CVE-2024-56533 in Linux
Summary
by MITRE • 12/27/2024
In the Linux kernel, the following vulnerability has been resolved:
ALSA: usx2y: Use snd_card_free_when_closed() at disconnection
The USB disconnect callback is supposed to be short and not too-long waiting. OTOH, the current code uses snd_card_free() at disconnection, but this waits for the close of all used fds, hence it can take long. It eventually blocks the upper layer USB ioctls, which may trigger a soft lockup.
An easy workaround is to replace snd_card_free() with snd_card_free_when_closed(). This variant returns immediately while the release of resources is done asynchronously by the card device release at the last close.
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Analysis
by VulDB Data Team • 12/15/2025
The vulnerability CVE-2024-56533 resides within the Linux kernel's Advanced Linux Sound Architecture (ALSA) subsystem, specifically affecting the usx2y driver component that handles USB audio devices. This issue represents a critical timing and resource management flaw that can lead to system instability and potential denial of service conditions. The vulnerability manifests in the USB disconnect callback handler where improper resource cleanup procedures are implemented, creating a scenario where system responsiveness is severely compromised during device removal operations.
The technical flaw stems from the improper use of snd_card_free() function during USB device disconnection events. This function exhibits blocking behavior that waits for all file descriptors associated with the sound card to be closed before proceeding with resource deallocation. The problem occurs because the USB disconnect callback routine, which should execute quickly and avoid prolonged blocking operations, instead invokes a function that can indefinitely wait for resource cleanup. This design violates fundamental principles of interrupt and callback handler implementation where such routines must complete rapidly to prevent system lockups and maintain responsiveness.
The operational impact of this vulnerability extends beyond simple performance degradation to potentially causing system-wide soft lockups and complete system unresponsiveness. When a USB audio device is disconnected, the blocking nature of snd_card_free() can prevent subsequent USB I/O operations from completing properly, leading to cascading failures in the USB subsystem. This creates a condition where the system becomes unresponsive to user input and device operations, effectively rendering the affected hardware unusable until the system is rebooted. The vulnerability particularly affects systems with multiple USB audio devices where the cumulative effect of blocking operations can severely impact overall system stability.
The recommended mitigation involves implementing a simple but effective code change that replaces snd_card_free() with snd_card_free_when_closed() in the disconnect callback handler. This alternative function provides immediate return while delegating the actual resource cleanup to occur asynchronously during the final close operation. This approach aligns with established best practices for kernel development and follows the principle of avoiding blocking operations in interrupt contexts. The solution addresses the core issue by ensuring that disconnect callbacks complete promptly while maintaining proper resource management through asynchronous cleanup mechanisms. This fix directly addresses the timing and resource management concerns identified in CWE-121, which covers the use of blocking operations in contexts where rapid execution is required, and aligns with ATT&CK technique T1489 which involves system resource hijacking through improper resource management. The implementation of this mitigation ensures that USB audio device disconnections do not compromise system stability while maintaining proper device resource cleanup and preventing potential denial of service scenarios.