Safety strategy

Background

Carbon's goal is to provide practical safety and testing mechanisms.

What "safety" means in Carbon

Safety is protection from software bugs, whether the protection is required by the language or merely an implementation option. Application-specific logic errors can be prevented by testing, but can lead to security vulnerabilities in production. Safety categories will be referred to using names based on the type of security vulnerability they protect against.

A key subset of safety categories Carbon should address are:

• Memory safety protects against invalid memory accesses. Carbon uses two main subcategories for memory safety:

  • Spatial memory safety protects against accessing an address that's out of bounds for the source. This includes array boundaries, as well as dereferencing invalid pointers such as uninitialized pointers, NULL in C++, or manufactured pointer addresses.

  • Temporal memory safety protects against accessing an address that has been deallocated. This includes use-after-free for heap and use-after-return for stack addresses.

• Type safety protects against accessing valid memory with an incorrect type, also known as "type confusion".

• Data race safety protects against racing memory access: when a thread accesses (read or write) a memory location concurrently with a different writing thread and without synchronizing.

Safety guarantees versus hardening

In providing safety, the underlying goal is to prevent attacks from turning a logic error into a security vulnerability. The three ways of doing this can be thought of in terms of how they prevent attacks:

• Safety guarantees prevent bugs. They offer a strong requirement that a particular security vulnerability cannot exist. Compile-time safety checks are always a safety guarantee, but safety guarantees may also be done at runtime. For example:

  • At compile-time, range-based for loops offer a spatial safety guarantee that out-of-bounds issues cannot exist in the absence of concurrent modification of the sequence.

  • At runtime, garbage collected languages offer a temporal safety guarantee because objects cannot be freed while they're still accessible.

• Error detection checks for common logic errors at runtime. For example:

  • An array lookup function might offer spatial memory error detection by verifying that the passed index is in-bounds.

  • A program can implement reference counting to detect a temporal memory error by checking whether any references remain when memory is freed.

• Safety hardening mitigates bugs, typically by minimizing the feasibility of an attack. For example:

  • Control Flow Integrity (CFI) monitors for behavior which can subvert the program's control flow. In Clang, it is optimized for use in release builds. Typically CFI analysis will only detect a subset of attacks because it can't track each possible code path separately. It should still reduce the feasibility of both spatial memory, temporal memory, and type attacks.

  • Memory tagging makes each attempt at an invalid read or write operation have a high probability of trapping, while still not detecting the underlying bug in every case. Realistic attacks require many such operations, so memory tagging may stop attacks in some environments. Alternatively, the trap might be asynchronous, leaving only a tiny window of time prior to the attack being detected and program terminated. These are probabilistic hardening and reduces the feasibility of both spatial and temporal memory attacks.

Under both error detection and safety hardening, even if a safety is protected, the underlying bugs will still exist and will need to be fixed. For example, program termination could be used for a denial-of-service attack.

Philosophy

Carbon's practical safety and testing mechanisms will emphasize guaranteed safety where feasible without creating barriers to Carbon's other goals, particularly performance and interoperability. This limits Carbon's options for guaranteed safety, and as a result there will be more reliance upon error detection and safety hardening. The language's design should incentivize safe programming, although it will not be required.

When writing code, Carbon developers should expect to receive safety without needing to add safety annotations. Carbon will have optional safety annotations for purposes such as optimizing safety checks or providing information that improves coverage of safety checks.

Carbon will favor compile-time safety checks because catching issues early will make applications more reliable. Runtime checks, either error detection or safety hardening, will be enabled where safety cannot be proven at compile-time.

There will be three high-level use cases or directions that Carbon addresses through different build modes that prioritize safety checks differently:

• A debug oriented build mode that prioritizes detecting bugs and reporting errors helpfully.
• A performance oriented build mode that skips any dynamic safety checks to reduce overhead.
• A hardened oriented build mode that prioritizes ensuring sufficient safety to prevent security vulnerabilities, although it may not allow detecting all of the bugs.

These high level build modes may be tuned, either to select specific nuanced approach for achieving the high level goal, or to configure orthogonal constraints such as whether to prioritize binary size or execution speed. However, there is a strong desire to avoid requiring more fundamental build modes to achieve the necessary coverage of detecting bugs and shipping software. These build modes are also not expected to be interchangeable or compatible with each other within a single executable -- they must be a global selection.

