[CVE-2026-64156][MODERATE REGULAR] netfs, afs: Fix write skipping in dir/link writepages [ Upstream

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

CVE: CVE-2026-64156
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfs, afs: Fix write skipping in dir/link writepages [ Upstream
Commit: 77bb293049d61e04c12b24ebbffafaf5ab36af90
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=77bb293049d61e04c12b24ebbffafaf5ab36af90
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64156
Analysis date: Sun, 19 Jul 2026 13:42:00 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline Moderate / manual review if AFS is deployed, not Important
Manual review required: NO

Summary:
AFS/netfs writeback could skip a WB_SYNC_NONE directory or link write under lock contention after the VFS cleared the dirty mark, potentially causing missed or delayed metadata persistence for local users with access to an AFS mount.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64156 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64156

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-2026-64156	MODERATE	CHECK WITH IMPACT FROM ORIG NN NONE	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:H/A:L';CWE-362;CWE-667;CWE-703;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '5.3';DESCR 'AFS and netfs single object writeback can skip a write under WB_SYNC_NONE when the writeback lock or validation lock is contended. If the VFS has already cleared the dirty state, the skipped write may not be retried correctly and directory or link metadata can remain unwritten or delayed. For the CVSS the PR:L is used because a local user or local process needs access to an AFS mount and must perform file system operations that create dirty directory or link state. The issue is not directly network reachable even though AFS is a network filesystem, because the trigger is local writeback behavior rather than remote packet processing. Impact is mainly integrity loss from missed or delayed metadata writeback, with limited availability impact only in the paranoid interpretation due to possible writeback inconsistency. There is no evidence of UAF, OOB access, arbitrary write, or privilege escalation from this patch context.';NO MANUAL CHECK; ,and ActionableScore result is Borderline Moderate / manual review if AFS is deployed, not Important (with actual score 4)  SKIP CVE-2026-64156 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 6dd80936618c4ff852d4db73aca400351d9bd9f0 YES  NO NO unknown 	MAYBE	DISK SIMPLEFIX SKIP LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE 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: **Borderline Moderate / manual review if AFS is deployed, not Important**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user or process with access to an AFS mount can plausibly create directory or link dirty state and rely on normal writeback paths.

* Filesystem/storage metadata corruption reachable by an unprivileged local user: +1
  The bug can cause skipped writeback of directory/link metadata after VFS has cleared the dirty mark.

* Important filesystem/storage path: +1
  The affected logic is in netfs/AFS writeback for non-pagecache directory/link objects.

* Integrity impact plausible: +1
  Missed or delayed writeback can leave metadata unwritten or inconsistent with expected server state.

* Hard or unreliable race / special timing required: -1
  Triggering depends on lock contention with WB_SYNC_NONE. The commit also says the directory path is mostly theoretical because afs_single_writepages() serializes with validate_lock.

Paranoid-only signal:

* Availability impact realistic: +1
  In the higher interpretation, repeated or persistent writeback inconsistency could cause operational disruption or metadata recovery issues, but this is not a strong kernel crash or memory corruption DoS.

Not awarded:

* No remote reachable signal. AFS is a network filesystem, but the trigger is local writeback behavior, not remote packet processing.
* No memory corruption signal. The patch shows dirty-state/writeback accounting correction, not UAF, OOB, double free, arbitrary write, or type confusion.
* No privilege escalation signal. There is no supported path to LPE in the patch context.
* No strong DoS signal. No panic, BUG, Oops, or reliable system-wide crash is shown.

## 3. Reachability analysis

The bug is reachable by a local user or local workload that can operate on an AFS mount and create directory or link metadata requiring writeback. Namespaces do not obviously make this a broader attack surface unless AFS mounts are exposed into containers or delegated environments. The path is not generally default-exposed on ordinary systems because it requires AFS/netfs usage. Realistic triggering also requires lock contention during WB_SYNC_NONE writeback, so exploitability is timing-dependent and relatively weak.

Call-site confidence: high. The relevant AFS caller and netfs helper behavior are both present in the patch.

## 4. Severity interpretation

This behaves like an ordinary-to-borderline Moderate filesystem correctness issue, not an Important-class kernel vulnerability. The main risk is metadata integrity loss or delayed persistence, not kernel memory corruption or privilege escalation. The paranoid score is slightly higher because filesystem metadata writeback bugs can have practical integrity consequences, but the patch context does not justify treating this as LPE, arbitrary corruption, or remote attack surface.

## 5. One-sentence report phrase

AFS/netfs writeback could skip a WB_SYNC_NONE directory or link write under lock contention after the VFS cleared the dirty mark, potentially causing missed or delayed metadata persistence for local users with access to an AFS mount.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: the issue is not Important-class and has no memory corruption or remote trigger evidence, but it touches filesystem metadata writeback and reaches ActionableScore 3 to 4 depending on whether persistent metadata disruption is considered plausible.

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

Patch: netfs, afs: Fix write skipping in dir/link writepages [ Upstream
Commit: 77bb293049d61e04c12b24ebbffafaf5ab36af90
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=77bb293049d61e04c12b24ebbffafaf5ab36af90

Changed files:
  fs/afs/dir.c
  fs/netfs/write_issue.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=77bb293049d61e04c12b24ebbffafaf5ab36af90

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

The original Linux kernel CVE announcement for CVE-2026-64156 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64156

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:H/PR:L/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L
  The Best / paranoid CVSS score: 5.3

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).