[CVE-2026-89787][MODERATE REGULAR] ext4: check dir entry fits before reading the hash trailer in ext4_search_dir() [ Upstream

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

CVE: CVE-2026-89787
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ext4: check dir entry fits before reading the hash trailer in ext4_search_dir() [ Upstream
Commit: 4d20106c536b73c4a8a02652dc85e35898baebf7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4d20106c536b73c4a8a02652dc85e35898baebf7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89787
Analysis date: Wed, 16 Sep 2026 05:19:44 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89787 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89787

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-2026-89787	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89787 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-416;*CWE-20;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An out-of-bounds read in ext4 directory lookup can occur for casefolded encrypted directories because ext4_match may read the hash trailer even when only the file name was proven to fit inside the directory block. A crafted directory entry whose name ends at the block boundary can pass the old pre-check while the rounded hash trailer address points past the block end. KASAN reports this as a use-after-free read when the adjacent memory belongs to a freed object. For the CVSS the PR:L is used for the paranoid score because a local user may be able to trigger lookup operations on an already mounted crafted or corrupted ext4 filesystem, while creating or mounting such content usually requires higher privileges. The issue is not network reachable. Impact is at least a local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact due to an out-of-bounds kernel read, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	READ OOB DANGER DISK UAF IMPROVEONLY LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* No floor or cap changed the score.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Conservative positive signals:

* +1 Read-only OOB kernel access: ext4_match() may read the hash trailer past the end of the checked directory block.
* +1 Kernel crash / DoS concern: KASAN reports a use-after-free style invalid read during ordinary lookup, and a production kernel may still fault depending on adjacent memory layout.
* +1 Important filesystem path: the bug is in ext4 directory lookup for casefolded encrypted directories.

Paranoid additional signal:

* +1 Local trigger after malformed filesystem state exists: an ordinary lookup such as statx can reach ext4_lookup() and ext4_search_dir() if the crafted or corrupted directory entry is already present.

Non-awarded signals:

* RealUAF not awarded: the root primitive is an out-of-bounds read past a directory block, not a stale reference intentionally kept to a freed ext4 object.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality not awarded because no user-visible disclosure path is established.
* Integrity not awarded because no unauthorized write or persistent metadata modification is established.
* Remote reachability not established.
* Strong LPE or strong exploitation leverage not awarded.
* Extra Availability not awarded because the same crash is already counted once.

3. Reachability analysis

The direct runtime trigger is a local filesystem lookup on an affected ext4 directory. The stack trace shows lookup/statx style execution reaching ext4_match() through ext4_search_dir().

The unresolved part is how the malformed directory entry is introduced. Creating or mounting a crafted ext4 image normally suggests privileged setup, raw block access, or prior filesystem corruption, but the supplied evidence does not positively prove that host-admin-equivalent privilege is required for every practical scenario. Therefore HostAdminRequired is UNKNOWN rather than YES.

Below-host-admin reachability is plausible only after the crafted or corrupted directory entry already exists on a mounted filesystem. Namespace-scoped privilege is not established. There is no network vector. No race is involved. No special hardware is required. The affected feature is narrower than generic ext4 because it depends on casefolded encrypted directories with hash-in-dirent behavior.

4. Primitive and severity interpretation

The exact primitive is a bounded read past the intended ext4 directory entry boundary when ext4_search_dir() checks that the name fits but does not check that the rounded hash trailer also fits before calling ext4_match().

This is best treated as CLASS_C: primarily invalid-state / OOB-read / crash behavior. KASAN labels the observed access as use-after-free because the adjacent memory may belong to a freed object, but the vulnerability is not a classic active-object UAF where a stale ext4 object reference survives its lifetime.

The demonstrated consequence is local denial of service through invalid kernel memory access. A limited confidentiality concern is theoretically possible for OOB reads in general, but no disclosure to userspace or attacker-visible output is established here. A limited integrity concern is also not established because the vulnerable operation is a read of hash trailer data, not a write or ownership/COW violation.

ImportantCandidate is NO because no Class A primitive, controlled corruption, repeated lifetime mutation, authorization bypass, or demonstrated LPE exists. AutoDowngradeAllowed is YES because the issue appears to be a constrained OOB-read crash case, though manual review is still recommended due to filesystem parsing and KASAN UAF-style evidence.

5. One-sentence report phrase

An ext4 casefolded encrypted directory entry can cause ext4_match() to read the hash trailer past the checked directory block during lookup, leading primarily to local kernel crash risk when crafted or corrupted filesystem content is present.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is a kernel filesystem OOB-read reported by KASAN as a UAF-style access, but the established primitive is still read-only and crash-oriented without demonstrated disclosure, write, LPE, or security-boundary bypass.

7. Call-site confidence

Call-site confidence: high

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

Patch: ext4: check dir entry fits before reading the hash trailer in ext4_search_dir() [ Upstream
Commit: 4d20106c536b73c4a8a02652dc85e35898baebf7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4d20106c536b73c4a8a02652dc85e35898baebf7

Changed files:
  fs/ext4/namei.c
  fs/namei.c
  fs/stat.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S

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=4d20106c536b73c4a8a02652dc85e35898baebf7

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

The original Linux kernel CVE announcement for CVE-2026-89787 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89787

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:L/I:N/A:H
  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: 3
  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).