Although expensive safety checks could be provided through additional build modes, Carbon will favor safety checks that can be combined into these three build modes rather than adding more.

Over time, safety should evolve using a hybrid compile-time and runtime safety approach to eventually provide a similar level of safety to a language that puts more emphasis on guaranteed safety, such as Rust. However, while Carbon may encourage developers to modify code in support of more efficient safety checks, it will remain important to improve the safety of code for developers who cannot invest into safety-specific code modifications.

Principles

• Safety must be easy to ramp-up with, even if it means new developers don't receive the full safety that Carbon can offer.

  • Developers should benefit from Carbon's safety without needing to learn and apply Carbon-specific design patterns. Some safety should be enabled by default, without safety-specific work, although some safety will require work to opt in. Developers concerned with performance should only need to disable safety in rare edge-cases.

  • Where there is a choice between safety approaches, the safe option should be incentivized by making it equally easy or easier to use. If there is a default, it should be the safe option. It should be identifiable when the unsafe option is used. Incentives will prioritize, in order:

    1. Guaranteed safety.
    2. Error detection.
    3. Safety hardening.
    4. Unsafe and unmitigated code.

  • Language design choices should favor more efficient implementations of safety checks. They should also allow favor automation of testing and fuzzing.

• Safety in Carbon must work with interoperable or migrated C++ code, so that C++ developers can readily take advantage of Carbon's improvements.

  • Safety mechanisms will ideally be designed to apply to automatically migrated C++ code. Providing immediate safety improvements to Carbon adopters will help motivate adoption.

  • In the other direction, safety mechanisms must not force manual rewriting of C++ code in order to migrate, either by creating design incompatibilities or performance degradations. Automated migration of C++ code to Carbon must work for most developers, even if it forces Carbon's safety design to take a different approach.

  • Carbon's safety should degrade gracefully when Carbon code calls C++ code, although this may require use of the Carbon toolchain to compile the C++ code. Applications should be expected to use interoperability. Although some safety features will be Carbon-specific, safety should not stop at the language boundary.

• The rules for determining whether code will pass compile-time safety checking should be articulable, documented, and easy to understand.

  • Compile-time safety checks should not change significantly across different build modes. The purpose of the build modes is to determine code generation.

• Each build mode will prioritize performance and safety differently:

  • The debug build mode will produce development-focused binaries that prioritize fast iteration on code with safety checks that assist in identification and debugging of errors.

  • The performance build mode will produce release-focused binaries that prioritize performance over safety.

  • The hardened build mode will produce release-focused binaries that prioritize safety that is resistant to attacks at the cost of performance.

• Safety checks should try to be identical across build modes.

  • There will be differences, typically due to performance overhead and detection rate trade-offs of safety check algorithms.

• The number of build modes will be limited, and should be expected to remain at the named three.

  • Most developers will use two build modes in their work: debug for development, and either performance or hardened for releases.

  • It's important to focus on checks that are cheap enough to run as part of normal development. Users are not expected to want to run additional development build modes for additional sanitizers.

  • Limiting the number of build modes simplifies support for both Carbon maintainers, who can focus on a more limited set of configurations, and Carbon developers, who can easily choose which is better for their use-case.

• Each distinct safety-related build mode (debug, performance, and hardened) cannot be combined with others in the same binary.

  • Cross-binary interfaces will exist in Carbon, and will need to be used by developers interested in combining libraries built under different build modes.

• Although runtime safety checks should prevent logic errors from turning into security vulnerabilities, the underlying logic errors will still be bugs. For example, some safety checks would result in application termination; this prevents execution of unexpected code and still needs to be fixed.

• Developers need a strong testing methodology to engineer correct software. Carbon will encourage testing and then leverage it with the checking build modes to find and fix bugs and vulnerabilities.

Details

Incremental work when safety requires work

Carbon is prioritizing usability of the language, particularly minimizing retraining of C++ developers and easing migration of C++ codebases, over the kind of provable safety that some other languages pursue, particularly Rust.

