CVE-2026-90266 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
btrfs: zoned: don't force read-only on transient -EAGAIN from reloc merge
On a zoned FS, btrfs_delayed_refs_rsv_refill() returns -EAGAIN whenever the over-committed metadata plus the zone_unusable bytes exceeds the usable size in a metadata block-group to avoid heavy over-commit of metadata and early ENOSPC in one transaction.
If this happens while doing reclaim, the transaction is getting aborted.
Treat -EAGAIN as a soft, retryable condition in case of block-group reclaim.
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability described involves a logic error within the btrfs file system implementation for zoned storage devices in the Linux kernel. Specifically, it addresses an issue where transient errors during metadata management lead to unnecessary and potentially disruptive state changes. In zoned filesystems like btrfs, data is written sequentially into zones that must be managed carefully to avoid over-commitment of resources. The function btrfs_delayed_refs_rsv_refill plays a critical role in this process by checking whether the combination of over-committed metadata and zone unusable bytes exceeds the usable size within a specific metadata block group. When this threshold is crossed, the system returns an -EAGAIN error code to signal that the operation should be retried later rather than failing immediately due to space constraints.
The core technical flaw lies in how this transient -EAGAIN condition was handled during transaction processing and reclaim operations. Previously, if the btrfs_delayed_refs_rsv_refill function returned -EAGAIN while a reclaim process was active, the kernel would interpret this as a hard failure rather than a temporary resource contention issue. Consequently, the entire transaction associated with that operation would be aborted. This behavior is problematic because it treats a soft, retryable condition indicative of momentary pressure on metadata resources as a fatal error requiring immediate termination of the current filesystem transaction. Such abrupt aborts can lead to unnecessary performance degradation or even data consistency issues if not handled gracefully through retries rather than hard failures.
From an operational impact perspective, this vulnerability primarily affects systems utilizing zoned block devices with btrfs file systems under heavy metadata load conditions. When these transient -EAGAIN errors occur during reclaim cycles, the forced transaction abortion can cause significant latency spikes or temporary unavailability of filesystem operations. In worst-case scenarios involving frequent occurrences, it could lead to perceived system instability or reduced throughput as transactions are repeatedly aborted and restarted without proper backoff mechanisms. This is particularly relevant for environments where zoned storage devices such as SMR hard drives or ZNS SSDs are used with btrfs, which relies heavily on precise metadata management to maintain performance and integrity across sequential write patterns.
The resolution involves modifying the error handling logic within the reclaim path of the btrfs codebase. Instead of treating -EAGAIN returned from btrfs_delayed_refs_rsv_refill as a condition that necessitates transaction abortion during block-group reclaim, the system now recognizes it as a soft and retryable state. This allows the filesystem to gracefully handle temporary resource contention by deferring or re-attempting operations rather than forcing immediate failure states. By distinguishing between transient space pressure conditions and genuine fatal errors, btrfs can maintain stability under load while preserving transactional integrity without unnecessary aborts that disrupt ongoing I/O patterns.
This fix aligns with best practices for handling retryable error codes in kernel subsystems where resource contention is expected during high-load scenarios. It reflects an understanding of the nuanced differences between hard failures requiring rollback and soft conditions warranting backoff or deferral strategies. The change ensures that btrfs behaves more predictably on zoned devices by avoiding aggressive transaction termination when faced with temporary metadata reservation limits being exceeded due to normal operational fluctuations rather than actual corruption or permanent space exhaustion.
In terms of industry standards, this issue relates broadly to CWE-390 which deals with detecting and handling error conditions improperly. The improper treatment of a transient -EAGAIN return value as a fatal condition constitutes an inadequate error detection mechanism that leads to unnecessary service disruption. Furthermore, the context involves resource management within storage subsystems where proper sequencing and retry logic are essential for maintaining availability guarantees. While not directly mapping to specific ATT&CK techniques since this is a stability rather than security exploit issue, it underscores the importance of robust error handling in kernel-level components to prevent denial-of-service-like effects caused by poor state machine transitions during resource contention events.
Mitigation strategies involve applying the provided patch that updates the btrfs reclaim logic to properly handle -EAGAIN conditions as retryable states. System administrators managing zoned storage devices with btrfs should ensure their kernels are updated to include this fix, particularly if they experience intermittent performance issues or unexpected transaction aborts under heavy metadata workloads. Regular monitoring of filesystem health and I/O patterns can help identify whether such transient errors were occurring prior to the patch application. Additionally, understanding the behavior of zoned devices in conjunction with btrfs metadata reservation mechanisms allows for better capacity planning and workload distribution to minimize occurrences where over-commitment thresholds are frequently approached during reclaim operations.