[CVE-2026-80899][MODERATE REGULAR] erofs: remove fscache backend entirely [ Upstream

[CVE-2026-80899][MODERATE REGULAR] erofs: remove fscache backend entirely [ Upstream

[AL-KERNEL]

CVE: CVE-2026-80899
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: erofs: remove fscache backend entirely [ Upstream
Commit: f6145794f17a27d25f8a84edb80731fb0e07c196
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f6145794f17a27d25f8a84edb80731fb0e07c196
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80899
Analysis date: Fri, 04 Sep 2026 14:27:34 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended / Actionable Moderate only if external issues confirm a concrete primitive
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-80899 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80899

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-2026-80899	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80899 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-664;CWE-703;CWE-693;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.8';DESCR 'The affected area is the deprecated EROFS over fscache on demand read backend, which was used for lazy image pulling through fscache and cachefiles integration. The visible diff does not show a concrete UAF, OOB write, arbitrary write, or information leak primitive, so the base impact is limited and mainly represents possible local denial of service or incorrect behavior when the feature is enabled. For the CVSS the PR:L is used for the paranoid score because delegated mount workflows, container image services, or reduced capability root contexts may expose this path without full host administrator control. The issue is not directly network reachable as packet traffic, but it may be reachable through a local or internal image management workflow where EROFS fscache based lazy pulling is used. Impact is at least availability risk in affected deployments and in the worst still defensible case may include limited confidentiality or integrity impact due to unspecified implementation issues. YES REQUIRES MANUAL CHECK.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate only if external issues confirm a concrete primitive (with actual score 3)	YES	DISK HARDWARE  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=1

1. ActionableScore
* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended / Actionable Moderate only if external issues confirm a concrete primitive**::

2. Signal breakdown
* +1 Important filesystem/storage path. EROFS fscache on demand is in filesystem data access and cache integration paths.
* +1 Availability impact plausible. The backend removal is explicitly motivated by newly found implementation issues, but the supplied diff does not show a concrete crash path.
* +1 Cross mount shared cache boundary concern, paranoid only. The removed feature involved trusted domain IDs, shared blobs, fscache, cachefiles, and page cache sharing across mounts. This is a policy sensitive area, but no concrete bypass is proven in the pasted patch.
* +1 Local reduced privilege trigger concern, paranoid only. In Nydus or container image lazy pulling deployments, a reduced privilege service or delegated workflow may expose this path without full host administrator intent.
* +1 Filesystem image and cache input control concern, paranoid only. Attackers may influence image contents or lazy pull inputs in some deployments, but the patch does not show a specific memory corruption primitive.
* -1 Deprecated and non default feature. EROFS_FS_ONDEMAND was deprecated for almost two years and must be enabled and used.
* Conservative subtraction: -2 for typical CAP_SYS_ADMIN or root controlled mount and configuration requirements.

3. Reachability analysis
Typical direct triggering requires EROFS fscache on demand support to be built and used, likely through privileged mount or container image infrastructure. It is not directly network reachable as packet processing. Namespaces and container workflows matter because Nydus style lazy pulling may let less privileged tenants influence images or pull behavior through a privileged service, but that is deployment dependent. The supplied patch removes a deprecated backend rather than fixing a visible UAF, OOB write, arbitrary write, or info leak.

4. Severity interpretation
This does not qualify as a Strong Important candidate from the provided patch alone. It is better treated as borderline manual review material because the affected code is a filesystem/storage cache backend with cross mount sharing semantics and the commit references external implementation issues. Theoretical impact could exceed ordinary DoS if those external issues involve cache ownership, lifetime, or page sharing bugs, but realistic evidence in the pasted diff is insufficient to score it as proven memory corruption or privilege escalation.

5. One-sentence report phrase
The deprecated EROFS fscache on demand backend was removed after newly found implementation issues in a filesystem cache and lazy image pulling path, with practical risk depending on whether EROFS_FS_ONDEMAND or Nydus style workflows are enabled.

6. Manual review recommendation
YES REQUIRES MANUAL CHECK. MANUAL CHECK REQUIRED. The visible diff does not prove a concrete exploit primitive, but the full backend removal and external issue references make this unsuitable for automatic closure without checking the linked reports and affected deployment usage.

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

Patch: erofs: remove fscache backend entirely [ Upstream
Commit: f6145794f17a27d25f8a84edb80731fb0e07c196
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f6145794f17a27d25f8a84edb80731fb0e07c196

Changed files:
  fs/erofs/data.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=f6145794f17a27d25f8a84edb80731fb0e07c196

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

The original Linux kernel CVE announcement for CVE-2026-80899 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80899

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:L
  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: 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).