[CVE-2026-99084][MODERATE REGULAR] readdir: take no inode lock on an immutable directory [ Upstream

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

CVE: CVE-2026-99084
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: readdir: take no inode lock on an immutable directory [ Upstream
Commit: 6d30b399ae28e5166d62484f81ccaefde6fe7996
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d30b399ae28e5166d62484f81ccaefde6fe7996
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99084
Analysis date: Mon, 26 Oct 2026 14:46:15 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A local user can use a delayed FUSE-backed `getdents()` fault to hold the shared nullfs inode lock and indefinitely stall filesystem operations in otherwise isolated empty mount namespaces.

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

The original Linux kernel CVE announcement for CVE-2026-99084 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99084

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-2026-99084	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-99084 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=3 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-667;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'iterate_dir can hold the shared inode lock while getdents copies directory entries into a user buffer. A local user can back that buffer with a file mapping on an attacker controlled FUSE mount and delay the resulting fault while the lock remains held. By queuing an exclusive directory operation behind that reader, later lookups, creates, and mounts using the globally shared nullfs inode can be stalled across otherwise isolated empty mount namespaces. For the CVSS the PR:L value reflects that local code execution and control of the FUSE backed mapping are required. The demonstrated primitive is cross user availability loss through lock starvation. There is no supported UAF, OOB access, information leak, or memory write primitive. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage MODERATE_KPANIC to MODERATE because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE and DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 3.3 and ActionableScore 3 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	DISK LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 4.
* Paranoid score: 4; no additional technically supported primitive.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +1 Local unprivileged trigger: a local user can issue `getdents()` with a destination mapping backed by an attacker-controlled FUSE server and indefinitely delay the fault.
* +1 Reliable strong DoS: the attacker can retain `i_rwsem` shared, queue an exclusive taker, and block later lookup, create, and mount operations.
* +1 Cross-boundary isolation interference: the nullfs inode is shared across otherwise unrelated empty mount namespaces, allowing one user to stall others.
* +1 Practical exploit chain: FUSE-delayed copy fault → retained shared lock → queued exclusive operation → subsequent operations blocked.
* No extra availability point: indefinite repetition is the same lock-starvation effect.
* No UAF, OOB access, memory write, disclosure, authorization bypass, or protected-state corruption is shown.
* NEW52/B5-R and NEW53/B3-PENDING do not apply.

3. Reachability analysis

The supplied evidence grounds a below-host-admin local trigger using an attacker-controlled FUSE mapping and an empty mount namespace. No host UID 0, host-admin capability, special hardware, boot option, or timing race is required. The attacker deliberately controls how long the FUSE fault remains unresolved. Namespace isolation is involved in the impact, but the trigger is not dependent on a namespace-scoped privileged capability.

4. Primitive and severity reading

The primitive is intentional retention of a globally contended inode read lock, followed by writer-induced reader starvation across shared nullfs users. It is a CLASS_C availability primitive, not memory corruption or an authorization decision bypass. The proven effect is cross-user and cross-namespace operational blocking; host panic, persistent damage, confidentiality loss, integrity loss, and privilege escalation are unsupported. ImportantCandidate is NO because no Important-preserving corruption or policy-bypass primitive exists. Auto-downgrade is allowed.

5. One-sentence report phrase

A local user can use a delayed FUSE-backed `getdents()` fault to hold the shared nullfs inode lock and indefinitely stall filesystem operations in otherwise isolated empty mount namespaces.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm product exposure to unprivileged FUSE and empty mount namespaces and assess the operational scope of the shared-inode stall.

7. Call-site confidence

Call-site confidence: high

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

Patch: readdir: take no inode lock on an immutable directory [ Upstream
Commit: 6d30b399ae28e5166d62484f81ccaefde6fe7996
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6d30b399ae28e5166d62484f81ccaefde6fe7996

Changed files:
  fs/nullfs.c
  fs/readdir.c
  include/linux/fs.h

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=6d30b399ae28e5166d62484f81ccaefde6fe7996

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

The original Linux kernel CVE announcement for CVE-2026-99084 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99084

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