CVE-2026-99058 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
workqueue: Fix NULL current_pwq deref in chained work check
current_wq_worker() only tells us that %current is a kworker. It does not guarantee that the worker is currently executing a work item, as worker->current_pwq is populated only while process_one_work() is running the work function.
is_chained_work() can be reached while queueing on a draining or destroying workqueue. If a kworker outside work-item execution reaches that path, current_wq_worker() returns a worker but worker->current_pwq is NULL, and the chained-work test can fault before it emits the intended warning.
Guard the chained-work test with worker->current_pwq. A kworker without current_pwq is not executing work on the target workqueue, so the helper should return false and let the draining/destroying warning fire.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel's workqueue subsystem manages asynchronous task execution through a pool of worker threads known as kworkers. These workers are responsible for processing various types of deferred work items scheduled by different parts of the kernel. A critical aspect of this system is the tracking of which specific work queue item each worker is currently executing, stored in the current_pwq field within the worker structure. This field serves as a crucial indicator that a worker is actively running a work function rather than being idle or performing other tasks. The integrity of checks relying on this state assumption is vital for maintaining system stability during complex lifecycle events such as queue draining and destruction.
A specific vulnerability was identified in the logic governing chained work detection, specifically within the is_chained_work() helper function. This function is designed to detect when a worker attempts to schedule new work while already processing another item on the same or related queues, which can lead to deadlocks if not handled correctly. The flaw arises because the preceding check using current_wq_worker() only confirms that the calling context belongs to a kworker thread but does not verify whether that worker is currently in the middle of executing a specific work item. Consequently, when queueing operations occur on a workqueue that is being drained or destroyed, it is possible for a kworker that is not actively running a work function to reach this code path. In such scenarios, while current_wq_worker() successfully returns a valid worker structure, the worker->current_pwq field remains NULL because process_one_work(), which populates this pointer during actual execution, has not been invoked or has already completed for that context.
This discrepancy leads to a null pointer dereference when is_chained_work() attempts to access current_pwq without verifying its validity first. The kernel subsequently triggers a fault before it can emit the intended warning about improper workqueue usage during draining or destruction phases. This crash represents a significant stability issue, potentially causing system panics or unexpected reboots under specific timing conditions involving concurrent queue management operations and worker state transitions. From a vulnerability classification perspective, this flaw aligns with CWE-476, which describes NULL pointer dereference vulnerabilities resulting from improper checks of pointers before use. The operational impact includes denial of service through kernel crashes, disrupting system availability and potentially affecting other running services dependent on the stability of the workqueue subsystem.
To mitigate this vulnerability, a defensive programming approach was implemented by guarding the chained-work test with an explicit check for worker->current_pwq being non-NULL. This ensures that is_chained_work() only proceeds with its logic if the worker is genuinely executing a work item on the target queue. If current_pwq is NULL, indicating that the kworker is not currently processing work in this context, the function returns false immediately. This allows the system to bypass the faulty dereference and correctly trigger the appropriate warnings for draining or destroying queues without crashing. This fix reinforces the principle of validating pointer states before access, a best practice aligned with secure coding standards such as those found in CWE-476 guidelines.
From an offensive security perspective, while this vulnerability primarily manifests as a stability issue rather than a direct privilege escalation vector, it could potentially be leveraged by local attackers to cause denial of service conditions if they can trigger the specific race condition leading to the null dereference during queue destruction sequences. This aligns with ATT&CK technique T1499, Endpoint Denial of Service, where an attacker disrupts normal system operations through resource exhaustion or crash-inducing inputs. Although exploitation requires precise timing and access to schedule work on vulnerable queues, it highlights the importance of robust state validation in kernel subsystems that handle high-concurrency scenarios. System administrators should ensure their kernels are updated with patches addressing this specific workqueue fix to maintain operational resilience against such stability threats.