CVE-2025-21859 in Linux
Summary
by MITRE • 03/12/2025
In the Linux kernel, the following vulnerability has been resolved:
USB: gadget: f_midi: f_midi_complete to call queue_work
When using USB MIDI, a lock is attempted to be acquired twice through a re-entrant call to f_midi_transmit, causing a deadlock.
Fix it by using queue_work() to schedule the inner f_midi_transmit() via a high priority work queue from the completion handler.
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Analysis
by VulDB Data Team • 12/14/2025
The vulnerability identified as CVE-2025-21859 represents a critical deadlock condition within the Linux kernel's USB gadget framework, specifically affecting the f_midi function driver responsible for MIDI device emulation. This issue manifests when USB MIDI functionality is utilized, creating a scenario where a lock acquisition attempt occurs twice through a re-entrant call path that leads to system-wide deadlock conditions. The flaw exists in the USB gadget subsystem's handling of MIDI data transmission, where the completion handler for USB transfers attempts to schedule additional MIDI processing work through a mechanism that inadvertently creates a recursive locking pattern. The root cause stems from the direct invocation of f_midi_transmit from within the completion handler context, which can result in nested lock acquisitions when the underlying USB transfer completes while another MIDI transmission is already in progress, thereby creating a circular dependency that prevents further execution.
The technical implementation of this vulnerability demonstrates a classic deadlock scenario within kernel-level USB device drivers, where the synchronization primitive used for protecting shared resources becomes corrupted through improper locking behavior. The fix implemented addresses this by introducing a work queue mechanism that decouples the immediate completion handler execution from the potentially blocking MIDI transmission logic. This approach follows the established pattern of using queue_work() to schedule work items on a high-priority work queue from interrupt context, ensuring that the inner f_midi_transmit function executes in a clean context separate from the completion handler. This solution aligns with the common security principle of avoiding nested lock acquisitions in kernel code and represents a standard approach to breaking re-entrant call chains that could lead to deadlock conditions. The implementation specifically addresses the issues outlined in CWE-362, which deals with concurrent execution using lock and unlock operations, and CWE-121, which covers stack-based buffer overflow conditions that can occur due to improper resource management in kernel code.
The operational impact of this vulnerability extends beyond simple system hangs or crashes, potentially affecting any system utilizing USB MIDI functionality through the Linux gadget framework, including embedded systems, industrial control devices, and multimedia applications that rely on USB MIDI interfaces. When triggered, the deadlock condition can cause complete system unresponsiveness, requiring manual intervention or system reboot to restore normal operation. This vulnerability particularly affects systems where USB gadget drivers are actively used for MIDI device emulation, such as audio production workstations, embedded audio devices, and industrial systems that communicate via USB MIDI protocols. The attack surface is significant within the Linux kernel ecosystem, as USB gadget drivers form a critical component of device emulation capabilities and are widely deployed across various hardware platforms and use cases. Systems running kernel versions containing this vulnerability are at risk of experiencing denial of service conditions that can persist until system reset, making this a critical security concern for production environments where system availability is paramount.
Mitigation strategies for CVE-2025-21859 focus primarily on applying the kernel update that incorporates the fixed implementation using queue_work() for scheduling MIDI transmission work items. Administrators should prioritize patching affected systems, particularly those running kernel versions that include the vulnerable USB gadget framework components. The fix implementation follows established best practices for kernel development and security, specifically addressing the ATT&CK technique T1489 which involves system shutdown or restart to disrupt operations. Organizations should also implement monitoring solutions to detect potential deadlock conditions in USB subsystems, though the primary defense remains the kernel-level fix. Additional defensive measures include limiting USB gadget functionality in environments where MIDI devices are not required, and implementing proper system resource monitoring to detect unusual lock contention patterns. The solution addresses the underlying architectural issue in the USB gadget framework and provides a robust mechanism for handling concurrent MIDI data transmission while maintaining system stability and responsiveness. This fix represents a standard security hardening approach that aligns with kernel security best practices and helps prevent similar vulnerabilities from manifesting in other parts of the USB subsystem.