CVE-2026-23109 in Linux
Summary
by MITRE • 02/04/2026
In the Linux kernel, the following vulnerability has been resolved:
fs/writeback: skip AS_NO_DATA_INTEGRITY mappings in wait_sb_inodes()
Above the while() loop in wait_sb_inodes(), we document that we must wait for all pages under writeback for data integrity. Consequently, if a mapping, like fuse, traditionally does not have data integrity semantics, there is no need to wait at all; we can simply skip these inodes.
This restores fuse back to prior behavior where syncs are no-ops. This fixes a user regression where if a system is running a faulty fuse server that does not reply to issued write requests, this causes wait_sb_inodes() to wait forever.
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Analysis
by VulDB Data Team • 05/03/2026
The vulnerability described in CVE-2026-23109 represents a critical issue within the Linux kernel's filesystem writeback subsystem that affects the synchronization behavior of certain filesystem implementations. This flaw specifically impacts the wait_sb_inodes() function which is responsible for ensuring data integrity during filesystem synchronization operations. The vulnerability stems from an overly aggressive approach to waiting for writeback operations to complete, particularly when dealing with filesystems that do not maintain traditional data integrity semantics. The issue manifests when the kernel attempts to wait for all pages under writeback to ensure data integrity, but fails to account for mappings that inherently lack these semantics, such as those used by the fuse filesystem implementation.
The technical root cause of this vulnerability lies in the improper handling of mappings that do not require data integrity guarantees during synchronization operations. The wait_sb_inodes() function contains a while loop that is documented to wait for all pages under writeback to ensure data integrity, but this approach does not properly distinguish between mappings that require such guarantees and those that do not. When a filesystem like fuse, which traditionally does not provide data integrity semantics, is involved in a synchronization operation, the kernel's current implementation forces a wait operation that can lead to indefinite blocking. This occurs because fuse-based filesystems may not properly respond to write requests from the kernel, causing the synchronization process to hang indefinitely. The vulnerability specifically affects the kernel's logic for determining when to skip waiting for certain types of mappings, which should be treated differently from regular filesystem mappings that require data integrity guarantees.
The operational impact of this vulnerability is significant, particularly in systems that rely on fuse-based filesystem implementations or that may be running faulty fuse servers. When a system experiences this regression, any attempt to synchronize filesystem state can result in indefinite blocking, effectively causing the system to become unresponsive during sync operations. This creates a denial of service condition where legitimate system operations cannot complete, as the kernel remains stuck waiting for responses from filesystems that do not provide the expected data integrity guarantees. The issue is particularly problematic in environments where fuse servers may be unreliable or unresponsive, as it can cause cascading failures throughout the system's filesystem synchronization mechanisms. This vulnerability directly impacts system availability and can lead to complete system hangs during critical synchronization events, making it a severe concern for production environments that depend on fuse-based filesystems.
The fix implemented addresses this issue by introducing a mechanism to identify and skip mappings that do not require data integrity guarantees during the wait_sb_inodes() operation. This approach restores the previous behavior where fuse filesystems could operate as no-ops during sync operations, preventing the indefinite waiting that was causing the regression. The solution specifically targets mappings with AS_NO_DATA_INTEGRITY flag, allowing the kernel to skip these inodes entirely during synchronization rather than attempting to wait for their completion. This change aligns with the principle that filesystem implementations should be allowed to operate according to their own semantics rather than being forced into a one-size-fits-all approach for data integrity. From a security perspective, this fix prevents potential denial of service conditions and ensures that kernel synchronization operations can complete properly even when dealing with non-standard filesystem implementations. The mitigation strategy follows established patterns for handling different filesystem semantics within the kernel and aligns with the broader principles of avoiding unnecessary blocking operations that could lead to system instability. This vulnerability demonstrates the importance of properly distinguishing between different types of filesystem mappings and their respective requirements for data integrity guarantees during system synchronization operations.
This vulnerability can be categorized under CWE-835, which deals with loop with Unstoppable Iteration, as the issue involves an infinite loop scenario where the wait operation cannot complete due to improper mapping handling. The fix aligns with ATT&CK technique T1489, which covers "Service Stop" and related system disruption techniques, as the vulnerability could cause system-wide service disruption through indefinite blocking. The resolution addresses the fundamental issue of proper filesystem semantics handling and ensures that the kernel's synchronization mechanisms can operate correctly across different filesystem implementations while maintaining system stability and preventing denial of service conditions.