Security

One of WebAssembly (and Wasmtime's) main goals is to execute untrusted code in a safe manner inside of a sandbox. WebAssembly is inherently sandboxed by design (must import all functionality, etc). This document is intended to cover the various sandboxing implementation strategies that Wasmtime has as they are developed. This has also been documented in a historical blog post too.

At this time Wasmtime implements what's necessary for the WebAssembly specification, for example memory isolation between instances. Additionally the safe Rust API is intended to mitigate accidental bugs in hosts.

Different sandboxing implementation techniques will also come with different tradeoffs in terms of performance and feature limitations, and Wasmtime plans to offer users choices of which tradeoffs they want to make.

WebAssembly Core

The core WebAssembly spec has several features which create a unique sandboxed environment:

• The callstack is inaccessible. Unlike most native execution environments, return addresses from calls and spilled registers are not stored in memory accessible to applications. They are stored in memory that only the implementation has access to, which makes traditional stack-smashing attacks targeting return addresses impossible.

• Pointers, in source languages which have them, are compiled to offsets into linear memory, so implementations details such as virtual addresses are hidden from applications. And all accesses within linear memory are checked to ensure they stay in bounds.

• All control transfers—direct and indirect branches, as well as direct and indirect calls—are to known and type-checked destinations, so it's not possible to accidentally call into the middle of a function or branch outside of a function.

• All interaction with the outside world is done through imports and exports. There is no raw access to system calls or other forms of I/O; the only thing a WebAssembly instance can do is what is available through interfaces it has been explicitly linked with.

• There is no undefined behavior. Even where the WebAssembly spec permits multiple possible behaviors, it doesn't permit arbitrary behavior.

Defense-in-depth

While WebAssembly is designed to be sandboxed bugs or issues inevitably arise so Wasmtime also implements a number of mitigations which are not required for correct execution of WebAssembly but can help mitigate issues if bugs are found:

• Linear memories by default are preceded with a 2GB guard region. WebAssembly has no means of ever accessing this memory but this can protect against accidental sign-extension bugs in Cranelift where if an offset is accidentally interpreted as a signed 32-bit offset instead of an unsigned offset it could access memory before the addressable memory for WebAssembly.

• Wasmtime uses explicit checks to determine if a WebAssembly function should be considered to stack overflow, but it still uses guard pages on all native thread stacks. These guard pages are never intended to be hit and will abort the program if they're hit. Hitting a guard page within WebAssembly indicates a bug in host configuration or a bug in Cranelift itself.

• Where it can Wasmtime will zero memory used by a WebAssembly instance after it's finished. This is not necessary unless the memory is actually reused for instantiation elsewhere but this is done to prevent accidental leakage of information between instances in the face of other bugs. This applies to linear memories, tables, and the memory used to store instance information itself.

• The choice of implementation language, Rust, for Wasmtime is also a defense in protecting the authors for Wasmtime from themselves in addition to protecting embedders from themselves. Rust helps catch mistakes when writing Wasmtime itself at compile time. Rust additionally enables Wasmtime developers to create an API that means that embedders can't get it wrong. For example it's guaranteed that Wasmtime won't segfault when using its public API, empowering embedders with confidence that even if the embedding has bugs all of the security guarantees of WebAssembly are still upheld.

• Wasmtime is in the process of implementing control-flow-integrity mechanisms to leverage hardware state for further guaranteeing that WebAssembly stays within its sandbox. In the event of a bug in Cranelift this can help mitigate the impact of where control flow can go to.

Filesystem Access

Wasmtime implements the WASI APIs for filesystem access, which follow a capability-based security model, which ensures that applications can only access files and directories they've been given access to. WASI's security model keeps users safe today, and also helps us prepare for shared-nothing linking and nanoprocesses in the future.

Wasmtime developers are intimately engaged with the WASI standards process, libraries, and tooling development, all along the way too.

Terminal Output

If untrusted code is allowed to print text which is displayed to a terminal, it may emit ANSI-style escape sequences and other control sequences which, depending on the terminal the user is using and how it is configured, can have side effects including writing to files, executing commands, injecting text into the stream as if the user had typed it, or reading the output of previous commands. ANSI-style escape sequences can also confuse or mislead users, making other vulnerabilities easier to exploit.

Our first priority is to protect users, so Wasmtime now filters writes to output streams when they are connected to a terminal to translate escape sequences into inert replacement sequences.

Some applications need ANSI-style escape sequences, such as terminal-based editors and programs that use colors, so we are also developing a proposal for the WASI Subgroup for safe and portable ANSI-style escape sequence support, which we hope to post more about soon.

Spectre

Wasmtime implements a few forms of basic spectre mitigations at this time:

• Bounds checks when accessing entries in a function table (e.g. the call_indirect instruction) are mitigated.

• The br_table instruction is mitigated to ensure that speculation goes to a deterministic location.

• Wasmtime's default configuration for linear memory means that bounds checks will not be present for memory accesses due to the reliance on page faults to instead detect out-of-bounds accesses. When Wasmtime is configured with "dynamic" memories, however, Cranelift will insert spectre mitigation for the bounds checks performed for all memory accesses.

Mitigating Spectre continues to be a subject of ongoing research, and Wasmtime will likely grow more mitigations in the future as well.