Impact
There was an invalid free and out-of-bounds read and write bug when running Wasm that uses externrefs in Wasmtime.
To trigger this bug, Wasmtime needs to be running Wasm that uses externrefs, the host creates non-null externrefs, Wasmtime performs a garbage collection (GC), and there has to be a Wasm frame on the stack that is at a GC safepoint where
- there are no live references at this safepoint, and
- there is a safepoint with live references earlier in this frame's function.
Under this scenario, Wasmtime would incorrectly use the GC stack map for the safepoint from earlier in the function instead of the empty safepoint. This would result in Wasmtime treating arbitrary stack slots as externrefs that needed to be rooted for GC. At the next GC, it would be determined that nothing was referencing these bogus externrefs (because nothing could ever reference them, because they are not really externrefs) and then Wasmtime would deallocate them and run <ExternRef as Drop>::drop on them. This results in a free of memory that is not necessarily on the heap (and shouldn't be freed at this moment even if it was), as well as potential out-of-bounds reads and writes.
Even though support for externrefs (via the reference types proposal) is enabled by default, unless you are creating non-null externrefs in your host code or explicitly triggering GCs, you cannot be affected by this bug.
We have reason to believe that the effective impact of this bug is relatively small because usage of externref is currently quite rare.
Patches
This bug has been patched and users should upgrade to Wasmtime version 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 at this time, you can avoid this bug by disabling the reference types proposal by passing false to wasmtime::Config::wasm_reference_types
References
For more information
If you have any questions or comments about this advisory:
Impact
There was an invalid free and out-of-bounds read and write bug when running Wasm that uses
externrefs in Wasmtime.To trigger this bug, Wasmtime needs to be running Wasm that uses
externrefs, the host creates non-nullexternrefs, Wasmtime performs a garbage collection (GC), and there has to be a Wasm frame on the stack that is at a GC safepoint whereUnder this scenario, Wasmtime would incorrectly use the GC stack map for the safepoint from earlier in the function instead of the empty safepoint. This would result in Wasmtime treating arbitrary stack slots as
externrefs that needed to be rooted for GC. At the next GC, it would be determined that nothing was referencing these bogusexternrefs (because nothing could ever reference them, because they are not reallyexternrefs) and then Wasmtime would deallocate them and run<ExternRef as Drop>::dropon them. This results in a free of memory that is not necessarily on the heap (and shouldn't be freed at this moment even if it was), as well as potential out-of-bounds reads and writes.Even though support for
externrefs (via the reference types proposal) is enabled by default, unless you are creating non-nullexternrefs in your host code or explicitly triggering GCs, you cannot be affected by this bug.We have reason to believe that the effective impact of this bug is relatively small because usage of
externrefis currently quite rare.Patches
This bug has been patched and users should upgrade to Wasmtime version 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 at this time, you can avoid this bug by disabling the reference types proposal by passing
falsetowasmtime::Config::wasm_reference_typesReferences
externrefFor more information
If you have any questions or comments about this advisory:
bytecodealliance/wasmtimerepository