GHSA-v4cp-h94r-m7xf Use after free passing `externref`s to Wasm in Wasmtime

• crates.io wasmtime: < 0.30.0 → fixed in 0.30.0 • PyPI wasmtime: < 0.30.0 → fixed in 0.30.0

Impact There was a use-after-free bug when passing `externref`s from the host to guest Wasm content. To trigger the bug, you have to explicitly pass multiple `externref`s from the host to a Wasm instance at the same time, either by • passing multiple `externref`s as arguments from host code to a Wasm function, • or returning multiple `externref`s to Wasm from a multi-value return function defined in the host. If you do not have host code that matches one of these shapes, then you are not impacted. If Wasmtime's `VMExternRefActivationsTable` became filled to capacity after passing the first `externref` in, then passing in the second `externref` could trigger a garbage collection. However the first `externref` is not rooted until we pass control to Wasm, and therefore could be reclaimed by the collector if nothing else was holding a reference to it or otherwise keeping it alive. Then, when control was passed to Wasm after the garbage collection, Wasm could use the first `externref`, which at this point has already been freed. We have reason to believe that the effective impact of this bug is relatively small because usage of `externref` is currently quite rare. Patches The bug has been fixed, and users should upgrade to Wasmtime 0.30.0. Additionally, we have updated our primary `externref` fuzz target such that it better exercises these code paths and we can have greater confidence in their correctness going forward. Workarounds If you cannot upgrade Wasmtime yet, you can avoid the bug by disabling reference types support in Wasmtime by passing `false` to `wasmtime::Config::wasm_reference_types`. References • The reference types Wasm proposal, which introduces `externref` For more information If you have any questions or comments about this advisory: • Reach out to us on the Bytecode Alliance Zulip chat • Open an issue in the `bytecodealliance/wasmtime` repository