[CVE-2026-98981][MODERATE 7.0] namespace: unhash a dentry before detaching the mounts on it [ Upstream

[CVE-2026-98981][MODERATE 7.0] namespace: unhash a dentry before detaching the mounts on it [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98981
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: namespace: unhash a dentry before detaching the mounts on it [ Upstream
Commit: db90eecbde319527aa9480c039d32a0f5fe80574
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=db90eecbde319527aa9480c039d32a0f5fe80574
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98981
Analysis date: Tue, 27 Oct 2026 02:07:57 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A race between mount detachment and dentry invalidation lets a local low-privilege lookup cross mount-namespace hiding and read an underlying file.

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

The original Linux kernel CVE announcement for CVE-2026-98981 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98981

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98981	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98981 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N';CWE-362;CWE-367;*CWE-200;CWE-668;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N';BEST CVSS score: '6.3';DESCR 'A race in unlink, rmdir, and rename handling leaves a positive hashed dentry visible after detach_mounts removes its covering mounts but before d_delete invalidates the name. A concurrent lookup in another mount namespace can therefore resolve and open the underlying object that the mount was intended to hide. The reproducer demonstrates repeated disclosure of SECRET_F through a path that should expose mounted content or return ENOENT. For the CVSS the PR:L value reflects that exploitation needs local process access, suitable filesystem permissions, and a shared mount namespace arrangement, but it does not normally require full host administration after that arrangement exists. AC:H reflects the need to win the lookup race. The confirmed primitive is cross namespace information disclosure and not memory corruption. Integrity impact is included only in the paranoid vector because a writable lookup might reach the same uncovered inode when normal inode permissions allow modification. The issue is not directly network reachable. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 6.3 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. 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 manual review requirement was applied. 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_REQUIREMANUALCHECK. The independent weaker Aegis shadow evaluated CVSS 1.9 and ActionableScore 2 with primitive class NONE and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	DISK RACE LEAK HARDWARE LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=CONFIRMED
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap applied.
* Raw paranoid score: 5; writable-file integrity effects are unproven and not scored.
* Paranoid score: 5.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 security-boundary bypass with practical access: after mount detachment, the still-hashed positive dentry lets another mount-namespace lookup reach the underlying object that the detached mount had hidden.
* +1 confirmed confidentiality impact: the supplied reproducer repeatedly opens and reads `SECRET_F` instead of mounted `MARK_A` or receiving `ENOENT`.
* +1 practical exploit chain: concurrent lookup → stale hashed dentry → mount no longer crossed → underlying file opened and read.
* +1 local below-host-admin trigger: supplied triage evidence identifies a PR:L actor with suitable filesystem access after the namespace/mount arrangement exists.
* No race penalty: despite timing dependence, the commit reports a broad window and 11,938 successful disclosures in 16,140 unlink operations.
* No integrity point: writable access to the underlying inode is only a theoretical extension.
* No memory-corruption, UAF, arbitrary-access, crash, or privilege-escalation point is awarded.

3. Reachability analysis

The exercising actor is local and needs access to the affected detached mount tree plus suitable lookup/unlink filesystem permissions. The reproducer’s initial mount construction uses operations that may require privileged or namespace-scoped mount authority, so prior setup is material. Supplied evidence nevertheless grounds a below-host-admin actor after that arrangement exists; host UID 0 is not required for that exercising path. Whether every setup operation can be performed solely with attacker-obtainable user/mount-namespace privileges is unresolved, hence `NamespaceScopedPrivilege=UNKNOWN`. The race is repeatedly exercisable rather than shown to be narrow or one-shot.

4. Primitive and severity reading

The primitive is a cross-namespace path-resolution isolation failure, not memory corruption: `detach_mounts()` removes covering mounts while the positive dentry remains discoverable. A later lookup consumes this active inconsistent state and opens the hidden underlying inode. The fix calls `d_drop()` before detaching mounts, preventing new lookups from finding that name.

This is CLASS_A because protected underlying content is demonstrably exposed across the mount-namespace boundary. It is not B4-PENDING: no mandatory credential or capability guard changes from DENY to ALLOW; the failure is lookup-state ordering. Important review remains warranted, but write access, privilege escalation, and broad confidentiality impact are not proven.

5. One-sentence report phrase

A race between mount detachment and dentry invalidation lets a local low-privilege lookup cross mount-namespace hiding and read an underlying file.

6. Manual review advice

MANUAL CHECK REQUIRED

The demonstrated cross-namespace disclosure is a concrete security-boundary effect, while deployment setup and writable-resource impact require confirmation.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: unhash a dentry before detaching the mounts on it [ Upstream
Commit: db90eecbde319527aa9480c039d32a0f5fe80574
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=db90eecbde319527aa9480c039d32a0f5fe80574

Changed files:
  fs/namei.c
  fs/namespace.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=db90eecbde319527aa9480c039d32a0f5fe80574

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

The original Linux kernel CVE announcement for CVE-2026-98981 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98981

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:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
  The Best / paranoid CVSS score: 6.3

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: 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: YES
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).