interoperability with C and C++ is sufficient for practical, convenient, and safe use within Android. Adding a new language has costs; we needed to demonstrate that Rust would be able to scale across the codebase and meet its potential in order to justify those costs. This post will cover the analysis we did more than a year ago while we evaluated Rust for use in Android. We also present a follow-up analysis with some insights into how the original analysis has held up as Android projects have adopted Rust.
Existing language interoperability in Android focuses on well defined foreign-function interface (FFI) boundaries, which is where code written in one programming language calls into code written in a different language. Rust support will likewise focus on the FFI boundary as this is consistent with how AOSP projects are developed, how code is shared, and how dependencies are managed. For Rust interoperability with C, the C application binary interface (ABI) is already sufficient.
Interoperability with C++ is more challenging and is the focus of this post. While both Rust and C++ support using the C ABI, it is not sufficient for idiomatic usage of either language. Simply enumerating the features of each language results in an unsurprising conclusion: many concepts are not easily translatable, nor do we necessarily want them to be. After all, we’re introducing Rust because many features and characteristics of C++ make it difficult to write safe and correct code. Therefore, our goal is not to consider all language features, but rather to analyze how Android uses C++ and ensure that interop is convenient for the vast majority of our use cases.
We analyzed code and interfaces in the Android platform specifically, not codebases in general. While this means our specific conclusions may not be accurate for other codebases, we hope the methodology can help others to make a more informed decision about introducing Rust into their large codebase. Our colleagues on the Chrome browser team have done a similar analysis, which you can find here.
This analysis was not originally intended to be published outside of Google: our goal was to make a data-driven decision on whether or not Rust was a good choice for systems development in Android. While the analysis is intended to be accurate and actionable, it was never intended to be comprehensive, and we’ve pointed out a couple of areas where it could be more complete. However, we also note that initial investigations into these areas showed that they would not significantly impact the results, which is why we decided to not invest the additional effort.
Exported functions from Rust and C++ libraries are where we consider interop to be essential. Our goals are simple:
While making Rust functions callable from C++ is a goal, this analysis focuses on making C++ functions available to Rust so that new Rust code can be added while taking advantage of existing implementations in C++. To that end, we look at exported C++ functions and consider existing and planned compatibility with Rust via the C ABI and compatibility libraries. Types are extracted by running objdump on shared libraries to find external C++ functions they use1 and running c++filt to parse the C++ types. This gives functions and their arguments. It does not consider return values, but a preliminary analysis2 of those revealed that they would not significantly affect the results.
objdump
c++filt
We then classify each of these types into one of the following buckets:
These are generally simple types involving primitives (including pointers and references to them). For these types, Rust’s existing FFI will handle them correctly, and Android’s build system will auto-generate the bindings.
These are handled by the cxx crate. This currently includes std::string, std::vector, and C++ methods (including pointers/references to these types). Users simply have to define the types and functions they want to share across languages and cxx will generate the code to do that safely.
std::string
std::vector,
These types are not directly supported, but the interfaces that use them have been manually reworked to add Rust support. Specifically, this includes types used by AIDL and protobufs.
We have also implemented a native interface for StatsD as the existing C++ interface relies on method overloading, which is not well supported by bindgen and cxx3. Usage of this system does not show up in the analysis because the C++ API does not use any unique types.
This is currently common data structures such as std::optional and std::chrono::duration and custom string and vector implementations.
std::optional
std::chrono::duration
These can either be supported natively by a future contribution to cxx, or by using its ExternType facilities. We have only included types in this category that we believe are relatively straightforward to implement and have a reasonable chance of being accepted into the cxx project.
Some types are exposed in today’s C++ APIs that are either an implicit part of the API, not an API we expect to want to use from Rust, or are language specific. Examples of types we do not intend to support include:
native_handle
std::locale&
cout
Overall, this category represents types that we do not believe a Rust developer should be using.
Android is in the process of deprecating HIDL and migrating to AIDL for HALs for new services.We’re also migrating some existing implementations to stable AIDL. Our current plan is to not support HIDL, preferring to migrate to stable AIDL instead. These types thus currently fall into the “We don't need/intend to support'' bucket above, but we break them out to be more specific. If there is sufficient demand for HIDL support, we may revisit this decision later.
