CVE-2023-30624: Undefined Behavior in Rust runtime functions
(updated )
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 use &mut self
methods where appropriate. Additionally verification tools for unsafe
code in Rust, such as cargo miri
, are planned to be executed on the main
branch soon to fix any Rust-level issues that may be exploited in future compiler versions.
Precomplied binaries available for Wasmtime from GitHub releases have been compiled with at most LLVM 15 so are not known to be vulnerable. As mentioned above, however, it’s still recommended to update.
References
- github.com/advisories/GHSA-ch89-5g45-qwc7
- github.com/bytecodealliance/wasmtime
- github.com/bytecodealliance/wasmtime/commit/0977952dcd9d482bff7c288868ccb52769b3a92e
- github.com/bytecodealliance/wasmtime/security/advisories/GHSA-ch89-5g45-qwc7
- groups.google.com/a/bytecodealliance.org/g/sec-announce/c/ecK-6G2yi90
- nvd.nist.gov/vuln/detail/CVE-2023-30624
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