[CVE-2026-89786][MODERATE REGULAR] ext4: fix out-of-bounds read in ext4_read_inline_dir() [ Upstream

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

CVE: CVE-2026-89786
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ext4: fix out-of-bounds read in ext4_read_inline_dir() [ Upstream
Commit: d1e7c186555ad65554fd2f2b02f5a539aa35ae48
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d1e7c186555ad65554fd2f2b02f5a539aa35ae48
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89786
Analysis date: Wed, 16 Sep 2026 05:25:55 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89786 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89786

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-2026-89786	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89786 CHECK WITH IMPACT FROM ORIG NN MODERATE	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:N/A:H';CWE-125;*CWE-20;CWE-130;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'An out-of-bounds read in ext4_read_inline_dir() can occur during getdents64() because the iterator checks positions against extra_size while the real dirent data lives in a kmalloc buffer of only inline_size bytes. A crafted or corrupted ext4 inline directory can make the code form a dirent pointer near the end of the buffer and then read de rec_len past the allocated object in __ext4_check_dir_entry(). For the CVSS the PR:L is used because a local user or process is needed to access the affected mounted filesystem and trigger directory iteration. The issue is not network reachable and requires local filesystem exposure or a mounted attacker controlled image. Impact is at least local denial of service or filesystem error handling. In the paranoid view it may also have limited confidentiality impact because the primitive is an out-of-bounds kernel read, but no integrity impact or privilege escalation primitive is evident from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	READ KASAN OOB DISK LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3. No floor or cap changed the score.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap changed the score.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +1 Limited kernel out-of-bounds read. The patch establishes that `ext4_read_inline_dir()` can form a dirent pointer near the end of a `kmalloc(inline_size)` buffer and read `de->rec_len` past the allocated object.
* +1 Local below-host-admin trigger for the read path. The vulnerable operation is reached through `getdents64()` / directory iteration on an affected mounted ext4 filesystem, which can be performed by a local user with access to the directory.
* +1 Important filesystem path. The bug is in ext4 directory parsing / iteration, a security-relevant filesystem path.

Non-awarded signals:

* RealUAF not awarded. No post-free or stale lifetime use is established.
* Class A / Class B not awarded. The primitive is a read-only OOB access, not controlled corruption, UAF, ownership violation, or authorization bypass.
* Reliable host-wide crash not awarded as a separate point. The supplied evidence shows KASAN slab-out-of-bounds detection, but a production panic or reliable non-debug crash is not independently established.
* Confidentiality not awarded. No user-visible disclosure path is shown for the bytes read past the buffer.
* Integrity not awarded. No OOB write, metadata overwrite, COW violation, or privilege-changing primitive is shown.
* Remote reachability not awarded. The path is local filesystem access, not packet-triggered or network-reachable.
* Manual-review escalation to Important not awarded. The patch supports an invalid read and possible DoS / filesystem error handling, but not LPE or meaningful exploitation leverage beyond the invalid read.

3. Reachability analysis

A local process can trigger the vulnerable read path by calling `getdents64()` on an affected ext4 inline directory. The attacker needs access to a mounted ext4 filesystem containing crafted or corrupted inline directory data. Creating such malformed on-disk state or mounting an attacker-controlled image may require prior privileged setup, but the runtime vulnerable operation itself is below host-admin once the affected directory is accessible.

No namespace-scoped privilege path is established. No network trigger is established. No race or timing condition is required. No special hardware is required. The relevant prerequisite is an ext4 filesystem with inline directory data in a malformed state that reaches the affected position and size mismatch.

4. Primitive and severity interpretation

The exact primitive is a bounded kernel out-of-bounds read of a dirent header field from beyond a `kmalloc(inline_size)` buffer during ext4 inline directory iteration. This is not an active structured UAF and not fixed stale storage. It is a filesystem parser bounds-checking bug.

The demonstrated consequence is KASAN-detected slab OOB read during `getdents64()`. Practical impact is local DoS or filesystem error handling depending on kernel configuration and runtime behavior. The speculative consequence is limited confidentiality impact if a downstream disclosure path existed, but the supplied patch and context do not establish such a path.

PrimitiveClass is CLASS_C because the demonstrated behavior is primarily invalid read / crash-or-error behavior without an independently useful corruption primitive. ImportantCandidate is NO because there is no UAF, write primitive, authorization bypass, ownership violation, or demonstrated privilege escalation. AutoDowngradeAllowed is YES because this is a constrained read-only OOB issue with no unresolved strong exploitation leverage.

5. One-sentence report phrase

An out-of-bounds read in ext4_read_inline_dir() can be triggered through getdents64() on crafted or corrupted inline directory data, causing KASAN detection or possible local DoS, but no UAF, write primitive, information disclosure path, or privilege escalation is established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a real kernel OOB read in an important filesystem parser, but current evidence supports only limited invalid-read / DoS impact and does not establish corruption leverage or a disclosure path.

7. Call-site confidence

Call-site confidence: high

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

Patch: ext4: fix out-of-bounds read in ext4_read_inline_dir() [ Upstream
Commit: d1e7c186555ad65554fd2f2b02f5a539aa35ae48
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d1e7c186555ad65554fd2f2b02f5a539aa35ae48

Changed files:
  fs/ext4/inline.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=d1e7c186555ad65554fd2f2b02f5a539aa35ae48

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

The original Linux kernel CVE announcement for CVE-2026-89786 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89786

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:L/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:N/A:H
  The Best / paranoid CVSS score: 5.3

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: unknown
  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).