This contains all types that do not fit into any of the above buckets. It is currently mostly std::string being passed by value, which is not supported by cxx.
One of the primary reasons for supporting interop is to allow reuse of existing code. With this in mind, we determined the most commonly used C++ libraries in Android: liblog, libbase, libutils, libcutils, libhidlbase, libbinder, libhardware, libz, libcrypto, and libui. We then analyzed all of the external C++ functions used by these libraries and their arguments to determine how well they would interoperate with Rust.
liblog
libbase
libutils
libcutils
libhidlbase
libbinder
libhardware
libz
libcrypto
libui
Overall, 81% of types are in the first three categories (which we currently fully support) and 87% are in the first four categories (which includes those we believe we can easily support). Almost all of the remaining types are those we believe we do not need to support.
In addition to analyzing popular C++ libraries, we also examined Mainline modules. Supporting this context is critical as Android is migrating some of its core functionality to Mainline, including much of the native code we hope to augment with Rust. Additionally, their modularity presents an opportunity for interop support.
We analyzed 64 binaries and libraries in 21 modules. For each analyzed library we examined their used C++ functions and analyzed the types of their arguments to determine how well they would interoperate with Rust in the same way we did above for the top 10 libraries.
Here 88% of types are in the first three categories and 90% in the first four, with almost all of the remaining being types we do not need to handle.
With almost a year of Rust development in AOSP behind us, and more than a hundred thousand lines of code written in Rust, we can now examine how our original analysis has held up based on how C/C++ code is currently called from Rust in AOSP.4
The results largely match what we expected from our analysis with bindgen handling the majority of interop needs. Extensive use of AIDL by the new Keystore2 service results in the primary difference between our original analysis and actual Rust usage in the “Native Support” category.
A few current examples of interop are:
Bindgen and cxx provide the vast majority of Rust/C++ interoperability needed by Android. For some of the exceptions, such as AIDL, the native version provides convenient interop between Rust and other languages. Manually written wrappers can be used to handle the few remaining types and functions not supported by other options as well as to create ergonomic Rust APIs. Overall, we believe interoperability between Rust and C++ is already largely sufficient for convenient use of Rust within Android.
If you are considering how Rust could integrate into your C++ project, we recommend doing a similar analysis of your codebase. When addressing interop gaps, we recommend that you consider upstreaming support to existing compat libraries like cxx.
Our first attempt at quantifying Rust/C++ interop involved analyzing the potential mismatches between the languages. This led to a lot of interesting information, but was difficult to draw actionable conclusions from. Rather than enumerating all the potential places where interop could occur, Stephen Hines suggested that we instead consider how code is currently shared between C/C++ projects as a reasonable proxy for where we’ll also likely want interop for Rust. This provided us with actionable information that was straightforward to prioritize and implement. Looking back, the data from our real-world Rust usage has reinforced that the initial methodology was sound. Thanks Stephen!Also, thanks to:
We used undefined symbols of function type as reported by objdump to perform this analysis. This means that any header-only functions will be absent from our analysis, and internal (non-API) functions which are called by header-only functions may appear in it. ↩
We extracted return values by parsing DWARF symbols, which give the return types of functions. ↩
Even without automated binding generation, manually implementing the bindings is straightforward. ↩
In the case of handwritten C/C++ wrappers, we analyzed the functions they call, not the wrappers themselves. For all uses of our native AIDL library, we analyzed the types used in the C++ version of the library. ↩
For this tutorial, you’ll need access to a GKE Kubernetes cluster and a GCR registry with push credentials. The cluster should already have Tekton Pipelines installed.
