CVE-2026-53017 in Linux
Summary
by MITRE • 06/24/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: fix data loss caused by incorrect use of nat_entry flag
Data loss can occur when fsync is performed on a newly created file (before any checkpoint has been written) concurrently with a checkpoint operation. The scenario is as follows:
create & write & fsync 'file A' write checkpoint - f2fs_do_sync_file // inline inode - f2fs_write_inode // inode folio is dirty - f2fs_write_checkpoint - f2fs_flush_merged_writes - f2fs_sync_node_pages - f2fs_flush_nat_entries - f2fs_fsync_node_pages // no dirty node - f2fs_need_inode_block_update // return false SPO and lost 'file A'
f2fs_flush_nat_entries() sets the IS_CHECKPOINTED and HAS_LAST_FSYNC flags for the nat_entry, but this does not mean that the checkpoint has actually completed successfully. However, f2fs_need_inode_block_update() checks these flags and incorrectly assumes that the checkpoint has finished.
The root cause is that the semantics of IS_CHECKPOINTED and HAS_LAST_FSYNC are only guaranteed after the checkpoint write fully completes.
This patch modifies f2fs_need_inode_block_update() to acquire the sbi->node_write lock before reading the nat_entry flags, ensuring that once IS_CHECKPOINTED and HAS_LAST_FSYNC are observed to be set, the checkpoint operation has already completed.
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Analysis
by VulDB Data Team • 06/24/2026
The vulnerability in the Linux kernel's f2fs filesystem implementation stems from a critical race condition involving concurrent file system operations that can result in permanent data loss. This issue specifically affects scenarios where a newly created file undergoes an fsync operation while a checkpoint process is simultaneously running, creating a complex interaction between multiple kernel subsystems including inode management, node page handling, and checkpoint synchronization mechanisms. The problem manifests when the filesystem attempts to determine whether inode block updates are necessary during the fsync operation, leading to incorrect assumptions about checkpoint completion status.
The technical flaw resides in the improper interpretation of nat_entry flags within the f2fs_need_inode_block_update() function, which directly violates fundamental concurrency control principles. When f2fs_flush_nat_entries() executes, it sets both IS_CHECKPOINTED and HAS_LAST_FSYNC flags on nat_entry structures, but these flags merely indicate that a checkpoint operation has been initiated rather than successfully completed. The function incorrectly treats these preliminary flag states as definitive proof of checkpoint completion, creating a dangerous assumption that leads to data loss scenarios. This violates the established principle that flag semantics should only be considered reliable after all associated operations have fully completed, not during their intermediate execution phases.
The operational impact of this vulnerability extends beyond simple data corruption to encompass complete file loss during what should be routine file system operations. The race condition occurs specifically when a file is created, written to, and synchronized using fsync before any checkpoint has been written to disk, making it particularly dangerous for applications requiring immediate data persistence guarantees. The sequence of events involves multiple kernel functions working in parallel including f2fs_do_sync_file, f2fs_write_inode, f2fs_write_checkpoint, and various node page management operations, where the timing window between flag setting and actual checkpoint completion creates the vulnerability. This scenario represents a classic concurrency bug that can affect any application relying on f2fs filesystem consistency guarantees, particularly those performing frequent fsync operations on newly created files.
The proposed patch addresses this issue by implementing proper locking mechanisms around the nat_entry flag reading operations within f2fs_need_inode_block_update(). By acquiring the sbi->node_write lock before examining these flags, the implementation ensures that any observation of IS_CHECKPOINTED and HAS_LAST_FSYNC being set guarantees that the checkpoint operation has already fully completed. This approach aligns with established security practices for concurrent system design and follows the principle of least privilege in kernel space operations. The solution directly addresses the root cause identified through CWE-362, which classifies this as a race condition vulnerability where proper synchronization is missing between concurrent access paths.
Security implications extend to potential exploitation scenarios where malicious actors could craft specific workloads to trigger this race condition repeatedly, leading to systematic data loss across multiple files. The vulnerability affects the integrity and availability aspects of the f2fs filesystem, particularly in high-concurrency environments where simultaneous fsync and checkpoint operations are common. The ATT&CK framework categorizes this under privilege escalation through resource exhaustion and data manipulation techniques, though the actual attack vector requires careful orchestration of specific system states. This vulnerability demonstrates the critical importance of proper synchronization primitives in kernel code and highlights the need for comprehensive testing of concurrent scenarios in filesystem implementations. The patch implementation ensures that all flag interpretations occur within proper critical sections, preventing the incorrect assumption that partially completed operations have fully succeeded.