CVE-2025-21859 is a vulnerability identified in the Linux kernel's USB gadget MIDI function (f_midi). The issue arises from a deadlock condition caused by a re-entrant call to the function f_midi_transmit, which attempts to acquire the same lock twice. Specifically, when using USB MIDI devices, the completion handler f_midi_complete calls f_midi_transmit in a way that leads to the lock being held recursively, causing the system to deadlock. The deadlock effectively halts the processing of USB MIDI data, potentially freezing the affected subsystem or the entire kernel thread handling USB MIDI operations. The fix implemented involves changing the completion handler to schedule the inner f_midi_transmit call via queue_work(), which defers the execution to a high priority work queue, thereby avoiding the re-entrant lock acquisition and preventing the deadlock. This vulnerability is present in certain Linux kernel versions identified by the commit hash d5daf49b58661ec4af7a55b277176efbf945ca05 and was publicly disclosed on March 12, 2025. There are no known exploits in the wild at this time, and no CVSS score has been assigned yet. The vulnerability affects the USB MIDI gadget subsystem, which is used in devices that emulate MIDI hardware over USB, commonly found in embedded systems, audio production equipment, and some specialized Linux-based devices.

For European organizations, the impact of this vulnerability depends largely on their use of Linux systems that employ USB MIDI gadget functionality. Organizations involved in professional audio production, music technology, embedded device manufacturing, or any sector relying on Linux-based USB MIDI devices could experience system instability or denial of service conditions due to kernel deadlocks. This could disrupt critical workflows, cause downtime in production environments, or affect embedded systems where USB MIDI is used for control or communication. While the vulnerability does not appear to allow privilege escalation or remote code execution, the deadlock can degrade system availability and reliability. In environments where Linux is used extensively, such as research institutions, media companies, or manufacturing plants in Europe, this could lead to operational interruptions. Since no known exploits exist yet, the immediate risk is moderate, but the potential for denial of service in critical systems warrants prompt attention.

To mitigate this vulnerability, European organizations should prioritize updating their Linux kernels to versions that include the fix for CVE-2025-21859. Since the fix involves kernel-level changes, applying official patches or upgrading to a patched kernel release is essential. Organizations using custom or embedded Linux distributions should coordinate with their vendors or maintainers to ensure timely patch integration. Additionally, organizations should audit their use of USB MIDI gadget functionality and assess whether it is necessary; disabling or restricting USB MIDI gadget support where not required can reduce exposure. Monitoring system logs for symptoms of deadlocks or USB MIDI subsystem errors can help detect potential issues early. For critical systems, implementing kernel live patching solutions where available can reduce downtime during patch deployment. Finally, maintaining strong change management and testing procedures for kernel updates will help ensure stability post-patch.