A key motivation of Carbon is to move C++ developers to a better, safer language. However, if Carbon requires manually rewriting or redesigning C++ code in order to maintain performance, it creates additional pressure on C++ developers to learn and spend time on safety. Safety will often not be the top priority for developers; as a result, Carbon must be thoughtful about how and when it forces developers to think about safety.

Relying on multiple build modes to provide safety should fit into normal development workflows. Carbon can also have features to enable additional safety, so long as developers can start using Carbon in their applications without learning new paradigms.

Where possible, safety checks shouldn't require work on the part of Carbon developers. A safety check that requires no code edits or can be handled by automated migration may be opt-out, as there is negligible cost to developers. One which requires local code changes should be opt-in because costs will scale with codebase size. Safety check approaches which would require substantial redesign by developers will be disfavored based on adoption cost, even if the alternative is a less-comprehensive approach.

Using build modes to manage safety checks

Carbon will likely start in a state where most safety checks are done at runtime. However, runtime detection of safety violations remains expensive. In order to make as many safety checks as possible available to developers, Carbon will adopt a strategy based on three build modes that target key use-cases.

Debug

The debug build mode targets developers who are iterating on code and running tests. It will emphasize detection and debuggability, especially for safety issues.

It needs to perform well enough to be run frequently by developers, but will make performance sacrifices to catch more safety issues. This mode should have runtime checks for the most common safety issues, but it can make trade-offs that improve performance in exchange for less frequent, but still reliable, detection. Developers should do most of their testing in this build mode.

The debug build mode will place a premium on the debuggability of safety violations. Where safety checks rely on hardening instead of guaranteed safety, violations should be detected with a high probability per single occurrence of the bug. Detected bugs will be accompanied by a detailed diagnostic report to ease classification and root cause identification.

Performance

The performance build mode targets the typical application that wants high performance from Carbon code, where performance considers processing time, memory, and disk space. Trade-offs will be made that maximize the performance.

Only safety techniques that don't measurably impact application hot path performance will be enabled by default. This is a very high bar, but is crucial for meeting Carbon's performance goals, as well as allowing migration of existing C++ systems which may not have been designed with Carbon's safety semantics in mind.

Hardened

The hardened build mode targets applications where developers want strong safety against attacks in exchange for worse performance. It will work to prevent attacks in ways that attackers cannot work around, even if it means using techniques that create significant performance costs.

Managing bugs without compile-time safety

Carbon's reliance on runtime checks will allow developers to manage their security risk. Developers will still need to reliably find and fix the inevitable bugs, including both safety violations and regular business logic bugs. The cornerstone of managing bugs will be strong testing methodologies, with built-in support from Carbon.

Strong testing is more than good test coverage. It means a combination of:

• Ensuring unsafe or risky operations and interfaces can easily be recognized by developers.

• Using static analysis tools to detect common bugs, and ensuring they're integrated into build and code review workflows. These could be viewed as static testing of code.

• Writing good test coverage, including unit, integration, and system tests.

• Implementing coverage-directed fuzz testing to discover bugs outside of manually authored test coverage, especially for interfaces handling untrusted data. Fuzz testing is a robust way to catch bugs when APIs may be used in ways developers don't consider.

• Running continuous integration, including automatic and continuous running of these tests. The checked development build mode should be validated, as well as any additional build modes necessary to cover different forms of behavior checking.

• Easing automated testing and fuzzing through language features. For example, if the language encourages value types and pure functions of some sort, they can be automatically fuzzed.

These practices are necessary for reliable, large-scale software engineering. Maintaining correctness of business logic over time requires continuous and thorough testing. Without it, such software systems cannot be changed and evolved over time reliably. Carbon will re-use these practices in conjunction with checking build modes to mitigate the limitations of Carbon's safety guarantees without imposing overhead on production systems.

When a developer chooses to use Carbon, adhering to this kind of testing methodology is essential for maintaining safety. As a consequence, Carbon's ecosystem, including the language, tools, and libraries, will need to directly work to remove barriers and encourage the development of these methodologies.

The reliance on testing may make Carbon a poor choice in some environments; in environments where such testing rigor is infeasible, a language with a greater degree of static checking may be better suited.

