Impact
There was a use-after-free bug when passing externrefs from the host to guest Wasm content.
To trigger the bug, you have to explicitly pass multiple externrefs from the host to a Wasm instance at the same time, either by
- passing multiple
externrefs as arguments from host code to a Wasm function,
- or returning multiple
externrefs 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
For more information
If you have any questions or comments about this advisory:
References
alexcrichton Analyst
fitzgen Analyst
cfallin Analyst
Impact
There was a use-after-free bug when passing
externrefs from the host to guest Wasm content.To trigger the bug, you have to explicitly pass multiple
externrefs from the host to a Wasm instance at the same time, either byexternrefs as arguments from host code to a Wasm function,externrefs 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
VMExternRefActivationsTablebecame filled to capacity after passing the firstexternrefin, then passing in the secondexternrefcould trigger a garbage collection. However the firstexternrefis 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 firstexternref, 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
externrefis currently quite rare.Patches
The bug has been fixed, and users should upgrade to Wasmtime 0.30.0.
Additionally, we have updated our primary
externreffuzz 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
falsetowasmtime::Config::wasm_reference_types.References
externrefFor more information
If you have any questions or comments about this advisory:
bytecodealliance/wasmtimerepositoryReferences