[CVE-2026-64065][MODERATE REGULAR] netfs: fix VM_BUG_ON_FOLIO() issue in netfs_write_begin() call [ Upstream

[CVE-2026-64065][MODERATE REGULAR] netfs: fix VM_BUG_ON_FOLIO() issue in netfs_write_begin() call [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64065
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfs: fix VM_BUG_ON_FOLIO() issue in netfs_write_begin() call [ Upstream
Commit: 51ffb788f074c0a61953086f49008028c1e7b645
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=51ffb788f074c0a61953086f49008028c1e7b645
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64065
Analysis date: Sun, 19 Jul 2026 18:32:09 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended / below Important
Manual review required: YES

Summary:
A race in netfs abandoned read cleanup can unlock a folio that netfs_write_begin still expects to remain locked, allowing a local writer on an affected netfs backed filesystem to trigger VM_BUG_ON_FOLIO and crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-64065 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64065

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: YES

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
======================================================================

CVE-2026-64065	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;CWE-755;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in netfs abandoned read cleanup can unlock a folio that netfs_write_begin still treats as protected by NETFS_RREQ_NO_UNLOCK_FOLIO. A local process writing to a netfs backed filesystem such as Ceph can hit the timing window where the folio is unlocked by netfs_unlock_abandoned_read_pages and then unlocked again by netfs_write_begin, causing VM_BUG_ON_FOLIO and a kernel crash. For the CVSS the PR:L is used because reliable triggering requires local code execution with access to files on the affected mounted filesystem, not administrator privileges. The issue is not directly network reachable, although it affects network filesystem clients and may require a reachable storage server in the deployment. Impact is at least local denial of service via kernel crash, and the paranoid score allows limited confidentiality or integrity impact because a folio lock invariant is violated, but there is no strong evidence of a stable privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended / below Important (with actual score 4)  SKIP CVE-2026-64065 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: ee4cdf7ba857a894ad1650d6ab77669cbbfa329e YES  NO NO unknown 	MAYBE	REMOTE KASAN DISK RACE HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended / below Important**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1
  A local process that can write to an affected netfs backed mount, for example Ceph, can exercise the path through write(), generic_perform_write(), ceph_write_begin(), and netfs_write_begin().

* Reliable kernel crash / strong DoS: +1
  The commit shows a concrete VM_BUG_ON_FOLIO followed by kernel BUG/Oops in folio_unlock(). The reproducer reaches about 30 percent probability in repeated generic/013 runs.

* Important filesystem/storage path: +1
  The bug is in netfs write_begin and folio/page-cache locking behavior, which is a sensitive filesystem write path.

Paranoid additional signal:

* Weak/indirect corruption candidate: +1
  The bug violates a folio locking invariant and can unlock a folio that another path still treats as protected. This is not a demonstrated UAF, OOB write, arbitrary write, or page-cache corruption primitive, but it is enough to justify manual review above pure DoS bookkeeping.

Signals not applied:

* Remote reachable: +0
  The issue affects network filesystems, but the trigger is local client-side write activity, not direct network packet processing.

* Strong LPE plausibility: +0
  No attacker-controlled reclaim, stale object reuse, write-after-free, type confusion, callback dispatch, or refcount takeover is shown.

* Generic strong memory corruption: +0
  The demonstrated failure is a double-unlock/lock-state assertion leading to BUG, not a concrete memory corruption primitive.

* Hard race subtraction: 0
  Although this is a race, it is reproducible under stress with meaningful probability, so I would not subtract the full race penalty.

## 3. Reachability analysis

The realistic trigger is a local user or process with write access to a file on an affected netfs-backed filesystem such as Ceph. Administrative privileges are not inherently required once the filesystem is mounted and writable by the attacker. Namespaces or containers can matter if a containerized workload has access to such a mount, because the trigger remains local filesystem I/O rather than privileged mount setup.

The path is not directly network reachable. A remote storage server or network filesystem deployment may be involved, but the attacker needs to cause local client write_begin behavior. This is therefore not an AV:N style issue for practical triage.

Call-site confidence: high. The provided trace shows the concrete call path from write() through Ceph and netfs into folio_unlock(), and the patch directly changes netfs_unlock_abandoned_read_pages().

## 4. Severity interpretation

This behaves primarily like an actionable local DoS in a filesystem/page-cache path. The demonstrated impact is a kernel BUG from unlocking an already unprotected folio. The theoretical concern is that folio lock invariant violations can sometimes hide deeper page-cache or concurrency corruption, but this patch does not show a stable privilege escalation primitive.

Conservative handling: ordinary Moderate to borderline manual-review issue.
Paranoid handling: manual review recommended because this is a race in filesystem memory handling, but it does not reach Important-class confidence without evidence of UAF, page-cache corruption, or attacker-controlled memory reuse.

## 5. One-sentence report phrase

A race in netfs abandoned read cleanup can unlock a folio that netfs_write_begin still expects to remain locked, allowing a local writer on an affected netfs backed filesystem to trigger VM_BUG_ON_FOLIO and crash the kernel.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a reproducible race in folio/page-cache locking on a filesystem write path, so it should not be auto-closed as a trivial DoS. However, current evidence supports crash/DoS first, with only weak paranoid concern for broader corruption and no demonstrated LPE primitive.

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

Patch: netfs: fix VM_BUG_ON_FOLIO() issue in netfs_write_begin() call [ Upstream
Commit: 51ffb788f074c0a61953086f49008028c1e7b645
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=51ffb788f074c0a61953086f49008028c1e7b645

Changed files:
  mm/filemap.c
  fs/netfs/read_retry.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=51ffb788f074c0a61953086f49008028c1e7b645

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

The original Linux kernel CVE announcement for CVE-2026-64065 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64065

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:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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).