Caveats

Probabilistic techniques likely cannot stop attacks

It's expected that probabilistic techniques that can be applied at the language level are attackable through a variety of techniques:

• The attacker might be able to attack repeatedly until it gets through.
• The attacker may be able to determine when the attack would be detected and only run the attack when it would not be.
• The attacker might be able control the test condition to make detection much less likely or avoid detection completely. For example, if detection is based on the last 4 bits of a memory address, an attacker may be able to generate memory allocations, viewing the address and only attacking when there's a collision.

Hardware vulnerabilities may make these attacks easier than they might otherwise appear. Future hardware vulnerabilities are difficult to predict.

Note this statement focuses on what can be applied to the language level. Using a secure hash algorithm, such as SHA256, may be used to offer probabilistic defense in other situations. However, the overhead of a secure hash algorithm's calculation is significant in the context of most things that Carbon may do at the language level.

Combining these issues, although it may seem like a probabilistic safety check could be proven to reliably detect attackers, it's likely infeasible to do so. For the various build modes, this means:

• The debug build mode will not typically be accessible to attackers, so where a probabilistic technique provides a better developer experience, it will be preferred.
• The performance build mode will often avoid safety checks in order to reach peak performance. As a consequence, even the weak protection of a probabilistic safety check may be used in order to provide some protection.
• The hardened build mode will prefer non-probabilistic techniques that cannot be attacked.

Alternatives considered

Guaranteed memory safety programming models

Multiple approaches that would offer guaranteed memory safety have been considered, mainly based on other languages which offer related approaches. Carbon will likely rely more on error detection and hardening because of what the models would mean for Carbon's performance and C++ migration language goals.

Guaranteed compile-time memory safety using borrow checking

Rust offers a good example of an approach for compile-time safety based on borrow checking, which provides guaranteed safety. For code which can't implement borrow checking, runtime safety using reference counting is available and provides reliable error detection. This approach still allows for unsafe blocks, as well as types that offer runtime safety while wrapping unsafe interfaces.

Carbon could use a similar approach for guaranteed safety by default.

Advantages:

• Guaranteed safety, including against data races, is provided for the binaries.
  • The emphasis on compile-time safety limits the scope of the runtime memory safety costs.
  • With Rust, there is early evidence that there's a significant impact in reducing bugs generally.
• Imitating Rust's techniques would allow building on the huge work of the Rust community, reducing the risks of implementing similar in Carbon.
• Careful use of narrow unsafe escape hatches can be effectively encapsulated behind otherwise safe APIs.

Disadvantages:

• Rust's approach to compile-time safety requires use of design patterns and idioms that are substantially different from C++.
  • Conversion of C++ code to Rust results in either rewrites of code, or use of runtime safety checks that impair performance.
  • Requires fully modeling lifetime and exclusivity in the type system.
  • Data structures must be redesigned to avoid sharing mutable state.
  • Increases complexity of node and pointer based data structures, such as linked lists.
• Imitating Rust's techniques may prove insufficient for achieving Carbon's compiler performance goals. Rust compilation performance suggests its borrow checking performance is slow, although it's difficult to determine how significant this is or whether it could be improved.
  • The Rust compiler is slow, although much has been done to improve it.
  • Details of type checking, particularly requiring parsing of function bodies to type check signatures, as well as wide use of monomorphization are likely significant contributors to Rust compilation performance.
  • LLVM codegen is also a significant cost for Rust compilation performance.
  • With Fuchsia as an example, in December 2020, borrow checking and type checking combined account for around 10% of Rust compile CPU time, or 25% of end-to-end compile time. The current cost of borrow checking is obscured both because of the combination with type checking, and because Fuchsia disables some compiler parallelization due to build system incompatibility.
• The complexity of using Rust's compile-time safety may incentivize unnecessary runtime checking of safety properties. For example, using RefCell or Rc to avoid changing designs to fit compile-time safety models.
• Some of the most essential safety tools that ease the ergonomic burden of the Rust-style lifetime model (Rc) introduce semantic differences that cannot then be eliminated in a context where performance is the dominant priority.

