[CVE-2026-64158][LOW] netfs: Fix write streaming disablement if fd open O_RDWR [ Upstream

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

CVE: CVE-2026-64158
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: netfs: Fix write streaming disablement if fd open O_RDWR [ Upstream
Commit: 9adf8e47d73d5e3c2fe77dea649dcde350ccd65c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9adf8e47d73d5e3c2fe77dea649dcde350ccd65c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64158
Analysis date: Sun, 19 Jul 2026 15:37:29 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
netfs incorrectly disabled write streaming for O_RDWR or readable file descriptors even though read_folio can fill missing folio ranges, causing local buffered write correctness issues without evidence of memory corruption, privilege escalation, or remote reachability.

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

The original Linux kernel CVE announcement for CVE-2026-64158 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64158

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: LOW
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-2026-64158	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';*CWE-664;CWE-682;CWE-404;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:N';BEST CVSS score: '3.6';DESCR 'netfs write streaming was disabled when the file descriptor was open for reading, including O_RDWR, even though netfs can now fill missing folio ranges through read_folio. This can cause incorrect buffered write behavior or unnecessary fallback paths in local file operations on netfs based filesystems. For the CVSS the PR:L is used because a local process must perform file operations on an affected mounted filesystem. The issue is not directly network reachable because it is triggered through local VFS write paths rather than remote packet input. No memory corruption, UAF, OOB access, information leak, or privilege escalation primitive is indicated by the patch context. Impact appears to be correctness or availability limited to the affected file operation, with no clear security boundary crossing.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-64158 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: c38f4e96e605f17990e871214e6ea1496bc4e65f YES  NO NO unknown 	MAYBE	REMOTE WRITE OOB DISK INIT SIMPLEFIX LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process can trigger the affected path by performing buffered writes on an affected netfs-backed mounted filesystem.

Paranoid-only signal:

* Important filesystem/storage path: +1
  The code is in netfs buffered write handling and touches folio/page-cache write behavior, but the patch does not show persistent metadata corruption or privileged file-backed data corruption.

Not triggered:

* Remote reachable: +0
  This is reached through local VFS write operations, not direct remote packet input.

* Memory corruption: +0
  No UAF, OOB access, double-free, type confusion, arbitrary write, refcount misuse, or stale pointer primitive is shown.

* Privilege escalation plausible: +0
  The patch context indicates correctness and write-streaming behavior, not a realistic LPE path.

* Reliable kernel crash / strong DoS: +0
  The commit mentions xfstest failures and problems, but not a reliable panic, Oops, filesystem shutdown, or system-wide DoS.

* Confidentiality impact plausible: +0
  No information disclosure or cross-user read primitive is shown.

* Integrity impact plausible: +0 conservative
  Paranoid interpretation may treat this as weak file-operation correctness risk, but there is no evidence of unauthorized modification of privileged data.

## 3. Reachability analysis

The bug is reachable by a local process that can access and write to files on an affected netfs-backed filesystem. It does not appear network-triggerable in the CVE-triage sense because the vulnerable decision is made in the local buffered write path. Namespaces and containers may make the filesystem visible to less privileged workloads, but the attacker still needs ordinary local file access to the mount and relevant file permissions. The path is not a generic default local privilege-escalation surface and depends on use of netfs-based filesystems and the specific write-streaming behavior.

Call-site confidence: high, because the relevant function and condition in netfs_perform_write() are present in the patch.

## 4. Severity interpretation

This behaves like a low-risk correctness issue or ordinary Moderate at most. The realistic impact is incorrect or suboptimal buffered write behavior on affected netfs filesystems. The theoretical concern is weak file data integrity or availability impact for the affected operation, but the patch does not support memory corruption, page-cache ownership bypass, privilege escalation, or reliable kernel crash. It should not be treated as an Important-class kernel vulnerability.

## 5. One-sentence report phrase

netfs incorrectly disabled write streaming for O_RDWR or readable file descriptors even though read_folio can fill missing folio ranges, causing local buffered write correctness issues without evidence of memory corruption, privilege escalation, or remote reachability.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

The issue appears to be a functional correctness fix in a filesystem write path with no demonstrated security boundary bypass, memory corruption primitive, strong DoS, or LPE path.

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

Patch: netfs: Fix write streaming disablement if fd open O_RDWR [ Upstream
Commit: 9adf8e47d73d5e3c2fe77dea649dcde350ccd65c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9adf8e47d73d5e3c2fe77dea649dcde350ccd65c

Changed files:
  fs/netfs/buffered_write.c

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=9adf8e47d73d5e3c2fe77dea649dcde350ccd65c

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

The original Linux kernel CVE announcement for CVE-2026-64158 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64158

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:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS score: 3.6

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: 1
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).