[CVE-2026-45999][MODERATE REGULAR] erofs: fix unsigned underflow in z_erofs_lz4_handle_overlap() [ Upstream

[CVE-2026-45999][MODERATE REGULAR] erofs: fix unsigned underflow in z_erofs_lz4_handle_overlap() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-45999
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: erofs: fix unsigned underflow in z_erofs_lz4_handle_overlap() [ Upstream
Commit: 43a878639b90e9721ffa5eb616a7e6d8454adef3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=43a878639b90e9721ffa5eb616a7e6d8454adef3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45999
Analysis date: Wed, 27 May 2026 11:29:03 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A crafted EROFS image can trigger unsigned underflow in the LZ4 inplace overlap path, causing an OOB read past the decompressed pages array and likely local DoS during filesystem parsing.

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

The original Linux kernel CVE announcement for CVE-2026-45999 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45999

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.

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

CVE-2026-45999	MODERATE	CHECK	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-191;CWE-125;*CWE-20;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A crafted EROFS image can make the LZ4 inplace overlap path run with outpages lower than inpages. This makes rq outpages minus rq inpages underflow and can read past the rq out decompressed_pages array during z_erofs_lz4_handle_overlap(). For the CVSS the PR:L is used in the paranoid score because some systems may let less trusted users provide filesystem images through automount, container image, Android, embedded update, appliance, or scanner workflows. The typical host case is closer to PR:H because mounting a crafted filesystem image usually requires administrative privileges. The issue is not network reachable by itself and does not involve UAF, out of bounds write, arbitrary write, or privilege escalation. Impact is primarily denial of service via kernel crash or filesystem corruption handling, with only limited confidentiality or integrity concern from kernel parser out of bounds read behavior.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	OOB DANGER DISK IMPROVEONLY LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Important filesystem/storage path: +1
  The bug is in EROFS decompression and filesystem image parsing.

* Reliable crash / DoS plausible: +1
  The crafted image can drive an out-of-bounds read past `decompressed_pages`, likely causing a crash, corruption report, or fault.

* Availability impact realistic: +1
  Mounting and reading the crafted image can disrupt the kernel path handling that filesystem.

Subtracting signals:

* Requires admin/CAP_SYS_ADMIN in typical deployments: -2
  Mounting a crafted EROFS image normally requires administrative privileges.

* Constrained read-only primitive: -1
  The supported primitive is OOB read due to unsigned underflow, not OOB write, UAF, arbitrary write, or object lifetime corruption.

Paranoid additions:

* Local delegated image-processing concern: +1
  Some environments process EROFS images from less trusted users through automount, container images, Android or embedded update flows, appliance pipelines, or scanners.

* Weak confidentiality/integrity concern: +1
  Kernel filesystem parser OOB reads can occasionally expose or misinterpret adjacent metadata, but no direct disclosure or write primitive is shown.

## 3. Reachability analysis

The bug is triggered by a crafted EROFS image with illegal compressed extents that force the LZ4 inplace overlap path with `outpages < inpages`. In normal host deployments, the attacker needs the ability to mount or otherwise make the kernel parse the crafted image.

Namespaces and containers can reduce effective privilege if image mounting or scanning is delegated. It is not network reachable by itself. Practical exposure is higher in Android, embedded, container-image, appliance-update, or automated filesystem scanning workflows.

## 4. Severity interpretation

This is a borderline Moderate filesystem parser issue. It is not an Important memory-corruption issue based on the patch alone because the visible primitive is an OOB read, not a write, UAF, refcount bug, or page-cache corruption.

Realistic impact is DoS or filesystem corruption handling after parsing a crafted image. Theoretical C/I impact is limited and should be manually checked only in environments that ingest untrusted EROFS images.

## 5. One-sentence report phrase

A crafted EROFS image can trigger unsigned underflow in the LZ4 inplace overlap path, causing an OOB read past the decompressed pages array and likely local DoS during filesystem parsing.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is an OOB read in kernel filesystem decompression code and may be relevant where untrusted EROFS images are mounted or processed, but no OOB write, UAF, or privilege-escalation primitive is shown.

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

Patch: erofs: fix unsigned underflow in z_erofs_lz4_handle_overlap() [ Upstream
Commit: 43a878639b90e9721ffa5eb616a7e6d8454adef3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=43a878639b90e9721ffa5eb616a7e6d8454adef3

Changed files:
  fs/erofs/decompressor.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=43a878639b90e9721ffa5eb616a7e6d8454adef3

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

The original Linux kernel CVE announcement for CVE-2026-45999 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45999

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:H/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: 1
  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).