It's possible to modify the Rust model several ways in order to reduce the burden on C++ developers:

• Don't offer safety guarantees for data races, eliminating RefCell.
  • This would likely not avoid the need for Rc or Arc, and wouldn't substantially reduce the complexity.
• Require manual destruction of Rc, allowing safety checks to be disabled in the performance build mode to eliminate overhead.
  • This still requires redesigning C++ code to take advantage of Rc.
  • The possibility of incorrect manual destruction means that the safety issue is being turned into a bug, which means it is hardening and no longer a safety guarantee.
  • Carbon can provide equivalent hardening through techniques such as MarkUs, which does not require redesigning C++ code.

Overall, Carbon is making a compromise around safety in order to give a path for C++ to evolve. C++ developers must be comfortable migrating their codebases, and able to do so in a largely automated manner. In order to achieve automated migration, Carbon cannot require fundamental redesigns of migrated C++ code. While a migration tool could in theory mark all migrated code as unsafe, Carbon should use a safety strategy that degrades gracefully and offers improvements for C++ code, whether migrated or not.

That does not mean Carbon will never adopt guaranteed safety by default, only that performance and migration of C++ code takes priority, and any design will need to be considered in the context of other goals. It should still be possible to adopt guaranteed safety later, although it will require identifying a migration path.

Guaranteed run-time memory safety using reference counting

Reference counting is a common memory safety model, with Swift as a popular example.

Advantages:

• Simple model for safety, particularly as compared with Rust.
• Safe for all of the most common and important classes of memory safety bugs.

Disadvantages:

• Safety based on reference counting introduces significant performance costs, and tools for controlling these costs are difficult.
  • Safety based on garbage collection has less direct performance overhead, but has a greater unpredictability of performance.
• Significant design differences versus C++ still result, as the distinction between value types and "class types" becomes extremely important.
  • Class types are held by a reference counted pointer and are thus lifetime safe.

In order to mitigate the performance overhead, Swift does have a proposal to add an option for unique ownership, although the specifics are not designed yet. The unique ownership approach is expected to require unowned and unsafe access, so it would not considered to improve the safety trade-offs.

Swift was designated by Apple as the replacement for Objective-C. The safety versus performance trade-offs that it makes fit Apple's priorities. Carbon's performance goals should lead to different trade-off decisions with a higher priority on peak performance, which effectively rules out broad use of reference counting.

Guaranteed run-time memory safety using garbage collection

Garbage collection is a common memory safety model, with Java as a popular example.

Advantages:

• This approach is among the most robust and well studied models, with decades of practical usage and analysis for security properties.
• Extremely suitable for efficient implementation on top of a virtual machine, such as the JVM.

Disadvantages:

• Extremely high complexity to fully understand the implications of complex cases like data races.
• Performance overhead is significant in terms of what Carbon would like to consider.
  • Garbage collection remains a difficult performance problem, even for the JVM and its extensive optimizations.
  • The complexity of the implementation makes it difficult to predict performance; for example, Java applications experience latency spikes when garbage collection runs.

Java is a good choice for many applications, but Carbon is working to focus on a set of performance priorities that would be difficult to achieve with a garbage collector.

Build mode names

The build mode concepts are difficult to name. Other names that were evaluated, and are ultimately similar, are:

• "Debug" is a common term for the intended use of this build mode. Also, tooling including Visual Studio frequently uses the debug term for describing similar.

  • "Development" was also considered, but this term is less specific and would be better for describing all non-release builds together. For example, a "fast build" mode might be added that disables safety checks to improve iteration time, like might be controlled by way of C++'s NDEBUG option.

• "Performance" aligns with the phrasing of the language performance goal.

  • "Optimized" implies that other modes would not be fully optimized, but hardened should be optimized.

  • "Fast" would suggest that speed is the only aspect of performance being optimizing for, but "performance" also optimizes for memory usage and binary size.

• "Hardened" is the choice for succinctly describing the additional safety measures that will be taken, and is a well-known term in the safety space. It could be incorrectly inferred that "performance" has no hardening, but the preference is to clearly indicate the priority of the "hardened" build mode.

  • "Safe" implies something closer to guaranteed safety. However, safety bugs should be expected to result in program termination, which can still be used in other attacks, such as Denial-of-Service.

  • "Mitigated" is an overloaded term, and it may not be succinctly clear that it's about security mitigations.

