CVE-2026-90032 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
media: usbtv: keep device alive while ALSA card exists
The ALSA PCM callbacks store the driver state in pcm->private_data. An open PCM file can outlive USB disconnect because usbtv_audio_free() uses snd_card_free_when_closed(). The disconnect path can then drop the V4L2 device reference and free struct usbtv before ALSA releases the substream, so a later close dereferences freed memory in snd_usbtv_pcm_close().
Take a V4L2 device reference for the ALSA card and drop it from the card private_free callback. This keeps struct usbtv valid until ALSA has closed the remaining files and freed the card.
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability identified within the Linux kernel's media subsystem, specifically in the usbtv driver, represents a critical use-after-free condition arising from improper reference counting between the USB core layer and the Advanced Linux Sound Architecture (ALSA) subsystem. This flaw is rooted in the asynchronous lifecycle management of hardware resources when dealing with hot-pluggable devices such as USB video capture adapters. The core issue stems from a race condition where the driver fails to maintain a valid reference count for the underlying V4L2 device structure while active audio streams are still open via ALSA PCM interfaces. When a user space application opens an ALSA PCM file descriptor, the kernel stores a pointer to the driver's internal state within the pcm private data field. This design allows the audio subsystem to manage its own lifecycle independently of the video capture interface, but it introduces complexity when the physical USB device is disconnected while these streams remain active.
The operational impact becomes severe during the USB disconnect sequence. The usbtv_audio_free function utilizes snd_card_free_when_closed, a mechanism designed to defer the release of sound card resources until all associated file descriptors are closed by user space processes. However, the standard USB disconnect path in the kernel is aggressive and proceeds immediately upon detecting physical detachment or link loss. In this specific implementation, the disconnect handler drops its reference to the V4L2 device structure without checking whether ALSA still holds active references through open PCM streams. Consequently, if a process has an open audio stream at the moment of disconnection, the kernel may free the struct usbtv data structure prematurely. This leaves dangling pointers in the ALSA subsystem that are subsequently accessed when the user space application attempts to close the file descriptor, triggering a use-after-free scenario.
This memory corruption vulnerability can lead to significant security and stability consequences. A local attacker with access to the device node could potentially trigger this race condition by opening an audio stream and then causing or simulating a USB disconnect event, such as through unbinding the driver via sysfs or physically removing the device during active playback. The resulting use-after-free in snd_usbtv_pcm_close allows for arbitrary code execution if the freed memory is reallocated and controlled by the attacker, kernel panics due to invalid pointer dereferences, or information disclosure of sensitive kernel data structures residing at the reclaimed address. This aligns with CWE-416, Use After Free, a common class of vulnerabilities in systems programming where resources are released while still being referenced. Furthermore, from an offensive security perspective, this vulnerability facilitates privilege escalation and system compromise, mapping to MITRE ATT&CK techniques related to exploitation for initial access or persistence through kernel-level code execution.
To mitigate this risk, the driver implementation was corrected by introducing a robust reference counting mechanism that ties the lifetime of the V4L2 device structure directly to the existence of active ALSA cards. The fix ensures that whenever an ALSA card is initialized and associated with the usbtv device, a new reference to the V4V2 device is acquired using standard kernel refcounting primitives. This reference is only released in the private_free callback of the sound card, which executes strictly after all PCM streams have been closed by user space applications. By decoupling the physical USB disconnect from the logical resource release for audio functionality, the driver guarantees that struct usbtv remains valid as long as any part of the media stack requires it. This approach adheres to best practices in kernel memory management and ensures compliance with secure coding standards that mandate explicit ownership transfer and reference counting for shared resources across subsystem boundaries.