$ kubectl apply --filename https://storage.googleapis.com/tekton-releases/chains/latest/release.yaml
$ export PROJECT_ID=<GCP Project ID>
$ gcloud iam service-accounts create tekton-chains
$ gcloud iam service-accounts keys create credentials.json --iam-account=tekton-chains@${PROJECT_ID}.iam.gserviceaccount.com
$ kubectl create secret docker-registry registry-credentials \
--docker-server=gcr.io \
--docker-username=_json_key \
--docker-email=tekton@chains.com \
--docker-password="$(cat credentials.json)" \
-n tekton-chains
$ kubectl patch serviceaccount tekton-chains-controller \
-p "{\"imagePullSecrets\": [{\"name\": \"registry-credentials\"}]}" -n tekton-chains
$ export CREDENTIALS_SECRET=kaniko-credentials
$ kubectl create secret generic $CREDENTIALS_SECRET --from-file credentials.json
$ kubectl apply -f https://raw.githubusercontent.com/tektoncd/chains/main/examples/kaniko/gcp/kaniko.yaml
$ cat <<EOF | kubectl apply -f -
apiVersion: tekton.dev/v1beta1
kind: TaskRun
metadata:
name: kaniko-run
spec:
taskRef:
name: kaniko-gcp
params:
- name: IMAGE
value: gcr.io/${PROJECT_ID}/kaniko-chains
workspaces:
- name: source
emptyDir: {}
- name: credentials
secret:
secretName: ${CREDENTIALS_SECRET}
EOF
In 2020 we launched Enhanced Safe Browsing, which you can turn on in your Chrome security settings, with the goal of substantially increasing safety on the web. These improvements are being built on top of existing security mechanisms that already protect billions of devices. Since the initial launch, we have continuously worked behind the scenes to improve our real-time URL checks and apply machine learning models to warn on previously-unknown attacks. As a result, Enhanced Safe Browsing users are successfully phished 35% less than other users. Starting with Chrome 91, we will roll out new features to help Enhanced Safe Browsing users better choose their extensions, as well as offer additional protections against downloading malicious files on the web.
Chrome extensions - Better protection before installation
Every day millions of people rely on Chrome extensions to help them be more productive, save money, shop or simply improve their browser experience. This is why it is important for us to continuously improve the safety of extensions published in the Chrome Web Store. For instance, through our integration with Google Safe Browsing in 2020, the number of malicious extensions that Chrome disabled to protect users grew by 81%. This comes on top of a number of improvements for more peace of mind when it comes to privacy and security.
Enhanced Safe Browsing will now offer additional protection when you install a new extension from the Chrome Web Store. A dialog will inform you if an extension you’re about to install is not a part of the list of extensions trusted by Enhanced Safe Browsing.
Any extensions built by a developer who follows the Chrome Web Store Developer Program Policies, will be considered trusted by Enhanced Safe Browsing. For new developers, it will take at least a few months of respecting these conditions to become trusted. Eventually, we strive for all developers with compliant extensions to reach this status upon meeting these criteria. Today, this represents nearly 75% of all extensions in the Chrome Web Store and we expect this number to keep growing as new developers become trusted.
Improved download protection
Enhanced Safe Browsing will now offer you even better protection against risky files.
When you download a file, Chrome performs a first level check with Google Safe Browsing using metadata about the downloaded file, such as the digest of the contents and the source of the file, to determine whether it’s potentially suspicious. For any downloads that Safe Browsing deems risky, but not clearly unsafe, Enhanced Safe Browsing users will be presented with a warning and the ability to send the file to be scanned for a more in depth analysis (pictured above).
If you choose to send the file, Chrome will upload it to Google Safe Browsing, which will scan it using its static and dynamic analysis classifiers in real time. After a short wait, if Safe Browsing determines the file is unsafe, Chrome will display a warning. As always, you can bypass the warning and open the file without scanning. Uploaded files are deleted from Safe Browsing a short time after scanning.
Integrating security into your app development lifecycle can save a lot of time, money, and risk. That’s why we’ve launched Security by Design on Google Play Academy to help developers identify, mitigate, and proactively protect against security threats.
The Android ecosystem, including Google Play, has many built-in security features that help protect developers and users. The course Introduction to app security best practices takes these protections one step further by helping you take advantage of additional security features to build into your app. For example, Jetpack Security helps developers properly encrypt their data at rest and provides only safe and well known algorithms for encrypting Files and SharedPreferences. The SafetyNet Attestation API is a solution to help identify potentially dangerous patterns in usage. There are several common design vulnerabilities that are important to look out for, including using shared or improper file storage, using insecure protocols, unprotected components such as Activities, and more. The course also provides methods to test your app in order to help you keep it safe after launch. Finally, you can set up a Vulnerability Disclosure Program (VDP) to engage security researchers to help.