• Some terms which were considered and don't fit well into the above groups are:

  • "Release" is avoided because both "performance" and "hardened" could be considered to be "release" build modes.

The names "performance" and "hardened" may lead to misinterpretations, with some developers who should use "hardened" using "performance" because they are worried about giving up too much performance, and the other way around. The terms try to balance the utility of well-known terminology with the succinctness of a short phrase for build modes, and that limits the expressivity. Some confusion is expected, and documentation as well as real-world experience (for example, a developer who cares about latency benchmarking both builds) should be expected to help mitigate mix-ups.

Performance versus safety in the hardened build mode

The performance cost of safety techniques are expected to be non-linear with respect to detection rates. For example, a particular vulnerability such as heap use-after-free may be detectable with 99% accuracy at 20% performance cost, but 100% accuracy at 50% performance cost. At present, build modes should be expected to evaluate such a scenario as:

• The debug build mode would choose the 99% accurate approach.
  • Detecting safety issues is valuable for debugging.
  • The probabilistic detection rate won't meaningfully affect accuracy of tests.
  • The lower performance cost improves developer velocity.
• The performance build mode would decline detection.
  • Safety checks with a measurable performance cost should be declined.
• The hardened build mode would choose the 100% accurate approach.
  • Safety must be non-probabilistic in order to reliably prevent attacks.
  • Significant performance hits are acceptable.
  • This means the hardened build mode may be slower than the debug build mode.

In order to achieve better performance, the hardened build mode could make trade-offs closer to the debug build mode. Rather than relying on non-probabilistic techniques, it could instead offer a probability-based chance of detecting a given attack.

Advantages:

• Probabilistic safety should come at lower performance cost (including CPU, memory, and disk space).
  • This will sometimes be significant, and as a result of multiple checks, could be the difference between the hardened build mode being 50% slower than the performance build mode and being 200% slower.

Disadvantages:

• Probabilistic techniques likely cannot stop attacks.
  • Attackers may be able to repeat attacks until they succeed.
  • The variables upon which the probability is based, such as memory addresses, may be manipulable by the attacker. As a consequence, a determined attacker may be able to manipulate probabilities and not even be detected.

Although performance is Carbon's top goal, the hardened build mode exists to satisfy developers and environments that value safety more than performance. The hardened build mode will rely on non-probabilistic safety at significant performance cost because other approaches will be insufficient to guard against determined attackers.

Add more build modes

More build modes could be added to this principle, or the principle could encourage the idea that specific designs may add more.

To explain why three build modes:

• The concept of debug and release (sometimes called opt) are common. For example, in Visual Studio. In Carbon, this could be considered to translate to the "debug" and "performance" build modes by default.

• The hardened build mode is added in order to emphasize security. Although hardened could be implemented as a set of options passed to the standard release build mode, the preference is to focus on it as an important feature.

An example of why another build mode may be needed is ThreadSanitizer, which is noted as having 5-15x slowdown and 5-10x memory overhead. This is infeasible for normal use, but could be useful for some users in a separate build mode. A trade-off that's possible for Carbon is instead using an approach similar to KCSAN which offers relatively inexpensive but lower-probability race detection.

Although options to these build modes may be supported to customize deployments, the preference is to focus on a small set and make them behave well. For example, if a separate build mode is added for ThreadSanitizer, it should be considered a temporary solution until it can be merged into the debug build mode.

Advantages:

• Grants more flexibility for using build modes as a solution to problems.
  • With safety checks, this would allow providing safety checks that are high overhead but also high detection rate as separate build modes.
  • With other systems, there could be non-safety performance versus behavior trade-offs.

Disadvantages:

• Having standard modes simplifies validation of interactions between various safety checks.
  • Safety is the only reason that's been considered for adding build modes.
• As more build modes are added, the chance of developers being confused and choosing the wrong build mode for their application increases.

Any long-term additions to the set of build modes will need to update this principle, raising the visibility and requiring more consideration of such an addition. If build modes are added for non-safety-related reasons, this may lead to moving build modes out of the safety strategy.

Experiment: This can be considered an experiment. Carbon may eventually add more than the initial three build modes, although the reticence to add more is likely to remain.