[CVE-2026-46329][MODERATE 7.0] erofs: handle end of filesystem properly for file-backed mounts [ Upstream

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

CVE: CVE-2026-46329
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: erofs: handle end of filesystem properly for file-backed mounts [ Upstream
Commit: 8d582d65d20bb4796db01b19e86909ad68cb337b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8d582d65d20bb4796db01b19e86909ad68cb337b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46329
Analysis date: Tue, 09 Jun 2026 12:27:15 -0400
ActionableScore: 5
ActionableScore lower bound: 1
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Not suitable for auto-close until the actual deployment model for EROFS file-backed mounts is checked.
Manual review required: YES

Summary:
EROFS file-backed mounts may fail to zero-fill reads beyond the backing filesystem image, potentially exposing stale folio data to a local actor who can cause crafted or truncated images to be mounted.

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

The original Linux kernel CVE announcement for CVE-2026-46329 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46329

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: YES

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
======================================================================

CVE-2026-46329	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N';*CWE-200;CWE-665;CWE-908;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:N';BEST CVSS score: '5.5';DESCR 'EROFS file-backed mounts can mishandle I/O beyond the end of the backing filesystem image. A short read or EOF from the backing file may complete the bio successfully without zero filling the remaining range, so folios can be marked up to date with stale data instead of zeros. This is best treated as an information disclosure risk rather than a crash bug because the visible impact is that reads from the mounted filesystem may return data that should not be exposed. For the CVSS the PR:L is used for the paranoid case because reduced capability root, container root, or an image mounting service may allow an untrusted local actor to trigger file-backed mounts without full host administrator control. The issue is not directly network reachable. Impact is confidentiality loss, with no clear integrity or availability impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 3)  SKIP CVE-2026-46329  SKIP The Fixes patch not applied yet, so unlikely that actual: ce63cb62d794c98c7631c2296fa845f2a8d0a4a1 YES  NO NO unknown 	MAYBE	DISK SIMPLEFIX HARDWARE LINUS  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=5
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Not suitable for auto-close until the actual deployment model for EROFS file-backed mounts is checked.

## 2. Signal breakdown

Paranoid score signals:

* Local unprivileged trigger: +1
  Applies only in the paranoid deployment model where an untrusted local user can provide or influence EROFS images through a container runtime, image mounting service, sandboxed root, or reduced-capability mount helper.

* Security-boundary bypass with practical sensitive-object access: +2
  The expected boundary is that reads beyond the filesystem image return zeros. If stale folio contents become user-visible, this can expose data that should not be reachable through the mounted filesystem.

* Confidentiality impact plausible: +1
  The main plausible impact is stale data disclosure. This is not a crash or privilege escalation primitive from the patch alone.

* Important filesystem/storage path: +1
  The bug is in filesystem I/O completion and page or folio population semantics, where incorrect zero-fill can become externally visible through normal file reads.

Conservative score signals:

* Confidentiality impact plausible: +1
* Important filesystem/storage path: +1
* Requires admin or CAP_SYS_ADMIN in typical deployments: -2
* Weak security-boundary concern from stale data exposure: +1

No score added for:

* Remote reachable: no evidence.
* Generic memory corruption: not supported.
* Privilege escalation plausible: not supported by the patch.
* Reliable kernel crash: not the described failure mode.
* Dirty-Pipe-like page-cache corruption: not the same primitive, because this is missing zero-fill on read completion rather than attacker-controlled writes into shared file-backed cache.

## 3. Reachability analysis

Typical triggering requires creating or mounting an EROFS file-backed filesystem image, which normally requires CAP_SYS_ADMIN or an equivalent privileged mount path. In ordinary host deployments this makes the conservative score low.

The risk increases if untrusted users can influence mounted EROFS images through container infrastructure, appliance image loading, sandboxed root workflows, or service-mediated mounting. In that model, the user may not have full host-root privileges but may still cause the kernel to process a crafted or truncated file-backed EROFS image. The issue is not network reachable by itself.

## 4. Severity interpretation

This behaves more like an actionable information disclosure issue than an ordinary DoS bug. The patch does not show UAF, OOB write, arbitrary write, or privilege escalation. However, missing zero-fill for I/O beyond EOF can plausibly expose stale page or folio contents through filesystem reads, which is security-relevant if untrusted images can be mounted.

Realistic severity depends strongly on deployment. With only full host-root mounting, it is low priority. With reduced-privilege image mounting or container image workflows, it becomes Actionable Moderate and should not be auto-closed without manual verification.

## 5. One-sentence report phrase

EROFS file-backed mounts may fail to zero-fill reads beyond the backing filesystem image, potentially exposing stale folio data to a local actor who can cause crafted or truncated images to be mounted.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because the patch indicates a plausible stale data disclosure path, and the practical severity depends on whether untrusted users can influence EROFS file-backed mounts through containers, image services, or reduced-capability mount workflows.

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

Patch: erofs: handle end of filesystem properly for file-backed mounts [ Upstream
Commit: 8d582d65d20bb4796db01b19e86909ad68cb337b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8d582d65d20bb4796db01b19e86909ad68cb337b

Changed files:
  fs/erofs/fileio.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=8d582d65d20bb4796db01b19e86909ad68cb337b

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

The original Linux kernel CVE announcement for CVE-2026-46329 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46329

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:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  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: 1
  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).