In the next course, you can learn how to integrate security at every stage of the development process by adopting the Security Development Lifecycle (SDL). The SDL is an industry standard process and in this course you’ll learn the fundamentals of setting up a program, getting executive sponsorship and integration into your development lifecycle.
Threat modeling is part of the Security Development Lifecycle, and in this course you will learn to think like an attacker to identify, categorize, and address threats. By doing so early in the design phase of development, you can identify potential threats and start planning for how to mitigate them at a much lower cost and create a more secure product for your users.
Improving your app’s security is a never ending process. Sign up for the Security by Design module where in a few short courses, you will learn how to integrate security into your app development lifecycle, model potential threats, and app security best practices into your app, as well as avoid potential design pitfalls.
The Android team has been working on introducing the Rust programming language into the Android Open Source Project (AOSP) since 2019 as a memory-safe alternative for platform native code development. As with any large project, introducing a new language requires careful consideration. For Android, one important area was assessing how to best fit Rust into Android’s build system. Currently this means the Soong build system (where the Rust support resides), but these design decisions and considerations are equally applicable for Bazel when AOSP migrates to that build system. This post discusses some of the key design considerations and resulting decisions we made in integrating Rust support into Android’s build system.
A RustConf 2019 meeting on Rust usage within large organizations highlighted several challenges, such as the risk that eschewing Cargo in favor of using the Rust Compiler, rustc, directly (see next section) may remove organizations from the wider Rust community. We share this same concern. When changes to imported third-party crates might be beneficial to the wider community, our goal is to upstream those changes. Likewise when crates developed for Android could benefit the wider Rust community, we hope to release them as independent crates. We believe that the success of Rust within Android is dependent on minimizing any divergence between Android and the Rust community at large, and hope that the Rust community will benefit from Android’s involvement.
rustc,
Rust provides Cargo as the default build system and package manager, collecting dependencies and invoking rustc (the Rust compiler) to build the target crate (Rust package). Soong takes this role instead in Android and calls rustc directly for several reasons:
rustc
Cargo.toml
build.rs
Using the Rust compiler directly allows us to avoid these issues and is consistent with how we compile all other code in AOSP. It provides the most control over the build process and eases integration into Android’s existing build system. Unfortunately, avoiding it introduces several challenges and influences many other build system decisions because Cargo usage is so deeply ingrained in the Rust crate ecosystem.
A build.rs script compiles to a Rust binary which Cargo builds and executes during a build to handle pre-build tasks, commonly setting up the build environment, or building libraries in other languages (for example C/C++). This is analogous to configure scripts used for other languages.
Avoiding build.rs scripts somewhat flows naturally from not relying on Cargo since supporting these would require replicating Cargo behavior and assumptions. Beyond this however, there are good reasons for AOSP to avoid build scripts as well:
/usr/lib
Android.bp
For instances in third-party code where a build script is used only to compile C dependencies, we either use existing cc_library Soong definitions (such as boringssl for quiche) or create new definitions for crate-specific code.
cc_library
When the build.rs is used to generate source, we try to replicate the core functionality in a Soong rust_binary module for use as a custom source generator. In other cases where Soong can provide the information without source generation, we may carry a small patch that leverages this information.
Why do we support proc_macros, which are compiler plug-ins that execute code on the host within the compiler context, but not build.rs scripts?
proc_macros
While build.rs code is written as one-off code to handle building a single crate, proc_macros define reusable functionality within the compiler which can become widely relied upon across the Rust community. As a result popular proc_macros are generally better maintained and more scrutinized upstream, which makes the code review process more manageable. They are also more readily sandboxed as part of the build process since they are less likely to have dependencies external to the compiler.
proc_macros are also a language feature rather than a method for building code. These are relied upon by source code, are unavoidable for third-party dependencies, and are useful enough to define and use within our platform code. While we can avoid build.rs by leveraging our build system, the same can’t be said of proc_macros.
There is also precedence for compiler plugin support within the Android build system. For example see Soong’s java_plugin modules.
