CVE-2026-98765 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
USB: gadget: dummy-hcd: Fix wait for outstanding request completions
The dummy-hcd driver emulates synchronize_irq() by waiting until its private callback_usage counter drops to 0 (with the private lock not held). This counter is incremented whenever a gadget driver callback occurs (which requires the private lock to be dropped), but not when a request completion handler is called.
This is an oversight. Request completion is triggered by timer interrupts (emulating device IRQs in a real UDC), and the interrupt handlers are supposed to have completed when the synchronize_irq() emulation routine returns -- they aren't supposed to be in the middle of a completion callback. If this happens it can lead to a gadget driver's unbind routine running before all outstanding request completions have finished, maybe even allowing the gadget driver's module to be unloaded while a completion handler is still running.
Fix the oversight by incrementing the callback_usage value across request completion callbacks.
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Analysis
by VulDB Data Team • 10/19/2026
The Linux kernel USB dummy host controller driver contains a synchronization flaw in its emulation of interrupt request handling that can lead to race conditions during device unbinding and module removal operations. The dummy-hcd driver is primarily used for testing and development purposes, emulating the behavior of real Universal Device Controller hardware by using software timers to simulate hardware interrupts. A critical component of this emulation involves ensuring that all pending interrupt service routines have completed before proceeding with cleanup tasks, a process typically managed through synchronize_irq semantics in physical drivers. In this specific implementation, the driver attempts to replicate this synchronization mechanism by monitoring a private counter known as callback_usage, which is intended to track active callbacks and ensure they finish before the system considers the device idle or ready for unloading.
The technical flaw arises from an inconsistency in how the callback_usage counter is updated relative to different types of function calls within the driver's interrupt handling logic. Specifically, the counter is incremented when a gadget driver callback occurs, but it fails to be incremented during request completion handlers. These completion handlers are triggered by timer interrupts that emulate device IRQs and represent the final stage of processing an USB transfer request. Because the synchronization routine waits for this counter to drop to zero, assuming all active operations have concluded, the omission means that ongoing request completions do not contribute to the count of in-flight operations. Consequently, the synchronize_irq emulation may return prematurely while completion handlers are still executing or queued to execute, creating a window where the driver believes it is safe to proceed with cleanup procedures despite pending work remaining.
This oversight has significant operational implications for system stability and security integrity. When the synchronization check fails to account for active request completions, there is a risk that the gadget driver's unbind routine may be invoked before all outstanding requests have finished processing. This race condition can result in use-after-free vulnerabilities or data corruption if memory structures associated with pending transfers are freed while still being accessed by completion handlers running in interrupt context. More critically, it allows for the possibility of the gadget driver module being unloaded from kernel space while a completion handler is still active within that same module's code. Such an event can lead to kernel panics, system crashes, or potentially exploitable conditions where control flow jumps to freed memory regions, undermining the reliability and security posture of the USB subsystem in embedded environments relying on this emulation driver.
To mitigate these risks, the vulnerability was addressed by modifying the dummy-hcd source code to increment the callback_usage counter across request completion callbacks as well. This adjustment ensures that the synchronization mechanism accurately reflects all active execution paths within the interrupt handling context, preventing premature unbinding and module removal until every pending operation has truly concluded. From a standards perspective, this issue aligns with CWE-362, which describes concurrent access to shared resources without proper synchronization, specifically manifesting here as a race condition in kernel thread management. Furthermore, it relates to ATT&CK technique T1059, Command and Scripting Interpreter, if an attacker could leverage the resulting instability to execute arbitrary code via memory corruption during module unloading. System administrators should ensure that kernels are updated with patches addressing this dummy-hcd synchronization fix, particularly in environments where USB gadget drivers are dynamically loaded or unloaded frequently, as well as in testing frameworks that rely heavily on this emulation driver for validating peripheral device behaviors under various interrupt timing scenarios.