[CVE-2026-64063][MODERATE REGULAR] netfs: Fix streaming write being overwritten [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-64063
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfs: Fix streaming write being overwritten [ Upstream
Commit: 20195925c768626dc901a4781a51e508702c88ad
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=20195925c768626dc901a4781a51e508702c88ad
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64063
Analysis date: Sun, 19 Jul 2026 16:16:23 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Ordinary Moderate / Borderline data-integrity issue, not Important-class
Manual review required: NO

Summary:
A stale netfs_folio dirty range state in netfs streaming writes can cause incorrect file contents or lost write data when a partially dirty folio is later overwritten by a full write on affected netfs clients such as CIFS.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64063 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64063

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE REGULAR
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64063	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L';*CWE-664;CWE-665;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.4';DESCR 'netfs streaming writes can leave stale netfs_folio dirty range metadata when a partially dirty folio is later overwritten by a full write. This can make the client track the wrong dirty region and can result in incorrect file contents or lost write data on affected netfs users such as CIFS with caching. For the CVSS the PR:L because a local user or process needs the ability to write to files on the mounted network filesystem, but administrator privileges are not normally required for ordinary file writes. The issue is not directly network reachable as an attack against the kernel from packet traffic. It is triggered through local filesystem write operations on an already mounted netfs filesystem. Impact is mainly integrity loss for file data and possible limited availability impact due to corrupted or unusable file contents. There is no strong evidence of kernel memory corruption, UAF after free, or privilege escalation from this patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Borderline data-integrity issue, not Important-class (with actual score 4)  SKIP CVE-2026-64063 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 8f52de0077ba3bf41e5d53d67a185700f41efce7 YES  NO NO unknown 	MAYBE	REMOTE WRITE OOB DISK LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Ordinary Moderate / Borderline data-integrity issue, not Important-class**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local user or process that can write to files on an already mounted netfs/CIFS filesystem can plausibly trigger the affected write pattern.
* Filesystem/storage data corruption reachable by an unprivileged local user: +1. The bug can produce incorrect file contents or lost write data through stale dirty-range tracking.
* Integrity impact plausible: +1. The demonstrated effect is client-side file data inconsistency.

Paranoid additional signal:

* Important filesystem/storage path: +1. The affected path is netfs buffered write handling, used by network filesystem clients such as CIFS, and impacts writeback/page-cache style data consistency.

Signals not applied:

* No generic memory corruption. The patch fixes stale netfs_folio bookkeeping and dirty range handling, not UAF, OOB write, type confusion, double-free, or arbitrary write.
* No Dirty-Pipe-like ownership/COW bypass. The issue corrupts data written through the same mounted filesystem context, not protected file-backed pages owned by another security domain.
* No remote/network-triggerable score. Although CIFS/netfs is network filesystem code, the trigger is local file write behavior, not attacker-controlled packets against the kernel.
* No privilege-escalation score. There is no supported path to kernel code execution or credential manipulation.

## 3. Reachability analysis

The realistic trigger is a local process performing specific write and copy-range patterns on a mounted netfs user such as CIFS with caching. Administrator privileges are not normally required for ordinary writes once the filesystem is mounted, so PR-like practical reachability is local unprivileged for users with write permission to the affected files.

Namespaces and containers may matter only if the mounted filesystem is exposed into the container and the attacker has write permission there. This does not make the bug remote in the kernel sense.

The path is not universally default for all systems, but CIFS/netfs deployments are common enough that no rare-subsystem penalty is warranted. Call-site confidence: high, because the affected function and surrounding write-state transitions are included.

## 4. Severity interpretation

This behaves like an ordinary Moderate filesystem data-integrity bug, with a borderline actionable score only because it affects a real write path and can corrupt file contents. The theoretical impact is limited to data integrity and possibly operational disruption from bad file contents. The patch does not support treating this as kernel memory corruption, LPE, or Important-class vulnerability.

## 5. One-sentence report phrase

A stale netfs_folio dirty range state in netfs streaming writes can cause incorrect file contents or lost write data when a partially dirty folio is later overwritten by a full write on affected netfs clients such as CIFS.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue is a local filesystem data-consistency bug without evidence of memory corruption, privilege escalation, sensitive-object theft, or remote kernel attack surface.

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: netfs: Fix streaming write being overwritten [ Upstream
Commit: 20195925c768626dc901a4781a51e508702c88ad
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=20195925c768626dc901a4781a51e508702c88ad

Changed files:
  fs/netfs/buffered_write.c
  include/trace/events/netfs.h

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=20195925c768626dc901a4781a51e508702c88ad

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DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-64063 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64063

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 4.4

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 3
  Paranoid ActionableScore: 4

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).