CVE-2026-80923 in Linux
Summary
by MITRE • 09/09/2026
In the Linux kernel, the following vulnerability has been resolved:
xhci: dbgtty: Fix unregister on tty_register_driver() failure
If tty_register_driver() fails, it drops the reference, but fails to set the global dbc_tty_driver to NULL, causing the unregister to be called again when module exits.
On module unload dbc_tty_exit() only gates its cleanup on the driver pointer being non-NULL, so it operates on the already-freed driver:
module_init(xhci_hcd_init) xhci_hcd_init() xhci_dbc_init() [return value ignored]
dbc_tty_init() tty_register_driver() fails tty_driver_kref_put() -> driver freed (dbc_tty_driver left dangling) ... module_exit(xhci_hcd_fini) xhci_hcd_fini() xhci_dbc_exit() dbc_tty_exit() if (dbc_tty_driver) -> true (dangling) tty_unregister_driver() -> use-after-free
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified in the Linux kernel's USB 3.0 eXtensible Host Controller Interface driver involves a critical memory management flaw within the debug terminal subsystem, specifically affecting the xhci_dbc module. This issue arises from an improper handling of error conditions during the initialization phase when attempting to register a tty device driver with the kernel core. The root cause lies in the interaction between the high-level driver registration functions and the internal state tracking mechanisms used by the specific hardware interface implementation. When the system attempts to initialize the debug console functionality for USB 3.0 devices, it relies on the standard Linux terminal layer API to register a new character device driver. This process involves allocating memory structures, initializing data fields, and registering the driver with the kernel's tty subsystem so that user-space applications can interact with the hardware via standard file descriptors.
The technical flaw occurs when the function tty_register_driver encounters an error condition during its execution. In such failure scenarios, the terminal layer correctly performs cleanup by releasing references to allocated memory structures through functions like tty_driver_kref_put, which eventually leads to the freeing of the driver structure in memory. However, the xhci_dbc implementation fails to update its internal global pointer variable, dbc_tty_driver, to reflect this change in state. Instead of setting this pointer to NULL after a failed registration or subsequent cleanup, the code leaves it pointing to the now-deallocated memory address. This creates a dangling pointer situation where the kernel believes the driver is still registered and active, while in reality, the underlying resources have been reclaimed by the system allocator.
The operational impact of this vulnerability manifests primarily during module unloading sequences rather than at runtime under normal operation conditions. When the xhci_hcd module is unloaded from the running kernel, typically through commands such as rmmod or during hot-unplug events for USB devices, the cleanup routine dbc_tty_exit is invoked. This function checks whether the global driver pointer is non-NULL to determine if any cleanup actions are necessary. Because the pointer was not cleared upon initialization failure, this check evaluates to true even though no valid driver exists. Consequently, the code proceeds to call tty_unregister_driver on a memory address that has already been freed and potentially reallocated for other purposes. This results in a use-after-free condition, where the kernel attempts to access or modify data structures using an invalid pointer.
From a security perspective, this vulnerability is classified under CWE-416, which denotes Use After Free vulnerabilities. The exploitation of such flaws can lead to severe consequences including kernel panics, system crashes, and potentially arbitrary code execution if an attacker can control the contents of the freed memory region before it is reused. Although this specific instance requires module unloading conditions that may be restricted by administrative privileges in many environments, the underlying pattern represents a significant integrity risk to the operating system's stability. The vulnerability aligns with ATT&CK techniques related to privilege escalation or denial of service through kernel exploitation, as compromising the kernel memory space allows for bypassing standard security controls and gaining full control over the host system.
Mitigation strategies focus on correcting the lifecycle management logic within the driver code. Developers must ensure that whenever a resource allocation fails or is explicitly released due to an error condition, all associated pointers are immediately set to NULL to prevent accidental dereference during subsequent cleanup phases. In this specific case, adding a statement to set dbc_tty_driver to NULL after tty_register_driver failure would resolve the issue by ensuring the exit routine correctly identifies that no active driver exists and skips the unregister call entirely. Additionally, implementing robust error handling paths in kernel drivers is essential to maintain memory safety guarantees. System administrators should apply available kernel patches or updates provided by their distribution vendors to address this flaw, thereby eliminating the risk of instability during device removal scenarios. Regular auditing of driver code for proper reference counting and pointer management practices serves as a preventive measure against similar vulnerabilities across the Linux ecosystem.