CVE-2026-98975 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
xfs: stop exchanging reflink flags when exchanging mappings
When an exchange covers both files from offset 0 to i_disk_size, xmi_can_exchange_reflink_flags() assumes every mapping moved and hands the reflink flag over to the other file.
That assumption is wrong. A file can still own shared blocks past i_disk_size, e.g. after EXCHANGE_RANGE_TO_EOF sets a smaller size without unmapping anything. The flag is cleared anyway, and the next write to such a block happens in place instead of CoW, corrupting the other file that shares it.
Commit a23eca88448e ("xfs: fix exchange-range reflink flag clearing issue with INO1_WRITTEN") fixed one instance of this. Rather than add another special case, stop moving the flags. xfs_exchmaps_ensure_reflink() already sets the flag on both inodes before the exchange, so both just keep it.
Nothing sets CLEAR_INO{1,2}_REFLINK anymore. Intents logged by older
kernels may still carry them; recovery already masks them off, so they stay in the log format mask and are ignored.
XFS_IOC_SWAPEXT has the same problem: it moves the reflink flag from one inode to the other along with the extents. Just removing that is not enough here: the exchange path flags both inodes up front in xfs_exchmaps_ensure_reflink(), but swapext has no such step, so the inode receiving the shared extents would be left without the flag. Set it on both inodes instead, and make sure both have a CoW fork, since an inode with the reflink flag is expected to have one.
The flag now stays until FALLOC_FL_UNSHARE_RANGE, xfs_scrub, or a truncate to zero removes it.
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Analysis
by VulDB Data Team • 10/23/2026
This vulnerability resides within the XFS filesystem implementation in the Linux kernel and specifically affects the handling of copy-on-write (CoW) semantics during file extent exchanges. The core issue arises from an incorrect assumption made by the xmi_can_exchange_reflink_flags function when processing exchange operations that cover the entire range from offset zero to i_disk_size. In these scenarios, the code erroneously transfers the reflink flag from one inode to another based on the premise that all mappings have been moved. This logic fails to account for files that retain shared blocks beyond their declared disk size, a state often created when operations like EXCHANGE_RANGE_TO_EOF reduce the file size without unmapping the underlying data structures. Consequently, the receiving inode loses its reflink flag despite still holding references to shared extents.
The operational impact of this flaw is severe and involves direct data integrity corruption. When an application subsequently writes to one of these shared blocks in the affected file, the system treats it as a standard write operation rather than initiating a copy-on-write process because the necessary reflink indicator has been incorrectly cleared. This results in overwriting data that is simultaneously referenced by another inode sharing those physical blocks. The consequence is silent corruption where changes intended for one file inadvertently alter the content of another unrelated or related file, violating fundamental isolation guarantees and potentially leading to application crashes, loss of critical data, or security breaches depending on the sensitivity of the shared information.
The root cause lies in a mismatch between how extent mappings are exchanged versus how metadata flags governing those extents are managed across different system calls. While XFS_IOC_SWAPEXT exhibits similar behavior by moving the reflink flag along with extents without ensuring both inodes retain appropriate CoW fork structures, the primary exchange path suffers from premature flag clearing. Previous attempts to mitigate this involved adding specific special cases for certain conditions, such as fixing issues related to INO1_WRITTEN status. However, these piecemeal fixes proved insufficient because they did not address the fundamental architectural flaw in assuming that extent movement necessitates flag transfer. The vulnerability persists until explicit unsharing or truncation operations occur, leaving a window of exposure during standard file exchange workflows.
To resolve this issue, the kernel logic has been modified to stop exchanging reflink flags entirely during mapping exchanges. Instead of moving the attribute from one inode to another, both inodes involved in the operation are now explicitly set with the reflink flag prior to the exchange via xfs_exchmaps_ensure_reflink. This ensures that any inode receiving shared extents retains the necessary metadata to trigger copy-on-write behavior upon subsequent writes. For XFS_IOC_SWAPEXT, which lacks this preparatory step, the fix involves manually setting the flag on both inodes and ensuring they possess a CoW fork if required by their new state. The reflink attribute now remains stable until explicitly removed through operations like FALLOC_FL_UNSHARE_RANGE, xfs_scrub utilities, or truncation to zero size, thereby maintaining consistency between physical block sharing and logical file attributes.
From a vulnerability classification perspective, this flaw aligns with CWE-20 Improper Input Validation as the kernel fails to correctly validate the relationship between extent mappings and metadata flags during complex I/O operations. It also relates to CWE-674 Uncontrolled Resource Consumption in terms of data integrity management, though more accurately it falls under CWE-835 Loop with Unreachable Exit Condition if viewed through the lens of state machine logic where the exit condition for flag clearing is incorrectly triggered. In the context of the MITRE ATT&CK framework, this vulnerability facilitates Data Manipulation techniques such as T1490 Inhibit System Recovery or more broadly data corruption tactics that undermine system reliability and integrity. The fix ensures adherence to proper resource management principles by decoupling metadata state transitions from physical extent movements unless explicitly required by unshare operations.
Mitigation strategies for administrators involve applying the latest kernel patches that include this specific XFS correction. Systems running older kernels without these fixes remain at risk of silent data corruption during high-volume file exchange or swapext operations involving shared extents. Regular use of xfs_scrub tools can help detect existing inconsistencies in filesystem metadata, although they do not prevent the initial occurrence of the bug. Developers integrating with XFS should ensure that applications handling sensitive data are aware of these CoW semantics and avoid relying on implicit flag states during atomic exchange operations until patched systems are deployed across all nodes in a cluster or storage environment.