Cloudanix VulnDB

CVE-2026-98844

None

CVSS

—

EPSS

0.2%

Published

Oct 18, 2026

CISA due date

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Cloudanix analysis

Schedule a fix SSVC: Track

A kernel file-handling error can leave old data visible after a file is made larger, potentially letting one user see information left by another.

Exploitation
Local Privilege
Attacker needs
Local User
Complexity
Moderate
Impact
Data Exposure
Component
OS kernel
Class
Information Disclosure
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

The vulnerability leaves stale pagecache contents from prior mmap writes beyond EOF uncleared during extending buffered or direct writes, allowing residual data to persist in the folio straddling the new EOF.

Why it matters to the business

Systems relying on netfs-based filesystems such as CIFS may expose residual file data to unauthorized readers after file-extension operations, increasing confidentiality risk in shared or multi-tenant storage environments.

How an attack unfolds

An attacker first creates a hole past EOF by writing through an mmap region, then triggers an extending write from another process or thread; because neither write path clears the straddling folio, the stale data remains readable or writable by subsequent operations.

How to fix it

Apply the upstream kernel commit that introduces netfs_clear_stale_post_isize() and zeros the tail of the straddling folio under the folio lock; CIFS users must also incorporate the exported helper. Rebuild and deploy the patched kernel; no configuration changes are required.

How to confirm the fix worked

Confirm the presence of netfs_clear_stale_post_isize() and the zeroing logic in the write paths; run fstests generic/363 to verify the stale-pagecache behavior is eliminated.

Typical attack path

  1. Map a region of a file and write data past the current EOF to create a dirty hole.
  2. Trigger a concurrent or subsequent extending write via buffered, DIO, or unbuffered path while i_rwsem is held shared.
  3. Read or otherwise access the newly extended region to observe the uncleared stale folio contents.

How to detect it

Examine kernel logs for unusual folio operations around file-extension writes; correlate with application-level mmap and write activity on netfs-backed filesystems; no public exploit artifacts are known.

What your security team should do

  • Prioritize kernel updates for any system using netfs or CIFS within the next maintenance window.
  • Monitor upstream stable branches for backports of the fix.
  • Review application code that performs mmap writes followed by extending writes for exposure to residual data.

The record

In the Linux kernel, the following vulnerability has been resolved: netfs: clear post-EOF pagecache when extending a file via write Fix netfs to erase the contents of a hole created after the EOF by an ordinary write if dirty data has been previously left there by writes through an mmapped region. Neither the buffered nor the unbuffered/DIO write path clears that stale pagecache. Zero the tail of the folio straddling the EOF before an extending write. That is the only folio that can hold data written past the EOF through an mmap, as pages wholly beyond the EOF can't be faulted in. The folio is zeroed rather than dropped so a concurrent extending write can't lose data. Both write paths downgrade the i_rwsem to shared, so extending writes can run concurrently and the i_size read by the caller may be stale by the time the folio is locked. Re-read i_size under the folio lock and clamp the zeroed range up to it, so a racing write that already put data into the folio isn't clobbered. Wait for any writeback on the folio to finish before zeroing it so that the pagecache isn't modified while it may still be read by the transport during transmission. Honour IOCB_NOWAIT by returning -EAGAIN rather than blocking on the folio lock, on writeback, or in folio_mkclean()'s rmap walk when the folio is mapped. truncate_pagecache() can't be used here: it must be called with the i_rwsem held exclusively, but these write paths only hold it shared, and it would block unconditionally, breaking IOCB_NOWAIT. Callers that hold i_rwsem exclusively for the whole resize (truncate, setattr, fallocate, clone) exclude any genuine concurrent buffered writer, so staleness can instead be decided from the folio's dirty state, as pagecache_isize_extended() already does for filesystems that serialise writes against truncate/setattr via a single i_rwsem. Export netfs_clear_stale_post_isize() helper to handle such case. The helper is required by the CIFS client to fix generic/363.

Affected packages

Ecosystem Package Introduced Fixed