Unlike C/C++ compilers, rustc only accepts a single source file representing an entry point to a binary or library. It expects that the source tree is structured such that all required source files can be automatically discovered. This means that generated source either needs to be placed in the source tree or provided through an include directive in source:
include!("/path/to/hello.rs");
The Rust community depends on build.rs scripts alongside assumptions about the Cargo build environment to get around this limitation. When building, the cargo command sets an OUT_DIR environment variable which build.rs scripts are expected to place generated source code in. This source can then be included via:
cargo
include!(concat!(env!("OUT_DIR"), "/hello.rs"));
This presents a challenge for Soong as outputs for each module are placed in their own out/ directory2; there is no single OUT_DIR where dependencies output their generated source.
out/
OUT_DIR
For platform code, we prefer to package generated source into a crate that can be imported. There are a few reasons to favor this approach:
As a result, all of Android’s Rust source generation module types produce code that can be compiled and used as a crate.
We still support third-party crates without modification by copying all the generated source dependencies for a module into a single per-module directory similar to Cargo. Soong then sets the OUT_DIR environment variable to that directory when compiling the module so the generated source can be found. However we discourage use of this mechanism in platform code unless absolutely necessary for the reasons described above.
By default, the Rust ecosystem assumes that crates will be statically linked into binaries. The usual benefits of dynamic libraries are upgrades (whether for security or functionality) and decreased memory usage. Rust’s lack of a stable binary interface and usage of cross-crate information flow prevents upgrading libraries without upgrading all dependent code. Even when the same crate is used by two different programs on the system, it is unlikely to be provided by the same shared object4 due to the precision with which Rust identifies its crates. This makes Rust binaries more portable but also results in larger disk and memory footprints.
This is problematic for Android devices where resources like memory and disk usage must be carefully managed because statically linking all crates into Rust binaries would result in excessive code duplication (especially in the standard library). However, our situation is also different from the standard host environment: we build Android using global decisions about dependencies. This means that nearly every crate is shareable between all users of that crate. Thus, we opt to link crates dynamically by default for device targets. This reduces the overall memory footprint of Rust in Android by allowing crates to be reused across multiple binaries which depend on them.
Since this is unusual in the Rust community, not all third-party crates support dynamic compilation. Sometimes we must carry small patches while we work with upstream maintainers to add support.
We support building all output types supported by rustc (rlibs, dylibs, proc_macros, cdylibs, staticlibs, and executables). Rust modules can automatically request the appropriate crate linkage for a given dependency (rlib vs dylib). C and C++ modules can depend on Rust cdylib or staticlib producing modules the same way as they would for a C or C++ library.
rlib
dylib
proc_macro
cdylib
staticlib
In addition to being able to build Rust code, Android’s build system also provides support for protobuf and gRPC and AIDL generated crates. First-class bindgen support makes interfacing with existing C code simple and we have support modules using cxx for tighter integration with C++ code.
The Rust community produces great tooling for developers, such as the language server rust-analyzer. We have integrated support for rust-analyzer into the build system so that any IDE which supports it can provide code completion and goto definitions for Android modules.
Source-based code coverage builds are supported to provide platform developers high level signals on how well their code is covered by tests. Benchmarks are supported as their own module type, leveraging the criterion crate to provide performance metrics. In order to maintain a consistent style and level of code quality, a default set of clippy lints and rustc lints are enabled by default. Additionally, HWASAN/ASAN fuzzers are supported, with the HWASAN rustc support added to upstream.
clippy
In the near future, we plan to add documentation to source.android.com on how to define and use Rust modules in Soong. We expect Android’s support for Rust to continue evolving alongside the Rust ecosystem and hope to continue to participate in discussions around how Rust can be integrated into existing build systems.
Thank you to Matthew Maurer, Jeff Vander Stoep, Joel Galenson, Manish Goregaokar, and Tyler Mandry for their contributions to this post.
