CVE-2026-89642
Summary
by MITRE • 09/11/2026
In the Linux kernel, the following vulnerability has been resolved:
cifs: call pagecache_isize_extended() in cifs_setsize() when extending
cifs_setsize() calls truncate_pagecache() but skips pagecache_isize_extended() on extension. truncate_setsize() shows the correct pattern:
i_size_write(inode, newsize); if (newsize > oldsize) pagecache_isize_extended(inode, oldsize, newsize); truncate_pagecache(inode, newsize);
pagecache_isize_extended() zeroes the tail of the page straddling old EOF. Without it, dirty bytes in that region can be written back to the server, exposing stale data in the newly extended range.
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Analysis
by VulDB Data Team • 09/11/2026
The Common Internet File System client within the Linux kernel contains a logic error in the cifs_setsize function that leads to potential information disclosure through exposure of stale file system data. This vulnerability arises from an incorrect sequence of operations when extending the size of a file on a CIFS share. Specifically, while the function correctly invokes truncate_pagecache to adjust the page cache boundaries, it fails to call pagecache_isize_extended before performing this truncation. In standard Linux kernel VFS patterns, such as those observed in truncate_setsize, the inode size is updated and any partial pages at the end of the file are zeroed out via pagecache_isize_extended prior to or during the cache adjustment process. The omission of this critical step means that the tail portion of a page straddling the old end-of-file boundary retains its previous contents rather than being initialized with zeros.
The operational impact of this flaw is significant for data integrity and confidentiality within networked file systems. When an application extends a file, it expects any newly allocated space to be zero-filled or explicitly written by the user. However, due to the missing call, dirty bytes from before the extension remain in memory associated with that page range. If these pages are subsequently flushed back to the remote CIFS server during write-back operations, the stale data previously residing beyond the old file size is transmitted and persisted on the storage backend. This results in a scenario where sensitive information or random kernel memory contents can be exposed through subsequent reads of the extended portion of the file by other processes or users with appropriate access rights.
This vulnerability aligns with CWE-200, which classifies exposure of sensitive information to an unauthorized actor, and specifically relates to improper initialization of data structures leading to leakage of prior state. From a threat modeling perspective using the MITRE ATT&CK framework, this behavior facilitates Information Discovery through Local Data Access, allowing an attacker who can trigger file size extensions on affected systems to read unintended memory contents that were not meant to be part of the current file content. The root cause is identified as CWE-665, improper initialization, where the kernel fails to properly zero out the newly extended region before making it accessible or writable in a way that persists stale data.
Mitigation for this issue requires applying the upstream Linux kernel patch that corrects the cifs_setsize function by inserting the call to pagecache_isize_extended at the appropriate point in the execution flow, ensuring that any partial pages are zeroed out consistent with VFS expectations. System administrators should ensure their systems are updated with the latest stable kernel versions containing this fix. Until patches are applied, limiting write access to files on CIFS mounts and monitoring for unusual file size extensions may reduce the attack surface, although complete prevention is only possible through code remediation as provided in the official vendor updates.