[CVE-2026-72132][MODERATE 7.0] NFS: Charge unstable writes by request size, not folio size [ Upstream

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

CVE: CVE-2026-72132
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: NFS: Charge unstable writes by request size, not folio size [ Upstream
Commit: a442c258320b689f13d2205eaeeddf8b0e630288
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a442c258320b689f13d2205eaeeddf8b0e630288
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72132
Analysis date: Sun, 16 Aug 2026 03:18:14 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
NFS unstable writeback accounting can overcharge each split request by the full folio size, allowing a local writer on an affected NFS mount to inflate dirty/writeback accounting and cause host-wide buffered write throttling without evidence of memory corruption or privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-72132 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72132

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 7.0
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-2026-72132	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:N/A:H';CWE-682;**CWE-400;CWE-770;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'NFS unstable write accounting can overcharge writeback by the full folio size for every split request instead of the request range size. With large folios and small request splits such as pNFS flexfiles striping or wsize limited splitting, dirty and writeback accounting can be inflated by large factors and can push the host into the hard throttle path for buffered writers. For the CVSS the PR:L is used for the paranoid score because a local user or local process with write access to the affected NFS mount can generate the write workload, while no administrative privilege is normally required for ordinary file writes. The issue is not directly network reachable in the CVSS sense because reliable triggering requires a local writer on an affected NFS client configuration rather than only sending packets to the host. Impact is denial of service through host wide write throttling and degraded write progress, with no evidence of memory corruption or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	DISK INIT SIMPLEFIX NETWORK LINUS MEMORY  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5 DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local user or process with write access to an affected NFS mount can generate buffered writes that create split sub-folio NFS requests. No admin privilege is normally needed for ordinary file writes.

* Reliable kernel crash / strong DoS: +1
  The issue can force buffered writers into the hard-throttle path by inflating dirty plus writeback accounting far beyond the real amount of data.

* Availability impact realistic: +1
  The impact can be host-wide, affecting unrelated buffered writers and even in-kernel NFS server threads sharing the same system. This is broader than a local per-process slowdown.

* Important filesystem/storage path: +1
  The bug is in NFS writeback and commit accounting, which is part of a core filesystem storage path.

Paranoid-only signal:

* Cross-boundary isolation or resource impact: +1
  In shared systems, inflated global dirty and writeback accounting can affect unrelated workloads. This is not a memory corruption boundary bypass, but it is a cross-workload resource availability concern.

Subtracting signal for conservative score:

* Specific runtime conditions required: -1
  The strongest impact depends on large folios combined with split NFS requests, such as pNFS flexfiles striping with small stripe units or wsize-limited splitting.

Not triggered:

* No generic memory corruption.
* No UAF, double free, OOB write, stale pointer, refcount takeover, or page-cache corruption primitive.
* No privilege escalation plausibility from the patch.
* No direct remote network trigger in CVSS or actionable-score terms.

## 3. Reachability analysis

The bug is reachable by a local workload writing to an affected NFS client mount. The strongest case requires NFS writeback request splitting, for example pNFS flexfiles striping with a small stripe unit or ordinary wsize-limited splitting, especially with large folios.

Namespaces or containers may matter if an untrusted container workload can write to a host-provided NFS mount and share the host dirty/writeback accounting pressure. However, mounting and configuring NFS itself is usually privileged. The trigger is not direct packet input from the network. A remote NFS server may influence layout or write splitting in some deployments, but reliable exploitation mainly requires a local writer on the NFS client.

Call-site confidence: high. The patch includes the relevant callers in `nfs_request_add_commit_list`, `pnfs_layout_mark_request_commit`, `nfs_clear_request_commit`, `nfs_retry_commit`, and `nfs_commit_release_pages`.

## 4. Severity interpretation

This behaves like an actionable resource-accounting DoS, not a memory corruption vulnerability. The realistic impact is host-wide write throttling and degraded progress for buffered writers. There is no supported confidentiality or integrity impact, and no plausible LPE primitive is shown by the patch.

Conservative handling is Borderline or Moderate because triggering the severe case requires specific NFS write-splitting conditions. Paranoid handling reaches Actionable Moderate because the resulting pressure can affect unrelated workloads and kernel NFS server threads on the same host.

## 5. One-sentence report phrase

NFS unstable writeback accounting can overcharge each split request by the full folio size, allowing a local writer on an affected NFS mount to inflate dirty/writeback accounting and cause host-wide buffered write throttling without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: no memory corruption or LPE primitive is visible, so this is not Important-class by itself. However, the host-wide availability effect and cross-workload dirty/writeback throttling make it an actionable Moderate candidate rather than a trivial auto-close.

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

Patch: NFS: Charge unstable writes by request size, not folio size [ Upstream
Commit: a442c258320b689f13d2205eaeeddf8b0e630288
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a442c258320b689f13d2205eaeeddf8b0e630288

Changed files:
  fs/nfs/internal.h
  fs/nfs/pnfs_nfs.c
  fs/nfs/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=a442c258320b689f13d2205eaeeddf8b0e630288

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

The original Linux kernel CVE announcement for CVE-2026-72132 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72132

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 4
  Paranoid ActionableScore: 5

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 7.0

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).