Planned changes to shared memory

There is standardization work ongoing that enables developers to create SharedArrayBuffer objects again, but changes are needed in order to be use these across threads (i.e., postMessage() for SharedArrayBuffer objects throws by default). These changes provide further isolation between sites and help reduce the impact of attacks with high-resolution timers, which can be created with shared memory.

Firefox's Nightly release from 73 onward has an experimental implementation of the features described in this document enabled by default.

Chrome intends to implement similar restrictions.

New HTTP header bonanza

As a baseline requirement, documents will need to be in a secure context.

For top-level documents, two headers will need to be set:

With these two headers set, postMessage() will no longer throw for SharedArrayBuffer objects and shared memory across threads is therefore available.

Nested documents and dedicated workers will need to set the Cross-Origin-Embedder-Policy header as well with the same value. No further changes are needed for same-origin nested documents and subresources. Same-site (but cross-origin) nested documents and subresources will need to set the Cross-Origin-Resource-Policy header with same-site as value. And their cross-origin (and cross-site) counterparts need to set the same header with cross-origin as value. Note that setting the Cross-Origin-Resource-Policy header to any other value than same-origin opens up the resource to potential attacks, such as Spectre.

Note that the Cross-Origin-Opener-Policy header limits your ability to retain a reference to popups. Direct access between two top-level window contexts will essentially only work if they are same-origin and carry the same two headers with the same two values.

API changes

As a result of this newly required environment, there are a couple API implications:

  • The Atomics object is always available.
  • SharedArrayBuffer objects are in principle always available, but unfortunately the constructor on the global object is hidden, unless the two headers mentioned above are set, for compatibility with web content. There is hope that this restriction can be removed in the future. WebAssembly.Memory can still be used to get an instance.
  • Unless the two headers mentioned above are set, the various postMessage() APIs will throw for SharedArrayBuffer objects. If they are set, postMessage() on Window objects and dedicated workers will function and allow for memory sharing.
  • To avoid having to check whether postMessage() throws, self.crossOriginIsolated is being standardized (a getter that returns a boolean; true if the headers are set), available in window and worker contexts.

WebAssembly Shared Memory

The WebAssembly Threads proposal allows WebAssembly.Memory objects to be created with a new shared constructor flag. When this flag is set to true, the constructed Memory object can be shared between workers via postMessage(), just like SharedArrayBuffer, and the backing buffer of the Memory object is a SharedArrayBuffer. Therefore, the requirements listed above for sharing a SharedArrayBuffer between workers also apply to sharing a WebAssembly.Memory.

The WebAssembly Threads proposal also defines a new set of atomic instructions. Just as SharedArrayBuffer and its methods are unconditionally enabled (and only sharing between threads is gated on the new headers), the WebAssembly atomic instructions are also unconditionally allowed.

Further reading