Undefined Behavior in Rust runtime functions
Date: 2023-04
Affected: rust/wasmtime, rust/wasmtime, rust/wasmtime
Attack vector: other
Category: vulnerability-disclosure
Severity: Low
CVE: CVE-2023-30624
CWE: CWE-758
CVSS: 3.9 (CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:L/I:L/A:L)

### Impact Wasmtime's implementation of managing per-instance state, such as tables and memories, contains LLVM-level undefined behavior. This undefined behavior was found to cause runtime-level issues when compiled with LLVM 16 which causes some writes, which are critical for correctness, to be optimized away. Vulnerable versions of Wasmtime compiled with Rust 1.70, which is currently in beta, or later are known to have incorrectly compiled functions. Versions of Wasmtime compiled with the current Rust stable release, 1.69, and prior are not known at this time to have any issues, but can theoretically exhibit potential issues. The underlying problem is that Wasmtime's runtime state for an instance involves a Rust-defined structure called `Instance` which has a trailing `VMContext` structure after it. This `VMContext` structure has a runtime-defined layout that is unique per-module. This representation cannot be expressed with safe code in Rust so `unsafe` code is required to maintain this state. The code doing this, however, has methods which take `&self` as an argument but modify data in the `VMContext` part of the allocation. This means that pointers derived from `&self` are mutated. This is typically not allowed, except in the presence of `UnsafeCell`, in Rust. When compiled to LLVM these functions have `noalias readonly` parameters which means it's UB to write through the pointers. Wasmtime's internal representation and management of `VMContext` has been updated to us


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