[CVE-2026-98992][LOW] namespace: don't copy an unmounted mount tree [ Upstream

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

CVE: CVE-2026-98992
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: namespace: don't copy an unmounted mount tree [ Upstream
Commit: 86a2d56b1960720bf1844441c0890621e9db2f25
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86a2d56b1960720bf1844441c0890621e9db2f25
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98992
Analysis date: Mon, 26 Oct 2026 14:52:48 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
Recursive bind mounting could incorrectly recreate child mounts attached to a lazily unmounted nsfs or pidfs mount, without demonstrated memory corruption or protected-resource access.

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

The original Linux kernel CVE announcement for CVE-2026-98992 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98992

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: LOW
Manual review required: NO

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
======================================================================

CVE-2026-98992	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98992 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, 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:L/I:L/A:L';CWE-668;CWE-284;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'may_copy_tree allowed an nsfs or pidfs mount root to pass before confirming that the mount still belonged to a mount namespace. With MS_REC or AT_RECURSIVE, copy_tree could then walk a lazily unmounted tree and copy every child mount that remained attached to it. The concrete primitive is unintended recursive reattachment and duplication of a detached mount stack, not UAF, OOB access, or arbitrary memory writes. For the CVSS the PR:L value reflects the need for local mount operations with CAP_SYS_ADMIN in an applicable mount or user namespace and avoids treating namespace scoped administration as full host PR:H. The base impact is limited availability loss from unexpected mount duplication. Limited confidentiality or integrity impact is plausible only if detached child mounts contain resources that should no longer be reachable through the recreated tree. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.3 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 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 LOW.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	DISK IMPROVEONLY LINUS KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; no stronger impact is sufficiently evidenced.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 weak/indirect active-state error: recursive `copy_tree()` can copy child mounts from a lazily unmounted nsfs/pidfs mount tree, producing unintended active mount-namespace state.
* No authorization-bypass points: the new check is structural validation of recursive copying, not evidence that a mandatory capability or permission decision changed from DENY to ALLOW.
* No availability point: extra mount duplication is concrete, but host-wide exhaustion, persistent leakage, or a reliable crash is not shown.
* No confidentiality or integrity point: detached children containing separately protected resources is only hypothetical; no protected victim mount or cross-authority access is demonstrated.
* No UAF, OOB access, arbitrary write, lifetime corruption, B5-R retry chain, or B3 shared-backing write is present.
* No privilege penalty: CAP_SYS_ADMIN scope for the complete sequence is not resolved well enough to prove host-admin-equivalent or delegated-only privilege.

3. Reachability analysis

The demonstrated sequence requires local bind-mount, lazy-unmount, and recursive bind operations. These require mount-management authority, but the evidence does not establish whether the complete sequence works with attacker-obtainable CAP_SYS_ADMIN in a user/mount namespace or instead requires host-level setup. Thus both host-admin and namespace-scoped reachability remain unknown. No special hardware, race, or timing condition is involved.

4. Primitive and severity reading

The primitive is unintended recursive reattachment and duplication of an unmounted mount subtree. It is an invalid active-state operation, not memory corruption or an authorization-decision bypass. The copied mounts are subsequently active VFS objects, but no protected-resource boundary crossing is shown. Accordingly this is CLASS_C with possible operational/resource impact, not an Important-preserving Class A/B primitive. ImportantCandidate is NO and auto-downgrade is allowed, while manual review remains recommended to verify namespace privilege and whether real detached child mounts can expose resources across an authority boundary.

5. One-sentence report phrase

Recursive bind mounting could incorrectly recreate child mounts attached to a lazily unmounted nsfs or pidfs mount, without demonstrated memory corruption or protected-resource access.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Verify the complete capability scope and whether any copied child mount can belong to a more privileged authority.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: don't copy an unmounted mount tree [ Upstream
Commit: 86a2d56b1960720bf1844441c0890621e9db2f25
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86a2d56b1960720bf1844441c0890621e9db2f25

Changed files:
  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=86a2d56b1960720bf1844441c0890621e9db2f25

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

The original Linux kernel CVE announcement for CVE-2026-98992 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98992

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:L/PR:L/UI:N/S:U/C:L/I:L/A:L
  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: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).