This can be mitigated to some extent with workspaces, but requires a very specific directory arrangement that AOSP does not conform to. ↩
This presents no problem for C/C++ and similar languages as the path to the generated source is provided directly to the compiler. ↩
Since include! works by textual inclusion, it may reference values from the enclosing namespace, modify the namespace, or use constructs like #![foo]. These implicit interactions can be difficult to maintain. Macros should be preferred if interaction with the rest of the crate is truly required. ↩
While libstd would usually be shareable for the same compiler revision, most other libraries would end up with several copies for Cargo-built Rust binaries, since each build would attempt to use a minimum feature set and may select different dependency versions for the library in question. Since information propagates across crate boundaries, you cannot simply produce a “most general” instance of that library. ↩
Chrome 90 for Windows adopts Hardware-enforced Stack Protection, a mitigation technology to make the exploitation of security bugs more difficult for attackers. This is supported by Windows 20H1 (December Update) or later, running on processors with Control-flow Enforcement Technology (CET) such as Intel 11th Gen or AMD Zen 3 CPUs. With this mitigation the processor maintains a new, protected, stack of valid return addresses (a shadow stack). This improves security by making exploits more difficult to write. However, it may affect stability if software that loads itself into Chrome is not compatible with the mitigation. Below we describe some exploitation techniques that are mitigated by stack protection, discuss its limitations and what we will do next to approach them. Finally, we provide some quick tips for other software authors as they enable /cetcompat for their Windows applications.
Imagine a simple use-after-free (UAF) bug where an attacker can induce a program to call a pointer of their choosing. Here the attacker controls an object which occupies space formerly used by another object, which the program erroneously continues to use. The attacker sets a field in this region that is used as a function call to the address of code the attacker would like to execute. Years ago an attacker could simply write their shellcode to a known location, then, in their overwrite, set the instruction pointer to this shellcode. In time, Data Execution Prevention was added to prevent stacks or heaps from being executable.
In response, attackers invented Return Oriented Programming (ROP). Here, attackers take advantage of the process’s own code, as that must be executable. With control of the stack (either to write values there, or by changing the stack pointer) and control of the instruction pointer, an attacker can use the `ret` instruction to jump to a different, useful, piece of code.
During an exploit attempt, the instruction pointer is changed so that instead of its normal destination, a small fragment of code, called an ROP gadget, is invoked instead. These gadgets are selected so that they do something useful (such as prepare a register for a function call) then call return.
These tiny fragments need not be a complete function in the normal program, and could even be found part-way through a legitimate instruction. By lining up the right set of “return” addresses, a chain of these gadgets can be called, with each gadget’s `ret` switching to the next gadget. With some patience, or the right tooling, an attacker can piece together the arguments to a function call, then really call the function.
Chrome has a multi-process architecture -- a main browser process acts as the logged-in user, and spawns restricted renderer and utility processes to host website code. This isolation reduces the severity of a bug in a renderer as its process cannot do much by itself. Attackers will then attempt to use another sandbox escape bug to run code in the browser, which lets them act as the logged-in user. As libraries are mapped at the same address in different processes by Windows, any bug that allows an attacker to read memory is enough for them to examine Chrome’s binary and any loaded libraries for ROP gadgets. This makes preventing ROP chains in the browser process especially useful as a mitigation.
Enter stack-protection. Along with the existing stack, the cpu maintains a shadow stack. This stack cannot be directly manipulated by normal program code and only stores return addresses. The CALL instruction is modified to push a return address (the instruction after the CALL) to both the normal stack, and the shadow stack. The RET (return) instruction still takes its return address from the normal stack, but now verifies that it is the same as the one stored in the shadow stack region. If it is, then the program is left alone and it continues to work as it always did. If the addresses do not match then an exception is raised which is intercepted by the operating system (not by Chrome). The operating system has an opportunity to modify the shadow region and allow the program to continue, but in most cases an address mismatch is the result of a program error so the program is immediately terminated.
In our example above, the attacker will be able to make their initial jump into a ROP gadget, but on trying to return to their next gadget they will be stopped.
Some software may be incompatible with this mechanism, especially some older security software that injects into a process and hooks operating system functions by overwriting the prelude with `rax = &hook; push rax; ret`.
Chrome does not yet support every direction of control flow enforcement. Stack protection enforces the reverse-edge of the call graph but does not constrain the forward-edge. It will still be possible to make indirect jumps around existing code as stack protection is only validated when a return instruction is encountered, and call targets are not validated. On Windows a technology called Control Flow Guard (CFG) can be used to verify the target of an indirect function call before it is attempted. This prevents calling into the middle of a function, significantly reducing the scope of useful instructions for attackers to use. Another approach is provided by Intel’s CET which includes an ENDBRANCH instruction to prevent jumps into arbitrary code locations. Memory tagging tools such as MTE can be used to make it more difficult to modify pointers to valid code sequences (and makes UAFs more difficult in general). We are working to introduce CFG to Chrome for Windows, and will add other techniques over time.
