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Wasmtime race condition could lead to WebAssembly control-flow integrity and type safety violations

Low severity GitHub Reviewed Published Oct 9, 2024 in bytecodealliance/wasmtime • Updated Oct 9, 2024

Package

cargo wasmtime (Rust)

Affected versions

>= 19.0.0, < 21.0.2
>= 22.0.0, < 22.0.1
>= 23.0.0, < 23.0.3
>= 24.0.0, < 24.0.1
>= 25.0.0, < 25.0.2

Patched versions

21.0.2
22.0.1
23.0.3
24.0.1
25.0.2

Description

Impact

Under certain concurrent event orderings, a wasmtime::Engine's internal type registry was susceptible to double-unregistration bugs due to a race condition, leading to panics and potentially type registry corruption. That registry corruption could, following an additional and particular sequence of concurrent events, lead to violations of WebAssembly's control-flow integrity (CFI) and type safety. Users that do not use wasmtime::Engine across multiple threads are not affected. Users that only create new modules across threads over time are additionally not affected.

Reproducing this bug requires creating and dropping multiple type instances (such as wasmtime::FuncType or wasmtime::ArrayType) concurrently on multiple threads, where all types are associated with the same wasmtime::Engine. Wasm guests cannot trigger this bug. See the "References" section below for a list of Wasmtime types-related APIs that are affected.

Wasmtime maintains an internal registry of types within a wasmtime::Engine and an engine is shareable across threads. Types can be created and referenced through creation of a wasmtime::Module, creation of wasmtime::FuncType, or a number of other APIs where the host creates a function (see "References" below). Each of these cases interacts with an engine to deduplicate type information and manage type indices that are used to implement type checks in WebAssembly's call_indirect function, for example. This bug is a race condition in this management where the internal type registry could be corrupted to trigger an assert or contain invalid state.

Wasmtime's internal representation of a type has individual types (e.g. one-per-host-function) maintain a registration count of how many time it's been used. Types additionally have state within an engine behind a read-write lock such as lookup/deduplication information. The race here is a time-of-check versus time-of-use (TOCTOU) bug where one thread atomically decrements a type entry's registration count, observes zero registrations, and then acquires a lock in order to unregister that entry. However, between when this first thread observed the zero-registration count and when it acquires that lock, another thread could perform the following sequence of events: re-register another copy of the type, which deduplicates to that same entry, resurrecting it and incrementing its registration count; then drop the type and decrement its registration count; observe that the registration count is now zero; acquire the type registry lock; and finally unregister the type. Now, when the original thread finally acquires the lock and unregisters the entry, it is the second time this entry has been unregistered.

Thread A Thread B
acquire(type registry lock)
decref(E) --> 0
block_on(type registry lock)
register(E') == incref(E) --> 1
release(type registry lock)
decref(E) --> 0
acquire(type registry lock)
unregister(E)
release(type registry lock)
acquire(type registry lock)
unregister(E)

This double-unregistration could then lead to a WebAssembly CFI violation under the following conditions: a new WebAssembly module X was loaded into the engine before the second, buggy unregistration occurs; X defined a function type F that was allocated in the same type registry slot where the original entry was allocated; the second, buggy unregistration incorrectly unregistered F; another new WebAssembly module Y was loaded into the engine; Y defined a function type G, different from F, but which is also allocated in the same type registry slot; a funcref of type G is created, either by the host or by Wasm; that funcref is passed to a WebAssembly instance of module X; that instance performs a call_indirect to that funcref; the call_indirect's dynamic type check, which preserves CFI, could incorrectly pass in this case, because F and G were assigned the same type registry slot. This would, ultimately, allow calling a function with too many, too few, or wrongly-typed arguments, violating CFI and type safety.

We were not able to reproduce this CFI violation in a vanilla Wasmtime build, although it remains theoretically possible. However, by modifying Wasmtime's source code to make losing the races described above more likely (by disabling certain assertions, inserting panic catches, and adding retry loops in a few places if we did not lose the race) we were able to incorrectly get a funcref to pass a type check that it should have failed, which would allow the CFI violation.

Patches

This bug was originally introduced in Wasmtime 19's development of the WebAssembly GC proposal. This bug affects users who are not using the GC proposal, however, and affects Wasmtime in its default configuration even when the GC proposal is disabled. Wasmtime users using 19.0.0 and after are all affected by this issue. We have released the following Wasmtime versions, all of which have a fix for this bug:

  • 21.0.2
  • 22.0.1
  • 23.0.3
  • 24.0.1
  • 25.0.2

Workarounds

If your application creates and drops Wasmtime types on multiple threads concurrently, there are no known workarounds. Users are encouraged to upgrade to a patched release.

References

The following APIs create or drop types, and therefore are affected by this race condition if performed on multiple threads concurrently and are all associated with the same wasmtime::Engine:

The change which introduced this bug was #7969

References

@alexcrichton alexcrichton published to bytecodealliance/wasmtime Oct 9, 2024
Published by the National Vulnerability Database Oct 9, 2024
Published to the GitHub Advisory Database Oct 9, 2024
Reviewed Oct 9, 2024
Last updated Oct 9, 2024

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Local
Attack Complexity Low
Attack Requirements Present
Privileges Required High
User interaction Active
Vulnerable System Impact Metrics
Confidentiality None
Integrity Low
Availability Low
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:L/AC:L/AT:P/PR:H/UI:A/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N

EPSS score

0.043%
(10th percentile)

Weaknesses

CVE ID

CVE-2024-47813

GHSA ID

GHSA-7qmx-3fpx-r45m

Source code

Credits

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