[wasmtime] Use after free in Wasmtime

Impact

There is a use after free vulnerability in Wasmtime when both running Wasm that uses externrefs and enabling epoch interruption in Wasmtime. If you are not explicitly enabling epoch interruption (it is disabled by default) then you are not affected. If you are explicitly disabling the Wasm reference types proposal (it is enabled by default) then you are also not affected.

The use after free is caused by Cranelift failing to emit stack maps when there are safepoints inside cold blocks. Cold blocks occur when epoch interruption is enabled. Cold blocks are emitted at the end of compiled functions, and change the order blocks are emitted versus defined. This reordering accidentally caused Cranelift to skip emitting some stack maps because it expected to emit the stack maps in block definition order, rather than block emission order. When Wasmtime would eventually collect garbage, it would fail to find live references on the stack because of the missing stack maps, think that they were unreferenced garbage, and therefore reclaim them. Then after the collection ended, the Wasm code could use the reclaimed-too-early references, which is a use after free.

This bug was discovered while extending our fuzz targets for externrefs and GC in Wasmtime. The updated fuzz target thoroughly exercises these code paths and feature combinations now. We have also added a regression test for this bug.

Patches

We have released versions 0.34.2 and 0.35.2, which fix the vulnerability. We recommend all Wasmtime users upgrade to these patched versions.

Workarounds

If upgrading is not an option for you at this time, you can avoid the vulnerability by either

• disabling the Wasm reference types proposal

  config.wasm_reference_types(false);
  

• or by disabling epoch interruption if you were previously enabling it.

  config.epoch_interruption(false);
  

References

• The WebAssembly reference types proposal, which introduces externrefs
• Documentation about epoch interruption in Wasmtime

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
  

  References

• https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-gwc9-348x-qwv2
• https://nvd.nist.gov/vuln/detail/CVE-2022-24791
• https://github.com/bytecodealliance/wasmtime/commit/666c2554ea0e1728c35aa41178cf235920db888a
• https://github.com/advisories/GHSA-gwc9-348x-qwv2