Guest-controlled out-of-bounds read/write on x86_64 in wasmtime (CVE-2023-26489)

What is affected, what to do

wasmtime is a fast and secure runtime for WebAssembly. In affected versions wasmtime's code generator, Cranelift, has a bug on x86_64 targets where address-mode computation mistakenly would calculate a 35-bit effective address instead of WebAssembly's defined 33-bit effective address.

Update cranelift-codegen to version 0.91.1, wasmtime to version 4.0.1.

Affected

• Bytecode Alliance wasmtime eq cranelift-codegen: >= 0.84.0, < 0.91.1..cranelift-codegen: >= 0.84.0, < 0.91.1
• Bytecode Alliance wasmtime eq cranelift-codegen: >= 0.92.0, < 0.92.1..cranelift-codegen: >= 0.92.0, < 0.92.1
• Bytecode Alliance wasmtime eq cranelift-codegen: >= 0.93.0, < 0.93.1..cranelift-codegen: >= 0.93.0, < 0.93.1
• Bytecode Alliance wasmtime eq wasmtime: >= 0.37.0, < 4.0.1..wasmtime: >= 0.37.0, < 4.0.1
• Bytecode Alliance wasmtime eq wasmtime: >= 5.0.0, < 5.0.1..wasmtime: >= 5.0.0, < 5.0.1
• Bytecode Alliance wasmtime eq wasmtime: >= 6.0.0, < 6.0.1..wasmtime: >= 6.0.0, < 6.0.1

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Metrics

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2

NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

Read in glossary

High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.

CRA

The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

Read in glossary

From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.

ISO/IEC 27001

ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

Read in glossary

Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

wasmtime is a fast and secure runtime for WebAssembly. In affected versions wasmtime's code generator, Cranelift, has a bug on x86_64 targets where address-mode computation mistakenly would calculate a 35-bit effective address instead of WebAssembly's 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 they're 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 Cranelift's 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_64's 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 Wasmtime's or Cranelift's AArch64 backend, for example.

Source: OSV (Open Source Vulnerabilities), NVD (NIST), cvelistv5

Vendors and products

• Bytecodealliance Cranelift-codegen
• Bytecodealliance Bytecode Alliance wasmtime

Weakness classes (CWE)

• CWE-125Out-of-bounds Read

  Base

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

  Entry at MITRE CWE-125

• CWE-787Out-of-bounds Write

  Base

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

  Entry at MITRE CWE-787

References & sources

• crates.iocrates/wasmtimepackage
• rustsec.orgadvisories/RUSTSEC-2023-0090.htmladvisory
• github.combytecodealliance/wasmtime/security/advisories/GHSA-ff4p-7xrq-q5…advisory, x_refsource_CONFIRM, Mitigation, Patch, Vendor advisory
• github.combytecodealliance/wasmtime/commit/63fb30e4b4415455d47b3da5a19d79…x_refsource_MISC, Patch
• docs.rswasmtime/latest/wasmtime/struct.Config.htmlx_refsource_MISC, Product
• docs.rswasmtime/latest/wasmtime/struct.Config.htmlx_refsource_MISC, Product
• groups.google.coma/bytecodealliance.org/g/sec-announce/c/Mov-ItrNJsQx_refsource_MISC, Mailing list, Release notes, Vendor advisory
• NVD (NIST)vuln/detail/CVE-2023-26489advisory
• github.combytecodealliance/wasmtimepackage