CVE-2026-89979: Linux Kernel ALSA PCM Race Condition Vulnerability

CVE-2026-89979 Overview

CVE-2026-89979 is a race condition in the Linux kernel's Advanced Linux Sound Architecture (ALSA) Pulse Code Modulation (PCM) subsystem. The flaw stems from insufficient synchronization between non-atomic PCM operations (hw_params, hw_free, and prepare) and atomic PCM operations, specifically the trigger-start command. A local attacker can exploit the race by concurrently issuing PREPARE or HW_PARAMS on a stream while another thread triggers a PCM start. The resulting inconsistent state has been observed by syzkaller as ODEBUG warnings and can lead to state corruption in the audio subsystem.

Critical Impact
Concurrent execution of atomic and non-atomic PCM operations can cause kernel state inconsistency, memory object corruption, and potential local privilege escalation on affected Linux systems.

Affected Products

Linux kernel (upstream, multiple stable branches)
Distributions shipping vulnerable kernel versions prior to the fix commits
Systems using the ALSA PCM subsystem for audio input/output

Discovery Timeline

2026-09-16 - CVE-2026-89979 published to NVD
2026-09-17 - Last updated in NVD database

Technical Details for CVE-2026-89979

Vulnerability Analysis

The vulnerability resides in the ALSA PCM core, which manages audio stream state transitions. ALSA differentiates between atomic operations (invoked from interrupt or lock-held contexts, such as trigger) and non-atomic operations (hw_params, hw_free, prepare) that may sleep.

While concurrent atomic-to-atomic state transitions are protected, the check between non-atomic and atomic ops occurs only at the beginning of the non-atomic path. Nothing prevents an atomic trigger from being dispatched mid-execution of a non-atomic operation. This gap creates a Time-of-Check to Time-of-Use (TOCTOU) window.

When thread A re-issues PREPARE or HW_PARAMS on an already-prepared stream while thread B issues a PCM start trigger during the prepare operation, the stream state becomes inconsistent. Syzkaller reproducers surface this as ODEBUG warnings tied to object lifecycle tracking.

Root Cause

The root cause is incomplete mutual exclusion between the atomic trigger-start path and the non-atomic buffer setup paths. The snd_pcm_buffer_access_lock() mechanism was used by hw_params, hw_free, and prepare, but the trigger path did not consult the associated runtime->buffer_accessing state before proceeding from the PREPARED state.

Attack Vector

Exploitation requires local access with the ability to open an ALSA PCM device (typically /dev/snd/pcmC*D*). An unprivileged user issues concurrent ioctl calls, one thread repeatedly invoking SNDRV_PCM_IOCTL_PREPARE or SNDRV_PCM_IOCTL_HW_PARAMS and another thread issuing SNDRV_PCM_IOCTL_START to race the trigger against the ongoing non-atomic operation. The fix updates the trigger path to inspect runtime->buffer_accessing; a negative value signals a concurrent non-atomic operation and aborts the trigger. See the upstream commits, including Kernel Git Commit b599e7f and Kernel Git Commit 50afadf, for the corrected synchronization logic.

Detection Methods for CVE-2026-89979

Indicators of Compromise

Unexpected ODEBUG warnings in dmesg or kernel logs referencing ALSA PCM objects or timer objects.
Kernel WARN or BUG splats originating from sound/core/pcm_native.c or sound/core/pcm_lib.c.
Unprivileged processes issuing high-frequency, concurrent ioctl calls against /dev/snd/pcmC*D* devices.

Detection Strategies

Audit installed kernel versions against the fixed commits listed in the NVD advisory to identify unpatched hosts.
Enable CONFIG_DEBUG_OBJECTS where feasible in non-production environments to surface object-lifecycle violations that indicate the race is being triggered.
Monitor for anomalous audio device access patterns from processes that do not typically use audio subsystems.

Monitoring Recommendations

Forward kernel ring buffer messages to a centralized logging platform and alert on ALSA PCM-related warnings.
Track process activity opening /dev/snd/* devices, correlating with parent process and user context.
Include kernel version telemetry in endpoint inventory to prioritize patch deployment.

How to Mitigate CVE-2026-89979

Immediate Actions Required

Identify Linux hosts running kernel versions that predate the fix commits referenced in the NVD advisory.
Apply vendor-supplied kernel updates from your distribution maintainer as soon as they are available.
Restrict access to audio devices on multi-user or shared-tenant systems where local users are untrusted.

Patch Information

The fix is available in multiple upstream stable branches through commits b599e7f, 50afadf, 5abeb4f, 7ad2ea7, acac7b5, ad29779, b1d1214, and b2edc01. The patch adds a check for runtime->buffer_accessing in the trigger path to block PCM start when a non-atomic operation is in progress.

Workarounds

Remove or blacklist unused ALSA kernel modules on servers that do not require audio functionality.
Tighten permissions on /dev/snd/* device nodes so only authorized users and groups can issue PCM ioctl calls.
Limit local shell access on multi-tenant systems until the kernel patch is applied.

bash

# Configuration example: restrict access to ALSA PCM devices
# Verify current permissions
ls -l /dev/snd/

# Restrict device group membership (adjust group as needed)
sudo chgrp audio /dev/snd/pcmC*
sudo chmod 660 /dev/snd/pcmC*

# Blacklist snd_pcm on systems that do not require audio
echo 'blacklist snd_pcm' | sudo tee /etc/modprobe.d/disable-alsa-pcm.conf
sudo update-initramfs -u