CVE-2025-21942 is a vulnerability in the Linux kernel's Btrfs filesystem implementation, specifically affecting the zoned block device support in the function cow_file_range(). The issue arises from improper unlocking of extent ranges during copy-on-write (COW) operations in zoned mode. The vulnerability manifests as a kernel hang due to tasks being blocked indefinitely while waiting on locked extents that are never unlocked. This occurs because the code path handling the case when no active zone is available (btrfs_reserve_extent() returning -EAGAIN) was changed in commit 06f364284794, moving the unlock logic outside a loop and causing allocated extents to remain locked if partial allocations had been made. The bug leads to kernel worker threads being stuck in a blocked state for extended periods, resulting in system hangs or degraded performance. The root cause is a missing unlock call for extents in the failure path where some allocations exist but the function must bail out to allow the caller to issue IOs to finish zones. The fix involves adjusting the unlock range to cover the allocated extents properly before exiting the loop, ensuring all locks are released. This vulnerability affects Linux kernel versions containing the specified commits and impacts systems using Btrfs with zoned block device support enabled, which is common in storage systems leveraging Zoned Namespace (ZNS) devices. No known exploits are reported in the wild as of publication.

For European organizations, the impact of CVE-2025-21942 can be significant in environments relying on Linux servers with Btrfs filesystems on zoned block devices, such as advanced storage arrays or data centers using ZNS SSDs for performance and endurance benefits. The vulnerability can cause kernel hangs and blocked worker threads, leading to system instability, degraded performance, or downtime. This can disrupt critical services, data processing, and storage operations, especially in sectors like finance, telecommunications, cloud service providers, and public infrastructure where Linux-based storage solutions are prevalent. The hang condition may necessitate system reboots, causing availability issues and potential data loss if not properly managed. Although no direct data corruption or privilege escalation is indicated, the denial of service effect on storage subsystems can indirectly impact confidentiality and integrity by interrupting normal operations and backups. The lack of a known exploit reduces immediate risk, but the complexity of the bug and its presence in kernel code means that targeted attackers or accidental triggers could cause outages. Organizations with high availability requirements and those using Btrfs on zoned devices should prioritize patching to maintain operational continuity.

1. Apply the official Linux kernel patches that fix the extent unlock logic in cow_file_range() as soon as they become available from trusted Linux kernel sources or distributions. 2. For organizations using custom or long-term support kernels, backport the fix to affected kernel versions to prevent hangs. 3. Monitor kernel logs for hung task messages related to btrfs worker threads and extent locking to detect potential exploitation or triggering of this vulnerability. 4. Consider temporarily disabling zoned block device support in Btrfs if feasible, or avoid using Btrfs on zoned devices until patched. 5. Implement robust system monitoring and alerting to detect kernel hangs early and automate safe reboots or failover procedures to minimize downtime. 6. Validate storage device firmware and drivers are up to date to reduce interaction issues with zoned devices. 7. Conduct thorough testing of kernel updates in staging environments that mirror production storage configurations to ensure stability before deployment. 8. Educate system administrators on the symptoms of this hang condition and response procedures to reduce incident response time.