CVE-2023-26489

wasmtime is a fast and secure runtime for WebAssembly. In affected versions wasmtimes code generator, Cranelift, has a bug on x86_64 targets where address-mode computation mistakenly would calculate a 35-bit effective address instead of WebAssemblys defined 33-bit effective address. This bug means that, with default codegen settings, a wasm-controlled load/store operation could read/write addresses up to 35 bits away from the base of linear memory. Due to this bug, however, addresses up to 0xffffffff * 8 + 0x7ffffffc = 36507222004 = ~34G bytes away from the base of linear memory are possible from guest code. This means that the virtual memory 6G away from the base of linear memory up to ~34G away can be read/written by a malicious module. A guest module can, without the knowledge of the embedder, read/write memory in this region. The memory may belong to other WebAssembly instances when using the pooling allocator, for example. Affected embedders are recommended to analyze preexisting wasm modules to see if theyre affected by the incorrect codegen rules and possibly correlate that with an anomalous number of traps during historical execution to locate possibly suspicious modules. The specific bug in Cranelifts x86_64 backend is that a WebAssembly address which is left-shifted by a constant amount from 1 to 3 will get folded into x86_64s addressing modes which perform shifts. For example (i32.load (i32.shl (local.get 0) (i32.const 3))) loads from the WebAssembly address $local0 << 3. When translated to Cranelift the $local0 << 3 computation, a 32-bit value, is zero-extended to a 64-bit value and then added to the base address of linear memory. Cranelift would generate an instruction of the form movl (%base, %local0, 8), %dst which calculates %base + %local0 << 3. The bug here, however, is that the address computation happens with 64-bit values, where the $local0 << 3 computation was supposed to be truncated to a a 32-bit value. This means that %local0, which can use up to 32-bits for an address, gets 3 extra bits of address space to be accessible via this movl instruction. The fix in Cranelift is to remove the erroneous lowering rules in the backend which handle these zero-extended expression. The above example is then translated to movl %local0, %temp; shl $3, %temp; movl (%base, %temp), %dst which correctly truncates the intermediate computation of %local0 << 3 to 32-bits inside the %temp register which is then added to the %base value. Wasmtime version 4.0.1, 5.0.1, and 6.0.1 have been released and have all been patched to no longer contain the erroneous lowering rules. While updating Wasmtime is recommended, there are a number of possible workarounds that embedders can employ to mitigate this issue if updating is not possible. Note that none of these workarounds are on-by-default and require explicit configuration: 1. The Config::static_memory_maximum_size(0) option can be used to force all accesses to linear memory to be explicitly bounds-checked. This will perform a bounds check separately from the address-mode computation which correctly calculates the effective address of a load/store. Note that this can have a large impact on the execution performance of WebAssembly modules. 2. The Config::static_memory_guard_size(1 << 36) option can be used to greatly increase the guard pages placed after linear memory. This will guarantee that memory accesses up-to-34G away are guaranteed to be semantically correct by reserving unmapped memory for the instance. Note that this reserves a very large amount of virtual memory per-instances and can greatly reduce the maximum number of concurrent instances being run. 3. If using a non-x86_64 host is possible, then that will also work around this bug. This bug does not affect Wasmtimes or Cranelifts AArch64 backend, for example.

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://docs.rs/wasmtime/latest/wasmtime/struct.Config.html#method.static_memory_guard_size
• https://docs.rs/wasmtime/latest/wasmtime/struct.Config.html#method.static_memory_maximum_size
• https://github.com/bytecodealliance/wasmtime/commit/63fb30e4b4415455d47b3da5a19d79c12f4f2d1f
• https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-ff4p-7xrq-q5r8
• https://groups.google.com/a/bytecodealliance.org/g/sec-announce/c/Mov-ItrNJsQ