[CVE-2026-72367][MODERATE 7.0] iomap: guard io_size EOF trim against concurrent truncate underflow [ Upstream

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

CVE: CVE-2026-72367
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: iomap: guard io_size EOF trim against concurrent truncate underflow [ Upstream
Commit: 1f38f65bf965fce9aa159d45c5347538f56c5973
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1f38f65bf965fce9aa159d45c5347538f56c5973
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72367
Analysis date: Sun, 16 Aug 2026 05:00:19 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
An iomap writeback race can underflow `io_size` during EOF trimming, causing a huge wrapped size that may corrupt completion-time EOF and unwritten/COW range handling, with local integrity impact and possible filesystem availability impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72367 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72367

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-2026-72367	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:H';CWE-191;CWE-362;CWE-682;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '6.3';DESCR 'IOMAP writeback can underflow io_size when EOF trimming races with truncate or when writeback covers a block aligned range beyond byte EOF. The negative trim result is stored in size_t and can wrap to a huge value, which may mislead append detection and completion time size handling in filesystem end_io paths. For the CVSS the PR:L is used because a local user or local process with permission to write and truncate files can potentially trigger the affected path without full administrator privileges. AC:H is used because reliable triggering depends on a race between writeback and EOF changes. The issue is not directly network reachable and is not a packet driven bug. Impact is primarily integrity loss through incorrect on disk EOF or unwritten and COW completion range handling, with possible availability impact if the huge range leads to filesystem errors or a crash.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-72367 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 51d20d1dacbec589d459e11fc88fbca419f84a99 YES  NO NO unknown 	MAYBE	OOB DISK LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	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

Conservative signals:

* Local unprivileged trigger: +1. A local user or process with ordinary permission to write and truncate files can potentially exercise the writeback path.
* Memory layout invariant restoration: +1. The patch restores consistency between `io_offset`, `io_size`, `map_len`, and `end_pos` in filesystem writeback accounting.
* Filesystem/storage metadata corruption reachable by an unprivileged local user: +1. A wrapped `io_size` can affect completion-time EOF, unwritten extent, and COW range handling.
* Important filesystem/storage path: +1. This is in iomap writeback completion accounting, used by important filesystems and storage paths.
* Integrity impact plausible: +1. The commit explicitly describes corrupt completion-time size handling and decisions affecting on-disk EOF and unwritten/COW completion ranges.
* Hard or unreliable race / special timing required: -1. Reliable triggering depends on EOF movement, writeback timing, and concurrent truncate or block-aligned writeback beyond byte EOF.

Conservative total: 4.

Paranoid additional signal:

* Availability impact realistic: +1. A huge wrapped `io_size` may cause filesystem errors, incorrect completion ranges, or crash/shutdown behavior in filesystem end_io handling.

Paranoid total: 5.

Not awarded:

* No remote/network-triggerable bonus. This is not packet driven.
* No generic strong memory corruption bonus. The bug is an integer underflow in writeback accounting, not a demonstrated UAF, OOB write, arbitrary write, or type confusion.
* No Dirty-Pipe-like ownership/COW violation bonus. The commit mentions COW completion ranges, but the patch does not sufficiently demonstrate cross-file unauthorized shared-block modification.
* No privilege escalation bonus. LPE is not demonstrated or strongly implied by the patch.

Call-site confidence: medium. The patch gives the affected helper and explains end_io consumers, but full filesystem-specific completion call sites are not included.

## 3. Reachability analysis

The realistic trigger is local. A user needs the ability to create writeback activity and race it with truncate or EOF changes on an affected iomap-based filesystem. Full root privileges are not inherently required if the attacker controls files on such a filesystem.

Namespaces and containers may matter if a container workload can write and truncate files on a host-backed affected filesystem. This does not automatically imply container escape, but it can make the local trigger available to less trusted workloads.

The path is not directly network reachable. Remote influence is only indirect, for example through a network service that performs attacker-controlled writes and truncates on the affected local filesystem.

The affected area is a common storage/writeback path, not a rare debugging interface. Exploitation still requires timing-sensitive interaction with writeback and EOF changes.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary low-priority correctness fix. The key issue is not kernel code execution but filesystem integrity risk from an unsigned underflow that can produce a huge `io_size` and mislead completion-time decisions.

The realistic evidence supports local integrity impact and possible availability impact. The theoretical worst case deserves manual review because COW and unwritten extent completion paths are sensitive, but the supplied patch does not prove a Dirty-Pipe-like cross-ownership write primitive or LPE.

## 5. One-sentence report phrase

An iomap writeback race can underflow `io_size` during EOF trimming, causing a huge wrapped size that may corrupt completion-time EOF and unwritten/COW range handling, with local integrity impact and possible filesystem availability impact.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This should not be auto-closed because it affects filesystem writeback accounting and has plausible persistent integrity impact, but the patch does not show a strong memory corruption or proven privilege-escalation primitive.

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

Patch: iomap: guard io_size EOF trim against concurrent truncate underflow [ Upstream
Commit: 1f38f65bf965fce9aa159d45c5347538f56c5973
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1f38f65bf965fce9aa159d45c5347538f56c5973

Changed files:
  fs/iomap/ioend.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=1f38f65bf965fce9aa159d45c5347538f56c5973

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

The original Linux kernel CVE announcement for CVE-2026-72367 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72367

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:H/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS score: 6.3

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: 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).