[CVE-2026-64159][MODERATE REGULAR] netfs: Fix zeropoint update where i_size > remote_i_size [ Upstream

[CVE-2026-64159][MODERATE REGULAR] netfs: Fix zeropoint update where i_size > remote_i_size [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64159
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfs: Fix zeropoint update where i_size > remote_i_size [ Upstream
Commit: 5cd5207de519ef0c085f4f559adf5eefcb4c5202
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5cd5207de519ef0c085f4f559adf5eefcb4c5202
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64159
Analysis date: Sun, 19 Jul 2026 13:36:52 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Ordinary Moderate / Low-like. Not an Important candidate
Manual review required: NO

Summary:
A netfs zero_point accounting bug can use local i_size instead of remote_i_size when releasing a folio, causing incorrect EOF/read-gap handling and limited file I/O consistency impact on CIFS/netfs mounts.

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

The original Linux kernel CVE announcement for CVE-2026-64159 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64159

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-2026-64159	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-682;CWE-754;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '4.4';DESCR 'A logic error in netfs_release_folio can update zero_point using local i_size instead of remote_i_size when local pagecache size is larger than the server EOF. This can make netfs handle the range between remote EOF and local dirty pagecache state incorrectly and can lead to short reads or read gap failures on filesystems such as CIFS with cache enabled. For the CVSS the PR:L is used because a local user or process needs access to a mounted netfs file and must perform file operations that create the i_size greater than remote_i_size condition. The issue is not directly network reachable as an attack against the kernel and is triggered through local filesystem operations on an existing network filesystem mount. Impact is limited to data consistency or availability of file I/O and does not indicate memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Borderline Ordinary Moderate / Low-like. Not an Important candidate (with actual score 3)  SKIP CVE-2026-64159 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: cce6bfa6ca0e30af9927b0074c97fe6a92f28092 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=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline Ordinary Moderate / Low-like. Not an Important candidate**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user or process with access to an already mounted netfs/CIFS filesystem can perform file operations such as truncate, write, copy_range, and mapread that create the i_size greater than remote_i_size condition.

* Important filesystem/storage path: +1
  The bug is in netfs folio release and affects CIFS/netfs pagecache EOF and read-gap behavior. This is a filesystem data consistency path, but not metadata corruption or page-cache ownership corruption.

Paranoid-only signal:

* Integrity impact plausible: +1
  The issue can cause incorrect file read behavior or short reads around EOF, which is a limited data consistency impact. This is not page-cache corruption of privileged files and does not imply Dirty-Pipe-like behavior.

Not triggered:

* Remote reachable / network-triggerable: +0
  The filesystem is network-backed, but the bug is triggered through local file operations on a mounted filesystem, not by unauthenticated remote packets.

* Generic memory corruption: +0
  The patch fixes incorrect zero_point arithmetic and state update. It does not show UAF, OOB access, double free, refcount misuse, type confusion, or arbitrary write.

* Privilege escalation plausible: +0
  No realistic LPE primitive is shown. The effect is incorrect EOF/read-gap handling, not credential, fd, namespace, or memory lifetime corruption.

* Reliable kernel crash / strong DoS: +0
  The described failure is short read/read-gap behavior, not kernel panic, filesystem shutdown, or host-wide DoS.

## 3. Reachability analysis

The likely trigger is a local user or workload operating on a mounted netfs/CIFS filesystem with caching behavior that allows local i_size to exceed remote_i_size. Namespaces or containers may matter only if the mount is exposed into them and the user can perform writes/truncates on files there. This does not require CAP_SYS_ADMIN once the filesystem is already mounted, but creating or configuring the mount is normally privileged.

The path is not directly network reachable as an attack against the kernel. A malicious server might influence remote EOF behavior, but the patch and reproducer point to local pagecache/file operation sequencing rather than remote protocol parsing. Realistic exploitation conditions are specific and correctness-oriented: CIFS/netfs cache behavior, uncommitted local data beyond server EOF, and folio release timing.

Call-site confidence: high, because the changed function and relevant state variables are visible in the patch.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like filesystem correctness issue, not an Important-class kernel vulnerability. The theoretical impact is limited data consistency or file I/O availability around EOF. There is no evidence of memory corruption, page-cache ownership violation, privilege escalation, sensitive data disclosure, or system-wide denial of service.

The paranoid score is only raised to 3 to avoid fully auto-downgrading a filesystem/pagecache consistency bug, but it remains below Actionable Moderate.

## 5. One-sentence report phrase

A netfs zero_point accounting bug can use local i_size instead of remote_i_size when releasing a folio, causing incorrect EOF/read-gap handling and limited file I/O consistency impact on CIFS/netfs mounts.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: the patch shows a bounded logic/accounting fix in filesystem cache state, with no memory corruption primitive, no direct remote trigger, no LPE path, and no strong DoS evidence.

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

Patch: netfs: Fix zeropoint update where i_size > remote_i_size [ Upstream
Commit: 5cd5207de519ef0c085f4f559adf5eefcb4c5202
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5cd5207de519ef0c085f4f559adf5eefcb4c5202

Changed files:
  fs/netfs/misc.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=5cd5207de519ef0c085f4f559adf5eefcb4c5202

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

The original Linux kernel CVE announcement for CVE-2026-64159 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64159

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:L
  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: 2
  Paranoid ActionableScore: 3

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).