CVE-2026-72367 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
iomap: guard io_size EOF trim against concurrent truncate underflow
iomap: fix zero padding data issue in concurrent append writes changed ioend accounting so that io_size tracks only valid data within EOF. This trims io_size when a writeback range extends past end_pos:
ioend->io_size += map_len; if (ioend->io_offset + ioend->io_size > end_pos) ioend->io_size = end_pos - ioend->io_offset;
However, if end_pos ends up below ioend->io_offset, the subtraction becomes negative and is stored in size_t io_size, causing an unsigned wrap to a huge value. This can happen when writeback continues past byte-level EOF up to a block-aligned range, or when a concurrent truncate shrinks the file after end_pos was sampled in iomap_writeback_handle_eof().
A wrapped io_size can mislead append detection and corrupt completion-time size handling, since filesystem end_io paths consume io_size for decisions such as on-disk EOF updates and unwritten/COW completion ranges.
Fix this by clamping io_size to zero when EOF has moved to or before the ioend start offset. This preserves the original intent of trimming io_size to valid in-EOF data while avoiding the underflow.
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Analysis
by VulDB Data Team • 08/15/2026
This vulnerability exists within the linux kernel's iomap subsystem and represents a critical flaw in how file writeback operations handle end-of-file boundaries during concurrent file modifications. The issue stems from improper handling of io_size calculations when dealing with concurrent truncate operations and writeback ranges that extend beyond the current file end position. The vulnerability manifests when multiple processes or threads simultaneously attempt to modify a file's contents while another operation is processing writeback, creating race conditions that can result in corrupted data structures and incorrect file size tracking.
The technical flaw occurs in the iomap_writeback_handle_eof() function where io_size is calculated based on map_len additions and subsequent trimming based on end_pos. When concurrent truncate operations occur between the time end_pos is sampled and when writeback completes, the end_pos value can become smaller than ioend->io_offset, causing integer underflow in the size_t data type. This unsigned integer wrap produces an extremely large positive value instead of a negative result, which then gets stored in io_size and propagates through subsequent filesystem operations. The vulnerability specifically affects the ioend accounting mechanism that tracks valid data within file boundaries, creating a scenario where invalid data ranges are treated as legitimate write operations.
The operational impact of this vulnerability is significant for systems relying on concurrent file access patterns, particularly in database applications, web servers, and storage systems where multiple processes frequently append data to files while other operations may truncate or modify file contents. When io_size wraps to an enormous value due to the underflow condition, it can cause incorrect append detection logic to misidentify valid data ranges, leading to corrupted on-disk file metadata and potentially causing filesystem inconsistencies. The corruption affects completion-time size handling where filesystem end_io paths use io_size values for decisions regarding on-disk EOF updates, unwritten/COW completion ranges, and other critical file system operations.
This vulnerability aligns with CWE-191, which addresses unsigned integer underflow conditions, and specifically relates to ATT&CK technique T1485, where adversaries might exploit file system inconsistencies to gain unauthorized access or cause data corruption. The fix implemented involves clamping io_size to zero when the end position moves to or before the ioend start offset, preventing the unsigned wrap while maintaining the original intent of trimming io_size to valid in-EOF data. This mitigation ensures that concurrent truncate operations cannot corrupt the writeback accounting mechanism, preserving file system integrity and preventing potential data loss scenarios.
The vulnerability demonstrates a classic race condition in concurrent systems where proper synchronization between file size modifications and ongoing writeback operations is insufficient. The fix addresses this by ensuring that io_size calculations remain within valid bounds regardless of concurrent modifications to file boundaries, thus maintaining the correctness of file size tracking throughout all stages of file I/O operations. This correction prevents malicious or accidental corruption of file metadata that could otherwise lead to data inconsistency issues and potential security vulnerabilities in systems processing concurrent file access patterns.