CVE-2026-98961 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
btrfs: unlock inode and extent in caller when io_uring read extent fails
btrfs_uring_read_extent() runs only after btrfs_encoded_read() has taken the inode shared lock and the extent lock. On failure it used to unlock in out_fail, and a pages-array allocation failure returned -ENOMEM without unlocking at all.
Unlock in the caller instead on all failure returns, matching the copy_to_user() error path. The deferred -EIOCBQUEUED path still unlocks in btrfs_uring_read_finished().
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Analysis
by VulDB Data Team • 10/23/2026
The vulnerability identified within the Linux kernel's Btrfs filesystem implementation involves a resource management flaw specifically affecting the io_uring read extent functionality. This issue arises from an inconsistency in lock handling during error conditions, which can lead to deadlocks or undefined behavior when operations fail unexpectedly. The core of the problem lies in the function btrfs_uring_read_extent(), which is invoked after btrfs_encoded_read() has already acquired both the inode shared lock and the extent lock. Under normal execution paths, these locks are properly released upon completion. However, during failure scenarios, particularly those involving memory allocation issues for page arrays, the code previously failed to release these critical synchronization primitives.
In earlier versions of the affected kernel components, error handling was fragmented across different parts of the call stack. Specifically, when btrfs_encoded_read() encountered a failure that required unlocking in out_fail, it would correctly release the locks. Yet, if the subsequent attempt to allocate an array for pages failed with an -ENOMEM return code, the function would exit immediately without invoking any unlock routine. This omission leaves the inode and extent locked indefinitely from the perspective of other kernel threads or processes attempting to access the same file system objects. Such a state effectively creates a denial-of-service condition where legitimate operations are blocked until the holding process terminates or the system is rebooted, severely impacting system stability and availability in multi-user environments.
The remediation strategy implemented resolves this by shifting the responsibility for unlocking from the internal error paths to the caller of btrfs_uring_read_extent(). This architectural change ensures that regardless of whether the failure stems from a standard I/O error or a memory allocation shortage, the inode and extent locks are consistently released in the calling context. This approach mirrors the existing error handling logic used for copy_to_user() failures, thereby creating a uniform and predictable pattern for resource cleanup across different types of input/output operations within the Btrfs io_uring interface. By centralizing the unlock logic at the caller level, the patch eliminates the possibility of missed unlocks due to specific internal allocation failures that were previously overlooked in separate error branches.
From a security perspective, this vulnerability is classified under CWE-404, which pertains to improper resource shutdown or release leading to denial-of-service conditions. The lack of proper cleanup on failure paths allows an attacker who can trigger repeated memory allocation failures during read operations to exhaust available locks associated with specific inodes and extents. This aligns with ATT&CK technique T1499, Endpoint Denial of Service, as it enables the degradation or complete halt of file system access for targeted files without requiring elevated privileges beyond those needed to initiate io_uring read requests. The impact is particularly significant in high-concurrency environments where lock contention can rapidly escalate into systemic stalls if resources are not promptly returned to the kernel's management pool upon error conditions.
Mitigation strategies primarily involve applying the upstream kernel patch that corrects this locking behavior. System administrators should ensure their systems are updated with versions of the Linux kernel containing this fix, as it addresses a fundamental flaw in synchronization logic rather than relying on external configuration changes. For environments where immediate patching is not feasible, monitoring for high lock contention or stalled I/O operations related to Btrfs file systems can serve as an indicator of potential exploitation attempts involving memory pressure scenarios. Additionally, enforcing strict resource limits and monitoring system stability during heavy I/O workloads can help detect anomalies resulting from such locking inconsistencies before they lead to prolonged service disruptions.