CVE-2026-89831 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: protect critical_task_priority updates with s_umount
The sysfs store path already takes s_umount for GC thread control entries, and ckpt_thread_ioprio is covered as well.
critical_task_priority also updates checkpoint or GC kthread scheduling state, but it is not covered by that serialization. It can race with remount or teardown paths that are stopping those threads.
Protect critical_task_priority sysfs writes with s_umount too.
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Analysis
by VulDB Data Team • 09/16/2026
The Linux kernel's F2FS file system contains a concurrency vulnerability related to the handling of scheduling priority updates for background tasks, specifically within the context of checkpoint and garbage collection thread management. This issue arises from an inconsistency in synchronization mechanisms used across different control paths. While existing sysfs store operations that manage Garbage Collection (GC) threads and checkpoint I/O priorities correctly acquire the super block's umount semaphore to ensure safe access during filesystem teardown or remounting, the code path responsible for updating critical_task_priority lacks this protection. This oversight creates a race condition where concurrent modifications can occur while the kernel is in the process of stopping these background threads due to an unmount or remount operation.
The technical flaw centers on the lack of mutual exclusion when writing to the critical_task_priority attribute via sysfs. The checkpoint and GC kthreads are responsible for maintaining filesystem integrity by managing metadata updates and reclaiming unused space, respectively. These threads operate with specific scheduling priorities that can be dynamically adjusted through this interface. However, because the update function does not hold the s_umount lock, it is possible for a user-space process to modify these priority settings at the exact moment the kernel's remount or teardown logic is attempting to halt and clean up these kthreads. This temporal overlap allows for unsynchronized access to shared thread control structures, potentially leading to undefined behavior such as use-after-free scenarios, data corruption of scheduling states, or system instability if a thread attempts to execute code that has been invalidated by the concurrent shutdown process.
The operational impact of this vulnerability is primarily related to system stability and potential denial-of-service conditions rather than direct privilege escalation or remote code execution. An attacker with local access who can write to sysfs could trigger this race condition repeatedly, causing kernel panics, freezes, or unpredictable filesystem behavior during critical maintenance operations like unmounting a volume. The severity is heightened because the affected components are fundamental to F2FS operation, meaning that any instability in these background threads can compromise the reliability of the entire storage subsystem on devices relying on this file system, such as many Android-based mobile devices and embedded systems.
To mitigate this vulnerability, kernel developers have implemented a fix by ensuring that all updates to critical_task_priority are serialized with other super block operations by acquiring the s_umount semaphore before modifying thread scheduling states. This aligns the protection mechanism for priority updates with those already in place for GC and checkpoint I/O controls, thereby eliminating the race window during filesystem teardown or remounting events. System administrators should ensure that their kernels are updated to include this patch from upstream Linux sources. For environments where immediate patching is not feasible, restricting access to sysfs entries related to F2FS configuration can reduce the attack surface by preventing unprivileged users from triggering the vulnerable code path until a permanent fix is deployed.
This vulnerability aligns with CWE-362, which describes concurrent execution errors resulting in race conditions due to insufficient synchronization of shared resources. In terms of offensive security frameworks, this type of flaw could be leveraged within the MITRE ATT&CK framework under techniques related to resource hijacking or denial of service via local exploitation, specifically where an attacker manipulates system resources to degrade performance or crash services. The fix reinforces the principle that all access paths modifying shared kernel state must adhere to consistent locking protocols to prevent temporal race conditions during lifecycle transitions like mount and unmount operations.