Instance host_state not required to be Sync, yet can be accessed from multiple threads at the same time.

[bjorn3]

wasmer/lib/vm/src/instance.rs

Line 98 in a2e744a

host_state: Box<dyn Any>,

1. Create new InstanceHandle
2. Clone InstanceHandle
3. Send clone to another thread
4. Call handle.host_state() on both handles
5. You now have a reference to a non-sync type in two threads. This is UB and may for example be exploited by using RefCell as host_state to get a race-condition.

[syrusakbary]

Thanks for opening the issue @bjorn3!

Here's the related bug reported in wasmtime: bytecodealliance/wasmtime#793 for future readers.

Possible solution

Since we are not longer using host_state perhaps we can get rid of it directly

[syrusakbary]

Thanks for creating the issue. We are working on refactoring our API for Wasmer 3.0

[syrusakbary]

This issue will be fixed in wasmer3 (already fixed in the wasmer3 branch)