Impact
There is a bug in Wasmtime's implementation of it's pooling instance allocator where when a linear memory is reused for another instance the initial heap snapshot of the prior instance can be visible, erroneously to the next instance. The pooling instance allocator in Wasmtime works by preallocating virtual memory for a fixed number of instances to reside in and then new instantiations pick a slot to use. Most conventional modules additionally have an initial copy-on-write "heap image" which is mapped in Wasmtime into the linear memory slot. When a heap slot is deallocated Wasmtime resets all of its contents back to the initial state but it does not unmap the image in case the next instance is an instantiation of the same module.
The bug in Wasmtime occurs when a slot in the pooling allocator previously was used for a module with a heap image, meaning that its current state of memory contains the initial heap contents of that module. If the next instantiation within that slot does not itself contain a heap image then Wasmtime would leave the old heap image in place erroneously and continue with instantiation. This means that instantiations of modules without a heap image can see the initial heap image of the prior instantiation within that slot.
Heap images in Wasmtime are created by precomputing WebAssembly data segments into one large mapping to be placed into linear memory at a particular offset. Most modules produced by toolchains today will have a heap image and an initialization snapshot. Creating a module without a heap image would require a hand-crafted .wat file or a specially crafted source program. This consequence means that this bug is highly unlikely to be accidentally triggered and would otherwise require an intentional trigger with a hand-crafted module.
One important part of this vulnerability is Wasmtime is highly likely to segfault when the slot is reused again with a module that itself has an initialization image. For example if module A has a heap initialization image and module B does not have a heap initialization image, then the following sequence of events could happen if they all are instantiated into the same instance slot:
Module A is instantiated, and then deallocated. This leaves A's heap image in place, reset to its initial contents. Module B is instantiated and erroneously can see the initial heap contents of A. Module B is then deallocated and the entire heap is unmapped and reset back to zero. Module A is instantiated again, but the state tracking the slot did not account for module B so it thinks the module image is still mapped and proceeds with instantiation. Any action on A's part to access linear memory will then trap and if the host accesses A's memory it will segfault because the data that's supposed to be mapped is all unmapped.
Adding this all together this means that in practice modules must be deliberately crafted to not have an initial heap image to view the contents of a prior image. If this module is instantiated though then when the slot is reused the next, likely image-using, module will believe its memory is mapped when it isn't, causing the host to segfault on unmapped memory it believed was mapped.
Patches
This bug has been patched and users should upgrade to Wasmtime 2.0.2.
Workarounds
Trigging this bug requires the pooling allocator to be configured and for copy-on-write heap images to also be enabled. Pooling allocation is not enabled by default but copy-on-write heap images are. Mitigations for this bug include:
Disabling the pooling allocator - note that pooling allocation is not enabled by default in Wasmtime Disabling the memory-init-cow feature or with Config::memoryinitcow
References
Config::allocationstrategy - configuration required to enable the pooling allocator. Config::memoryinitcow - configuration required to enable or disable copy-on-write (this is enabled by default). Mailing list announcement Patch for release-2.0.0 branch Patch for main
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
Impact
There is a bug in Wasmtime's C API implementation where the definition of the wasmtimetrapcode does not match its declared signature in the wasmtime/trap.h header file. This discrepancy causes the function implementation to perform a 4-byte write into a 1-byte buffer provided by the caller. This can lead to three zero bytes being written beyond the 1-byte location provided by the caller.
Patches
This bug has been patched and users should upgrade to Wasmtime 2.0.2.
Workarounds
This can be worked around by providing a 4-byte buffer casted to a 1-byte buffer when calling wasmtimetrapcode. Users of the wasmtime crate are not affected by this issue, only users of the C API function wasmtimetrapcode are affected.
References
Definition of wasmtimetrapcode Mailing list announcement Patch to fix for main branch
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
Impact
There is a bug in Wasmtime's implementation of its pooling instance allocator when the allocator is configured to give WebAssembly instances a maximum of zero pages of memory. In this configuration the virtual memory mapping for WebAssembly memories did not meet the compiler-required configuration requirements for safely executing WebAssembly modules. Wasmtime's default settings require virtual memory page faults to indicate that wasm reads/writes are out-of-bounds, but the pooling allocator's configuration would not create an appropriate virtual memory mapping for this meaning out of bounds reads/writes can successfully read/write memory unrelated to the wasm sandbox within range of the base address of the memory mapping created by the pooling allocator.
This bug can only be triggered by setting InstanceLimits::memorypages to zero. This is expected to be a very rare configuration since this means that wasm modules cannot allocate any pages of linear memory. All wasm modules produced by all current toolchains are highly likely to use linear memory, so it's expected to be unlikely that this configuration is set to zero by any production embedding of Wasmtime, hence the low severity of this bug despite the critical consequences.
Patches
This bug has been patched and users should upgrade to Wasmtime 2.0.2.
Workarounds
One way to mitigate this issue is to disable usage of the pooling allocator. Note that the pooling allocator is not enabled by default.
This bug can also only be worked around by increasing the memorypages allotment when configuring the pooling allocator to a value greater than zero. If an embedding wishes to still prevent memory from actually being used then the Store::limiter method can be used to dynamically disallow growth of memory beyond 0 bytes large. Note that the default memorypages value is greater than zero.
This bug is not applicable with the default settings of the wasmtime crate.
References
Config::allocationstrategy - configuration required to enable the pooling allocator. InstanceLimits::memorypages - configuration field that, when zero, exhibits this bug. Store::limiter - means of limiting memory without using memorypages Mailing list announcement Patch for the release-2.0.0 branch
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