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. Released versions 0.34.2 and 0.35.2, which fix the vulnerability. We recommend all Wasmtime users upgrade to these patched versions. If upgrading is not an option for you at this time, you can avoid the vulnerability by either disabling the Wasm reference types proposal or by disabling epoch interruption if you were previously enabling it.

Impact

There exists a bug in the pooling instance allocator in Wasmtime's runtime where a failure to instantiate an instance for a module that defines an externref global will result in an invalid drop of a VMExternRef via an uninitialized pointer.

As instance slots may be reused between consecutive instantiations, the value of the uninitialized pointer may be from a previous instantiation and therefore under the control of an attacker via a module's initial values for its globals. If the attacker can somehow determine an address under their control inside the mapped memory representing the instance pool, it is possible to trick the runtime to call dropinplace on a trait object under the attacker's control and therefore cause remote code execution.

Exploiting the bug to cause remote code execution would be very difficult as attackers cannot determine the addresses of globals from code executing within the WebAssembly VM and the memory space for the instance pool cannot be statically determined. Operating system mitigations, such as address space layout randomization, would additionally increase the difficulty for attackers to determine useful executable code to target with an exploit. It is also very unlikely that attackers will be able to directly influence the conditions that trigger the bug as described below.

When the conditions to trigger the bug are met, however, it is much easier to exploit this bug to cause a denial of service by crashing the host with an invalid memory read.

The following engine configuration (via Config) is required to be impacted by this bug:

support for the reference types proposal must be enabled (this is the default for Config). a pooling allocation strategy must be configured via Config::allocationstrategy, which is not the default allocation strategy.

A module must be instantiated with all the following characteristics:

The module defines at least one table or memory. The module defines at least one externref global.

During instantiation, one of the following must occur to cause the instantiation to fail:

a call to mprotect or VirtualAlloc fails (e.g. out-of-memory conditions). a resource limiter was configured in the associated Store (via Store::limiter or Store::limiterasync) and the limiter returns false from the initial call to memorygrowing or tablegrowing. Stores do not have a resource limiter set by default.

This results in a partially-initialized instance being dropped and that attempts to drop the uninitialized VMExternRef representing the defined externref global.

We have reason to believe that the effective impact of this bug is relatively small because the usage of externref is still uncommon and without a resource limiter configured on the Store, which is not the default configuration, it is only possible to trigger the bug from an error returned by mprotect or VirtualAlloc.

Note that on Linux with the uffd feature enabled, it is only possible to trigger the bug from a resource limiter as the call to mprotect is skipped; if no resource limiter is used, then this configuration is not vulnerable.

Patches

The bug has been fixed in 0.34.1 and 0.33.1; users are encouraged to upgrade as soon as possible.

Workarounds

If it is not possible to upgrade to 0.34.1 or 0.33.1 of the wasmtime crate, it is recommend that support for the reference types proposal be disabled by passing false to Config::wasmreferencetypes.

Doing so will prevent modules that use externref from being loaded entirely.

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