CVE-2026-98245 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
btrfs: take commit root semaphore when iterating in mark_block_group_to_copy()
mark_block_group_to_copy() iterates over the commit root with skip_locking=true. A concurrent transaction commit can swap and free the commit root during iteration, causing use-after-free when accessing extent buffers.
Fix it by using path->need_commit_sem to protect the commit root search.
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Analysis
by VulDB Data Team • 10/06/2026
The vulnerability identified in the Linux kernel's Btrfs filesystem implementation involves a critical concurrency flaw within the mark_block_group_to_copy function. This specific routine is responsible for iterating over the file system's metadata structures, specifically targeting extent buffers associated with block groups that require copying during operations such as balance or scrub tasks. The core of the issue lies in how this iteration was previously managed under conditions where locking mechanisms were intentionally bypassed to improve performance or handle specific internal states by passing a skip_locking parameter set to true. While skipping locks can be necessary for certain non-blocking paths, it introduces significant risk when accessing shared data structures that are subject to modification by other kernel threads, particularly those involved in transaction commits.
The technical flaw manifests as a use-after-free condition resulting from race conditions between the iteration process and concurrent transaction commit operations. During normal file system operation, transactions may be committed asynchronously, which involves swapping out the current commit root for a new one and potentially freeing memory associated with the old commit root's extent buffers. Because mark_block_group_to_copy was iterating over this commit root without holding the appropriate semaphore protection, it remained vulnerable to accessing memory that had already been freed by a concurrent transaction commit thread. This race condition allows an attacker or faulty workload to trigger access to invalid memory addresses, leading to kernel panics, data corruption, or potentially arbitrary code execution if the freed memory is reallocated and manipulated in specific ways before being accessed again.
From a security classification perspective, this vulnerability aligns with CWE-416, which describes Use After Free errors. The lack of proper synchronization primitives during iteration over shared resources creates an opportunity for exploitation through timing attacks or high-concurrency workloads that maximize the window between checking a pointer and dereferencing it. In terms of adversarial tactics, this flaw relates to ATT&CK techniques involving resource hijacking or denial of service via kernel instability, as successful exploitation can destabilize the entire operating system by crashing the kernel process responsible for managing file system integrity. The impact extends beyond simple availability loss; data consistency within Btrfs volumes could be compromised if partial writes occur due to corrupted metadata structures accessed during this race condition.
The resolution implemented in the Linux kernel addresses this issue by enforcing stricter synchronization controls specifically around the commit root search and iteration logic. By utilizing path->need_commit_sem, the fix ensures that the necessary semaphore is acquired before accessing or iterating over the commit root's extent buffers. This change effectively serializes access to these critical data structures, preventing concurrent transaction commits from freeing memory while it is still being referenced by ongoing operations like block group copying. The patch restores consistency between iteration and modification phases of metadata management without requiring a complete overhaul of the locking strategy for all paths, thereby maintaining performance where safe while eliminating the race condition at its source.
Mitigation strategies for organizations running affected versions of the Linux kernel involve applying the latest available security patches that include this specific fix. Since Btrfs is widely used in enterprise storage solutions and cloud infrastructure, ensuring timely updates to both the kernel and associated user-space tools like btrfs-progs is essential. Administrators should monitor vendor advisories for patch availability corresponding to their distribution's supported lifecycle. Additionally, implementing strict resource limits on file system operations can reduce the likelihood of triggering high-concurrency scenarios that might expose this race condition in production environments until patches are fully deployed across all nodes. Regular auditing of kernel logs for memory corruption errors or unexpected panics related to Btrfs operations can also serve as an early warning mechanism for potential exploitation attempts or instability caused by unpatched vulnerabilities.