CVE-2026-98695 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
USB: serial: fix ioctl hangup race
The tty ioctls can race with hangup and end up calling into a tty driver for a device that is already gone or powered down.
Serialise the callbacks using the tty port mutex and add the missing checks to make sure the port has not been hung up before accessing port driver data or hardware.
This specifically avoids dereferencing a NULL-pointer if a device is disconnected during lengthy break signalling.
Note that TIOCGICOUNT and TIOCMIWAIT do not access port driver data or hardware and are therefore not affected by the race.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel USB serial subsystem contains a concurrency vulnerability related to the handling of terminal control ioctls, specifically involving race conditions between ioctl operations and device hangup procedures. This issue arises when user-space applications invoke tty ioctls on a serial port while the underlying physical USB device is being disconnected or powered down by the system. In such scenarios, the kernel may attempt to execute driver-specific callbacks for a terminal line discipline that no longer has valid hardware backing because the device has already been removed from the bus or suspended. This race condition stems from insufficient synchronization between the tty layer and the usb-serial driver core during state transitions involving port detachment.
The technical root cause lies in the lack of proper serialization when accessing port-specific data structures and hardware registers during ioctl execution. When a hangup sequence is initiated, typically due to device removal or explicit close operations, the associated tty structure may be freed or marked as invalid before all pending ioctls have completed their processing. Without adequate locking mechanisms, subsequent ioctl calls can dereference pointers that point to already-freed memory regions or attempt to communicate with hardware interfaces that are no longer responsive. This leads to undefined behavior within the kernel space, potentially resulting in a NULL pointer dereference if the device is disconnected during lengthy break signaling operations where the driver attempts to maintain state for extended periods without verifying current port validity.
The operational impact of this vulnerability includes potential system instability and denial of service conditions. A successful exploitation could cause a kernel panic due to the null pointer dereference, effectively crashing the affected Linux instance or container environment. For systems running critical infrastructure services that rely on serial communication over USB adapters, such as industrial control systems, telecommunications equipment, or legacy hardware interfaces, this vulnerability poses a significant risk of service interruption. The instability can manifest unpredictably depending on timing and workload characteristics, making it difficult to reproduce consistently but highly disruptive when triggered in production environments where continuous availability is required.
To mitigate this issue, the resolution involves serializing access to tty port callbacks using the tty port mutex, ensuring that only one operation can modify or read critical state variables at any given time during sensitive operations like hangup sequences. Additionally, explicit checks have been added to verify whether a port has already been hung up before attempting to access driver data structures or interact with hardware components. These safeguards prevent the kernel from proceeding with operations on invalid ports and eliminate the possibility of dereferencing freed memory blocks. It is important to note that certain ioctls such as TIOCGICOUNT for getting interrupt counts and TIOCMIWAIT for waiting on modem status changes do not access port driver data or hardware directly, so they remain unaffected by this race condition and were excluded from the synchronization fixes applied to other ioctl handlers.
From a classification perspective, this vulnerability aligns with CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, commonly known as a race condition. The specific mechanism involves accessing shared resources without proper locking during state transitions between active operation and device removal. In terms of the MITRE ATT&CK framework, while not directly exploitable for remote code execution in most standard configurations, this type of kernel-level concurrency flaw can be leveraged for local privilege escalation or denial of service attacks by triggering the race condition through carefully timed ioctl calls from a user-space process with appropriate permissions to open serial devices. Organizations should ensure that their Linux kernels are updated to include patches addressing this usb-serial tty synchronization issue and review configurations involving USB-to-serial adapters in critical systems to minimize exposure windows during device hot-plug events or unexpected disconnections.