[CVE-2026-46078][MODERATE REGULAR] erofs: fix the out-of-bounds nameoff handling for trailing dirents

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

CVE: CVE-2026-46078
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: erofs: fix the out-of-bounds nameoff handling for trailing dirents
Commit: 222055e6b4063abd2d9e13c3d49bbd1724c50789
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=222055e6b4063abd2d9e13c3d49bbd1724c50789
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46078
Analysis date: Wed, 27 May 2026 10:07:10 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A crafted EROFS directory entry can make `strnlen()` read past the directory block during `readdir`, requiring local malformed filesystem metadata processing and primarily causing DoS or limited OOB-read risk.

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

The original Linux kernel CVE announcement for CVE-2026-46078 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46078

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-46078	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-125;CWE-126;*CWE-20;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.8';DESCR 'An out of bounds read can occur in the EROFS readdir path because a crafted trailing dirent can set nameoff outside the valid name area and make maxsize - nameoff underflow before strnlen(). This can make the kernel scan past the directory block while parsing filesystem metadata. For the CVSS the PR:L is used in the paranoid score because some deployments may allow less trusted local users or delegated services to provide EROFS images, while the normal case often requires mount or block device control. The issue is not network reachable and requires local processing of a malformed EROFS filesystem image. Impact is primarily denial of service or invalid kernel memory read behavior. In the worst still defensible case it may cause limited confidentiality or integrity concern from parser state confusion, but the patch does not support an out of bounds write, UAF, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 2)	YES	WRITE OOB DANGER DISK SIMPLEFIX KERNEL_PANIC_PLUS_UAF LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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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 directory metadata parsing during `readdir`.

* Reliable kernel crash / DoS plausible: +1
  `strnlen()` may scan past the directory block after `maxsize - nameoff` underflows.

* Availability impact realistic: +1
  A crafted or corrupted filesystem image can trigger invalid kernel memory reads and filesystem error handling or crash behavior.

* Requires admin/root/CAP_SYS_ADMIN in typical deployments: -2
  In a normal system, triggering usually requires mounting a crafted EROFS image or controlling the backing storage.

* Constrained read-only primitive: -1
  The demonstrated primitive is OOB read via `strnlen()`, with no OOB write, UAF, double-free, or controlled overwrite.

Paranoid additional signals:

* Local lower-privilege trigger concern: +1
  Some environments may process EROFS images from less trusted users, such as embedded update flows, Android images, container image pipelines, appliance firmware, or service-managed mounts.

* Filesystem metadata corruption reachable by delegated local workflow: +1
  A crafted filesystem image is the relevant input, and delegated mount or image-processing setups may reduce the practical privilege barrier.

* Confidentiality concern weakly plausible: +1
  OOB read in a kernel parser can theoretically expose adjacent memory behavior or affect parser decisions, but no direct disclosure path is shown.

* Hard/special precondition: -1
  Requires malformed EROFS directory metadata and a path that causes the kernel to parse the crafted trailing dirent.

## 3. Reachability analysis

The bug is triggered by a malformed EROFS filesystem where a trailing directory entry has `nameoff >= maxsize`, causing the length argument to `strnlen()` to underflow. In typical deployments, an attacker needs administrative mount privileges or control over a block device or filesystem image source.

Namespaces and containers can matter if a container runtime, image scanner, update service, or delegated mount workflow processes attacker-supplied EROFS images. The issue is not network reachable by itself. EROFS is not as universally exposed as ext4, but it is relevant in embedded, Android, immutable rootfs, and image-based deployment environments.

## 4. Severity interpretation

This behaves like a borderline Moderate filesystem parser issue. It is not an Important-class vulnerability based on the patch alone because the visible primitive is a constrained OOB read, not memory corruption with write control, UAF, refcount corruption, page-cache corruption, or a demonstrated privilege-escalation path.

Theoretical risk is higher than a simple local assertion because malformed filesystem metadata is parsed in kernel context. Realistic evidence supports DoS or invalid read behavior, with only weak C/I concern unless an actual disclosure or corruption sink is demonstrated.

## 5. One-sentence report phrase

A crafted EROFS directory entry can make `strnlen()` read past the directory block during `readdir`, requiring local malformed filesystem metadata processing and primarily causing DoS or limited OOB-read risk.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is an OOB read in a kernel filesystem metadata parser and may be relevant in image-based deployment workflows, but the patch does not show an OOB write, UAF, or practical privilege-escalation primitive.

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

Patch: erofs: fix the out-of-bounds nameoff handling for trailing dirents
Commit: 222055e6b4063abd2d9e13c3d49bbd1724c50789
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=222055e6b4063abd2d9e13c3d49bbd1724c50789

Commit description:

Currently we already have boundary-checks for nameoffs, but the trailing
dirents are special since the namelens are calculated with strnlen()
with unchecked nameoffs.

If a crafted EROFS has a trailing dirent with nameoff >= maxsize,
maxsize - nameoff can underflow, causing strnlen() to read past the
directory block.

nameoff0 should also be verified to be a multiple of
`sizeof(struct erofs_dirent)` as well [1].

[1] https://sashiko.dev/#/patchset/20260416063511.3173774-1-hsiangkao%40linux.alibaba.com

Fixes: 3aa8ec7 ("staging: erofs: add directory operations")
Fixes: 33bac91 ("staging: erofs: keep corrupted fs from crashing kernel in erofs_readdir()")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Reported-by: Junrui Luo <moonafterrain@outlook.com>
Closes: https://lore.kernel.org/r/A0FD7E0F-7558-49B0-8BC8-EB1ECDB2479A@outlook.com
Cc: stable@vger.kernel.org
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/erofs/dir.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=222055e6b4063abd2d9e13c3d49bbd1724c50789

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

The original Linux kernel CVE announcement for CVE-2026-46078 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46078

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: 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).