CVE-2021-47113 in Linux
Summary
by MITRE • 03/15/2024
In the Linux kernel, the following vulnerability has been resolved:
btrfs: abort in rename_exchange if we fail to insert the second ref
Error injection stress uncovered a problem where we'd leave a dangling inode ref if we failed during a rename_exchange. This happens because we insert the inode ref for one side of the rename, and then for the other side. If this second inode ref insert fails we'll leave the first one dangling and leave a corrupt file system behind. Fix this by aborting if we did the insert for the first inode ref.
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Analysis
by VulDB Data Team • 03/13/2025
The vulnerability identified as CVE-2021-47113 resides within the Linux kernel's btrfs file system implementation, specifically addressing a critical flaw in the rename_exchange operation that can lead to file system corruption and data integrity issues. This vulnerability manifests when the btrfs file system attempts to exchange two file system objects through a rename operation while simultaneously managing inode references. The flaw occurs during the complex process of maintaining file system metadata consistency across multiple operations, where the kernel's handling of reference insertion fails to properly account for partial operation failures.
The technical root cause of this vulnerability stems from the btrfs file system's implementation of the rename_exchange function, which involves inserting inode references for both sides of the exchange operation. When the first inode reference is successfully inserted but the second insertion fails due to resource constraints, memory allocation issues, or other operational failures, the system leaves behind a dangling inode reference. This creates an inconsistent state where the file system metadata becomes corrupted, as the first reference remains in place while the second reference fails to materialize. The error injection stress testing methodology used to uncover this vulnerability demonstrates how systematic failure conditions can expose race conditions and improper state management in file system operations, particularly in high-concurrency environments where multiple processes may be attempting similar operations simultaneously.
The operational impact of CVE-2021-47113 extends beyond simple data corruption to potentially compromise entire file system structures and render data inaccessible. When a btrfs file system becomes corrupted in this manner, it can lead to data loss, system instability, and the need for extensive recovery procedures including file system checks, journal replay operations, and potentially complete data restoration from backups. The vulnerability affects systems running Linux kernels with btrfs file systems that support the rename_exchange functionality, which is commonly used in various file management operations including file moves, directory renames, and atomic file system exchanges. This flaw particularly impacts enterprise environments and server deployments where btrfs is actively used for data storage, as it can lead to service interruptions and data integrity breaches during routine file system operations.
Security implications of this vulnerability align with CWE-119 Improper Access to Resources Through Violation of Locking Protocol, as the flaw represents a failure in proper resource management during atomic operations. The vulnerability also demonstrates characteristics consistent with ATT&CK technique T1485 Data Destruction, as corrupted file systems can result in data loss and system instability. Mitigation strategies should focus on applying the kernel patch that implements proper abort behavior when inode reference insertion fails, ensuring that all reference operations within rename_exchange are either fully completed or completely rolled back. System administrators should prioritize updating affected Linux kernels to versions containing the fix, while also implementing monitoring for file system corruption indicators and maintaining regular backup procedures to minimize impact from potential exploitation of this vulnerability. The fix requires careful implementation of transactional semantics where the system aborts the entire rename_exchange operation if any individual reference insertion fails, preventing partial state modifications that could lead to file system inconsistency.