[CVE-2026-80631][MODERATE 7.0] btrfs: lzo: reject compressed segment that overflows the compressed input [ Upstream

[CVE-2026-80631][MODERATE 7.0] btrfs: lzo: reject compressed segment that overflows the compressed input [ Upstream

[AL-KERNEL]

CVE: CVE-2026-80631
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: btrfs: lzo: reject compressed segment that overflows the compressed input [ Upstream
Commit: 1641d058adfbd50cf95d54581ed5d142ee82c07f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1641d058adfbd50cf95d54581ed5d142ee82c07f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80631
Analysis date: Fri, 28 Aug 2026 06:30:03 -0400
ActionableScore: 5
ActionableScore lower bound: 2
Actionable bucket: Actionable Moderate. Manual check recommended
Manual review required: NO

Summary:
A crafted Btrfs LZO compressed extent can make `lzo_decompress_bio()` walk beyond the compressed bio input and dereference a NULL folio, causing a local kernel crash when the affected extent is read.

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

The original Linux kernel CVE announcement for CVE-2026-80631 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80631

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.

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

CVE-2026-80631	MODERATE	CHECK WITH IMPACT FROM ORIG NN NONE	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-20;CWE-125;**CWE-476;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A NULL pointer dereference in the Btrfs LZO decompression path can occur because lzo_decompress_bio validated each on disk segment length against the workspace buffer but not against the total compressed input length. A crafted compressed extent can make copy_compressed_segment walk past the last bio folio, after which bio_next_folio can return a NULL folio and folio_size(NULL) can crash the kernel. For the CVSS the PR:L is used in the paranoid score because a local user may be able to trigger reads from an already mounted malicious or corrupted Btrfs filesystem, although creating or mounting such crafted on disk metadata usually requires higher privileges or control of the storage image. The issue is not network reachable and is triggered through local filesystem I/O. Impact is denial of service via kernel crash. No direct arbitrary write, UAF, or privilege escalation primitive is supported by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate. Manual check recommended (with actual score 4)  SKIP CVE-2026-80631 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ KASAN NULLPTR DISK SKIP IMPROVEONLY  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=5
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate. Manual check recommended**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1
  The patch description shows a concrete NULL pointer dereference in `lzo_decompress_bio()` via `folio_size(NULL)`, with KASAN trace.
* Important filesystem/storage path: +1
  This is in Btrfs compressed read/decompression handling for on-disk extents.
* No generic memory corruption bonus: +0
  The demonstrated primitive is NULL pointer dereference after walking past the last bio folio, not UAF, OOB write, double free, arbitrary write, or type confusion.
* No confidentiality or integrity impact: +0
  The patch supports kernel crash only. It does not show data disclosure, attacker-controlled overwrite, CoW bypass, or page-cache corruption.
* No remote/network exposure: +0
  Trigger is via local filesystem/storage input, not packet processing.

Paranoid additional signals:

* Local unprivileged trigger: +1
  If a crafted or corrupted Btrfs filesystem is already mounted, a local user may be able to trigger decompression by reading the affected extent.
* Filesystem/storage metadata corruption reachable by an unprivileged local user: +1
  The malformed on-disk compressed extent is filesystem metadata/data-format input consumed by the kernel during read I/O.
* Availability impact realistic: +1
  Once the crafted extent is reachable, the crash path appears direct and host-wide.

Not awarded:

* Privilege escalation plausible: +0
  No reclaim, stale object reuse, write-after-free, callback control, arbitrary write, or type confusion is shown.
* Dirty-Pipe-like / CoW ownership violation: +0
  The patch does not indicate unauthorized cross-file modification, shared-page corruption, or CoW bypass.
* Generic memory corruption: +0
  The supported failure mode is NULL pointer dereference, not a strong corruption primitive.

## 3. Reachability analysis

The conservative interpretation assumes the attacker must create, modify, or mount a crafted Btrfs image or otherwise control corrupted on-disk extents, which normally requires administrative privileges or control over the storage image. Under that view this is closer to an ordinary Moderate or Low-like local DoS.

The paranoid interpretation assumes the malicious or corrupted Btrfs filesystem is already mounted or exposed through a storage trust boundary, and a local unprivileged user can trigger reads from the crafted compressed extent. In that case no further special privileges are needed to reach the crash path.

Namespaces and containers do not automatically make this unprivileged unless the container or service can access the mounted Btrfs content that contains the crafted extent. The path is not network reachable by itself, but remote storage provisioning, disk image ingestion, removable media, VM image handling, backup restore, or shared storage workflows may make the crafted filesystem input relevant in practice.

Call-site confidence: high. The commit gives the concrete failing path through `lzo_decompress_bio()`, `end_bbio_compressed_read()`, `btrfs_bio_end_io()`, and `simple_end_io_work`.

## 4. Severity interpretation

This behaves primarily as a filesystem-triggered denial of service, not as a demonstrated privilege escalation issue. The bug is caused by insufficient validation of an on-disk compressed segment length against the total compressed input length. The concrete observed result is a NULL pointer dereference and kernel crash.

Theoretical memory corruption potential is weak here because the described overrun reaches a missing folio and faults on NULL rather than showing attacker-controlled memory access, overwrite, UAF, or object confusion. Realistic exploitation evidence supports DoS only. The paranoid score is elevated because crafted filesystem inputs can be security-relevant and because local unprivileged triggering may be possible once the malicious filesystem is mounted.

## 5. One-sentence report phrase

A crafted Btrfs LZO compressed extent can make `lzo_decompress_bio()` walk beyond the compressed bio input and dereference a NULL folio, causing a local kernel crash when the affected extent is read.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

This should not be treated as an Important candidate based on the supplied patch alone, because no UAF, write primitive, information leak, or privilege escalation path is supported. Manual review is still recommended because the issue is triggered by crafted filesystem input and practical exposure depends heavily on mount policy, storage trust boundaries, and whether unprivileged users can read from already-mounted malicious Btrfs content.

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

Patch: btrfs: lzo: reject compressed segment that overflows the compressed input [ Upstream
Commit: 1641d058adfbd50cf95d54581ed5d142ee82c07f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1641d058adfbd50cf95d54581ed5d142ee82c07f

Changed files:
  fs/btrfs/lzo.c
  mm/kasan/report.c
  fs/btrfs/compression.c
  fs/btrfs/bio.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=1641d058adfbd50cf95d54581ed5d142ee82c07f

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

The original Linux kernel CVE announcement for CVE-2026-80631 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80631

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:L/PR:H/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: 2
  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).