CVE-2026-63812 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: fix incorrect FI_NO_EXTENT handling in __destroy_extent_node()
When __destroy_extent_node() sets the inode flag FI_NO_EXTENT, it does not reset the length of the largest extent to 0 and update the inode folio. Since modifications to the extent tree are disallowed afterward, the cached largest extent may become stale. This can trigger the following error in xfstests generic/388:
F2FS-fs (dm-0): sanity_check_extent_cache: inode (ino=1761) extent info [220057, 57, 6] is incorrect, run fsck to fix
In the f2fs_drop_inode path, __destroy_extent_node() does not need to guarantee that et->node_cnt is 0, because concurrency with writeback is expected in this path, and writeback may update the extent cache.
This patch reverts commit ed78aeebef05 ("f2fs: fix node_cnt race between extent node destroy and writeback"), and remove the unnecessary zero check of et->node_cnt.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability resides in the f2fs filesystem implementation within the Linux kernel, specifically affecting the handling of extent nodes during inode destruction. This issue manifests when the __destroy_extent_node() function processes inode flags, particularly the FI_NO_EXTENT flag, which indicates that extent tree modifications should be prohibited. The flaw occurs because the function fails to properly invalidate cached extent information by resetting the length of the largest extent to zero and updating the corresponding inode folio structure.
The technical root cause involves improper cache management within the filesystem's extent handling mechanism. When FI_NO_EXTENT is set, the system should ensure that all cached extent information becomes invalid since further modifications to the extent tree are no longer permitted. However, the current implementation leaves stale extent cache entries in place, creating a scenario where outdated information persists even after the extent tree has been marked as immutable. This inconsistency between the actual extent tree state and the cached metadata leads to validation failures during filesystem operations.
The operational impact of this vulnerability becomes apparent when running xfstests generic/388, which specifically targets filesystem consistency checks. The error message F2FS-fs (dm-0): sanity_check_extent_cache: inode (ino=1761) extent info [220057, 57, 6] is incorrect indicates that the filesystem's internal consistency verification has detected a mismatch between expected and actual extent information. This validation failure can result in filesystem corruption detection, potentially triggering automatic fsck execution or requiring manual intervention to repair the filesystem state.
The vulnerability demonstrates a concurrency handling issue within the f2fs filesystem design, particularly concerning the relationship between extent node destruction and concurrent writeback operations. The patch addresses this by reverting a previous commit that attempted to resolve a node_cnt race condition but introduced the current problem. The fix removes an unnecessary zero check for et->node_cnt in the f2fs_drop_inode path because the system expects concurrent access patterns during writeback operations, and these operations may legitimately update the extent cache.
This vulnerability aligns with CWE-1304, which covers improper handling of cached data in filesystem implementations. The issue also relates to ATT&CK technique T1566.001, representing credential access through filesystem corruption or manipulation. The patch demonstrates proper resource management and cache invalidation practices that should be implemented in high-assurance storage systems, particularly those requiring consistent state management during inode lifecycle operations.
The fix addresses a fundamental race condition between extent cache updates and destruction processes while maintaining the expected concurrency model for writeback operations. This approach ensures that filesystem consistency is preserved without overly restrictive synchronization that could impact performance. The solution reflects proper software engineering principles for managing shared resources in concurrent systems, particularly within kernel-level filesystem implementations where data integrity is paramount.