CVE-2016-9794 in Linux
Summary
by MITRE • 01/25/2023
Race condition in the snd_pcm_period_elapsed function in sound/core/pcm_lib.c in the ALSA subsystem in the Linux kernel before 4.7 allows local users to cause a denial of service (use-after-free) or possibly have unspecified other impact via a crafted SNDRV_PCM_TRIGGER_START command.
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Analysis
by VulDB Data Team • 01/25/2023
The vulnerability identified as CVE-2016-9794 represents a critical race condition within the Advanced Linux Sound Architecture (ALSA) subsystem of the Linux kernel. This flaw exists in the snd_pcm_period_elapsed function located in sound/core/pcm_lib.c and affects kernel versions prior to 4.7. The race condition manifests when processing audio streams through the PCM (Pulse Code Modulation) framework, creating a window where concurrent operations can lead to unpredictable system behavior. The vulnerability specifically arises during the handling of the SNDRV_PCM_TRIGGER_START command, which initiates audio playback operations. When multiple threads or processes attempt to manipulate PCM stream states simultaneously, the timing-dependent nature of the function execution creates opportunities for improper resource management and memory access patterns.
The technical exploitation of this vulnerability leverages a use-after-free condition that occurs when the PCM period elapsed function attempts to access memory regions that have already been freed or deallocated. This race condition allows a local attacker with minimal privileges to manipulate the audio subsystem in such a way that memory corruption occurs during concurrent PCM operations. The flaw stems from inadequate synchronization mechanisms within the ALSA subsystem, particularly around how the kernel manages PCM stream state transitions and buffer handling. When the snd_pcm_period_elapsed function executes concurrently with other PCM operations, it can access freed memory structures, leading to system instability. This vulnerability aligns with CWE-367, which specifically addresses Time-of-Check to Time-of-Use (TOCTOU) race conditions, and more broadly with CWE-416, concerning the use of freed memory.
The operational impact of CVE-2016-9794 extends beyond simple denial of service, though that remains a primary concern. Local users can potentially trigger system crashes that result in complete system hangs or reboots, effectively rendering the audio subsystem unusable and causing broader system instability. In some cases, the memory corruption patterns associated with use-after-free conditions could theoretically be exploited to achieve privilege escalation or information disclosure, though the primary vector remains denial of service. The vulnerability affects systems running Linux kernel versions before 4.7, making it particularly relevant for older systems and embedded devices that may not have received timely kernel updates. The attack surface is primarily limited to local users who have access to audio devices, but the impact on system stability makes this a significant concern for any system where audio functionality is critical.
Mitigation strategies for this vulnerability focus on both immediate kernel updates and system hardening measures. The most effective solution involves upgrading to Linux kernel version 4.7 or later, where the race condition has been addressed through proper synchronization mechanisms and memory management improvements. System administrators should prioritize patching affected systems, particularly those running older kernel versions that may not receive security updates. Additional protective measures include implementing proper access controls to audio devices, limiting local user privileges where possible, and monitoring for unusual audio subsystem behavior that might indicate exploitation attempts. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving privilege escalation through kernel flaws and denial of service attacks targeting system services. Organizations should also consider implementing automated patch management systems to ensure timely deployment of kernel security updates and maintain audit logs of audio subsystem usage to detect potential exploitation attempts. The vulnerability demonstrates the importance of proper concurrency control in kernel-level code and highlights the need for thorough testing of multi-threaded operations in system-critical components.