[CVE-2026-64059][MODERATE REGULAR] netfs: Fix folio->private handling in netfs_perform_write() [ Upstream

[CVE-2026-64059][MODERATE REGULAR] netfs: Fix folio->private handling in netfs_perform_write() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64059
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfs: Fix folio->private handling in netfs_perform_write() [ Upstream
Commit: 7f040243c74d72b45b22246c7d9e621fbeab44ac
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7f040243c74d72b45b22246c7d9e621fbeab44ac
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64059
Analysis date: Sun, 19 Jul 2026 14:43:25 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Low-like, with weak Actionable Moderate concern only in sustained-abuse scenarios
Manual review required: NO

Summary:
Incorrect folio private-state handling in netfs buffered writes can leak folio references and netfs metadata references during repeated local write paths, potentially causing resource-exhaustion DoS on affected network filesystem clients.

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

The original Linux kernel CVE announcement for CVE-2026-64059 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64059

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-64059	MODERATE	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:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'netfs_perform_write could mishandle folio private state between NULL, NETFS_FOLIO_COPY_TO_CACHE, netfs_group, and netfs_folio metadata. A local user with write access to an affected netfs backed mount could trigger repeated writes or mkwrite paths that attach new private data without correctly releasing the previous object, causing folio reference leaks and netfs metadata or group reference leaks. For the CVSS the PR:L is used because reliable triggering requires a local user or process able to write to the mounted filesystem, but does not normally require full administrator privileges. The issue is not directly network reachable because the vulnerable operation is a local filesystem write path on the client, even though the filesystem data may be backed by a network service. Impact is primarily denial of service through memory or reference exhaustion, with no concrete UAF, OOB access, info leak, or arbitrary write primitive visible from the patch.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, with weak Actionable Moderate concern only in sustained-abuse scenarios (with actual score 3)  SKIP CVE-2026-64059 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 8f52de0077ba3bf41e5d53d67a185700f41efce7 YES  NO NO unknown 	MAYBE	REMOTE WRITE OOB DISK INIT LEAK LINUS  DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_GUESSCVSS_AND_CIANNH INCREASED_FROM_LOW_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN3 DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like, with weak Actionable Moderate concern only in sustained-abuse scenarios**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user or process with write access to an affected netfs-backed mount can plausibly exercise `netfs_perform_write()` or `netfs_page_mkwrite()`.

* Availability impact realistic: +1
  The bug can leak folio references, `netfs_folio` objects, or `netfs_group` references. Repeated triggering may cause memory or reference exhaustion.

* Important filesystem/storage path: +1, paranoid only
  The bug is in the netfs buffered write and page-mkwrite path, which is relevant to network filesystems and page-cache-backed write handling.

Subtracted or not awarded:

* Hard or special triggering conditions: -1 conservative
  The commit describes some cases as unlikely and dependent on specific folio private-state transitions, streaming writes, group handling, or third-party network filesystem changes.

* No generic memory corruption bonus
  The patch describes leaks and multiple private-data attachments, but not UAF, double-free, OOB write, arbitrary write, type confusion, or attacker-controlled reuse.

* No privilege escalation bonus
  There is no concrete primitive for LPE. The issue behaves like reference and memory leakage rather than exploitable corruption.

* No confidentiality or integrity bonus
  No information disclosure, page-cache corruption, COW bypass, or privileged file-backed data corruption is shown.

## 3. Reachability analysis

A local user with write permission to an affected mounted network filesystem can potentially trigger the vulnerable paths through normal write or mmap write-fault activity. Root privileges are not inherently required, but the relevant filesystem must already be mounted and must use the affected netfs helpers.

This is not directly network reachable. The vulnerable code runs on the client-side local write path, even though the backing filesystem may be remote. A malicious server might influence timing or content state in some network filesystem scenarios, but the patch does not show a direct remote trigger.

Namespaces and containers may matter only if an untrusted workload is given access to an affected mount. The bug does not itself bypass namespace, credential, or capability boundaries.

Call-site confidence: high, because the affected write and mkwrite paths are included in the provided patch.

## 4. Severity interpretation

This behaves more like an ordinary Moderate or Low-like resource leak than an Important kernel vulnerability. The theoretical worst case is sustained local DoS through memory or reference exhaustion. The realistic evidence does not support memory corruption, privilege escalation, sensitive data exposure, or page-cache ownership violation.

The paranoid score is slightly higher because the bug is in a filesystem write path and repeated abuse may produce stronger availability impact. It should still not be treated as Important without additional evidence of UAF, stale object reuse, double-free, or file-backed data corruption.

## 5. One-sentence report phrase

Incorrect folio private-state handling in netfs buffered writes can leak folio references and netfs metadata references during repeated local write paths, potentially causing resource-exhaustion DoS on affected network filesystem clients.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a local resource leak without a demonstrated memory-corruption primitive, privilege-escalation path, confidentiality impact, or direct network trigger. It can be handled as non-urgent unless the affected product heavily exposes writable netfs mounts to untrusted users.

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

Patch: netfs: Fix folio->private handling in netfs_perform_write() [ Upstream
Commit: 7f040243c74d72b45b22246c7d9e621fbeab44ac
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7f040243c74d72b45b22246c7d9e621fbeab44ac

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=7f040243c74d72b45b22246c7d9e621fbeab44ac

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

The original Linux kernel CVE announcement for CVE-2026-64059 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64059

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:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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).