By itself, stack protection can be bypassed in some contexts. For instance, stack protection does not prevent an attacker tricking a program into calling an existing function by entirely replacing an object containing a function pointer. This approach does not involve ROP as the function call happens instead of the expected call, and returns to the address it was originally called from, so must be allowed. However, the called function must be useful to an attacker, and most functions will not be. An example of an attack using this method is to craft a call to add the `--no-sandbox` argument to Chrome’s command line. This results in future renderers being launched without normal protections. Over time we will identify and remove such useful tools.
In the renderer, for performance reasons, our javascript and wasm engines may use memory that is both writable and executable at the same time. This allows an attacker to modify code that v8 is already going to execute, saving them the trouble of constructing a ROP chain. This explains why it was not our first priority to make v8 CET compatible, and why stack-protection is not yet enabled in the renderer.
Finally, stack protection doesn’t stop the bugs in the first place. Everything we have discussed above is a mitigation that makes it more difficult to execute arbitrary code. If a programming error allows arbitrary writes then it is very unlikely that we can prevent this being used to run arbitrary code. Attackers will adapt and find new ways to turn memory safety errors into code execution.
You can see if Hardware-enforced Stack Protection is enabled for a process using the Windows Task Manager. Open task manager, open the Details Tab, Right Click on a heading, Select Columns & Check the Hardware-enforced Stack Protection box. The process display will then indicate if a process is opted-in to this mitigation. ‘Compatible Modules Only’ indicates that any dll marked as /cetcompat at build time will raise an exception if a return address is invalid.
You can see which Chrome processes are opted-out of CET by consulting the Mitigations field of chrome://sandbox and clicking ‘+’. All processes are included unless the mitigation CET_USER_SHADOW_STACKS_ALWAYS_OFF is present in the expanded details view.
If you are developing software, or debugging a problem in Chrome the shadow stack can be helpful as it includes only return addresses, and these cannot be corrupted by rogue writes elsewhere in the process. To see these registers use the `r` command in windbg with the mask option:
0:159> rM 8002
rax=00000000c000060a rbx=000000fa5bbfeff0 rcx=0000000000000030
rdx=0000000000000000 rsi=00007ffba4118924 rdi=000000fa5bbff1a0
rip=00007ffc1847b4a1 rsp=000000fa5bbfc0a0 rbp=000000fa5bbfc0a0
r8=000000fa5bbfc098 r9=0000000000000000 r10=0000000000000000
r11=0000000000000246 r12=000000fa5bbfe230 r13=000002c3450b5830
r14=000002c3450b7850 r15=000000fa5bbfc260
iopl=0 nv up ei pl zr na po nc
ssp=000000fa5c3fef10 cetumsr=0000000000000001
`ssp` points to the shadow stack region, `cetumsr` indicates if cet is enabled for the process.
You can then see the call stack within the shadow region using `dps @ssp`. Values are not overwritten so you can also see where you came from by looking a bit deeper: `dps @ssp-20`.
If a process is not compatible with Hardware-enforced Stack Protection, the system event log (Application Log) will include brief error reports (Id:1001). You can filter those related to cetcompat using the following powershell snippet:-
Get-WinEvent -MaxEvents 128 -FilterHashtable @{ LogName='Application'; Id='1001' } `
| Where-Object {$_.Message -match 'chrome.exe'} `
| Select-Object -First 8 `
| fl
These will include the following parameters:-
P1: application.exe
P2: application version
P3: application build ts
P4: faulting module .dll
P5: faulting module version
P6: faulting module build ts
P7: faulting offset in P4 from base_address
P8: exception code (c0000409)
P9: subcode (00...000030)
If Chrome is misbehaving and you think it might be because of cetcompat, it is possible to disable it using Image File Execution Options - we do not recommend this except for a limited period of testing. If you find you have to do this, please raise an issue on https://crbug.com so that we can investigate the failure.
/cetcompat is enabled for most processes for Chrome M90 on Windows. Enabling Hardware-enforced Stack Protection will layer with existing and future measures to make exploitation more difficult and so more expensive for an attacker, ultimately protecting the people who use Chrome every day.