[CVE-2026-98985][MODERATE 7.0] namespace: check the mounts before reading their parents in pivot_root() [ Upstream

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

CVE: CVE-2026-98985
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: namespace: check the mounts before reading their parents in pivot_root() [ Upstream
Commit: a56d4f8f8f4c5d9ac339d5c03a9d90eecd3697fc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a56d4f8f8f4c5d9ac339d5c03a9d90eecd3697fc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98985
Analysis date: Mon, 26 Oct 2026 17:13:22 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A namespace-capable local user can race `pivot_root()` against mount-parent cleanup to cause a four-byte slab UAF read and possible kernel fault, without a demonstrated write, disclosure, or policy bypass.

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

The original Linux kernel CVE announcement for CVE-2026-98985 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98985

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-2026-98985	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98985 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (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:N/I:N/A:H';*CWE-416;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A race in path_pivot_root can read the parent of a detached mount after the parent has been freed. The demonstrated primitive is a 4 byte slab use after free read in IS_MNT_SHARED while another task closes the final parent reference. For the CVSS the PR:L value is used because triggering requires local namespace operations and pivot_root capability in the governing user namespace. An unprivileged user may obtain that capability inside a new user and mount namespace when such namespace creation is enabled. AC:H is used because exploitation requires winning a race between pivot_root and final reference cleanup. The affected checks return EINVAL in either case and no controlled write or information disclosure primitive is established. Impact is normally limited to availability and may become a kernel crash on diagnostic or panic configured systems. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 4.7 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 IMPORTANT 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. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule 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 LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS unavailable and ActionableScore 2 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	MAYBE	READ DANGER DISK UAF IMPROVEONLY LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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

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

1. ActionableScore

* Raw base score: 3.
* Conservative score: 4. Real-UAF FLOOR 4 applied: 3 -> 4.
* Raw paranoid score: 3.
* Paranoid score: 4. The same floor applies; no additional exploit primitive is supported.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 — Confirmed slab use-after-free read: `new_mnt->mnt_parent` or `root_mnt->mnt_parent` can be freed before `IS_MNT_SHARED()` reads its flags.
* +1 — Plausible kernel crash/DoS, demonstrated as a KASAN slab-UAF report; fatality outside diagnostic/panic configurations is not guaranteed.
* +1 — Below-host-admin local trigger is supported through attacker-created user and mount namespaces when unprivileged namespace creation is enabled.
* -1 — A materially timing-sensitive race is required between `pivot_root()` and final parent-reference cleanup.
* Floor 4 — Real stale-object use occurs without host-admin-equivalent privilege.
* No controlled reclaim, write, disclosure, repeated release, callback use, or structured post-reuse processing is shown.
* No extra availability point: it would duplicate the same possible crash.
* No C/I impact: both check outcomes return `-EINVAL`, so no successful state change or authorization bypass occurs.

3. Reachability analysis

The grounded attacker is a local user able to create a user namespace and mount namespace, obtain the namespace-scoped capability needed for `pivot_root()`, retain a detached mount through an FD, and race another task closing the final parent reference. This supports below-host-admin, namespace-scoped reachability where unprivileged user namespaces are enabled. No special hardware or prior host provisioning is required beyond that system policy. The race is explicitly narrow enough to justify the timing penalty.

4. Primitive and severity reading

The primitive is a four-byte slab UAF read from a freed mount-parent object. Although the mount is a structured lifetime-managed object, the stale operation only reads the shared-mount flag and immediately returns `-EINVAL` regardless of the value. There is no second lifetime mutation or later consumption of attacker-shaped state.

This is constrained CLASS_B lifetime corruption with crash potential, not CLASS_A exploitation leverage. ImportantCandidate is NO because no useful effect remains after excluding the fault. Auto-downgrade is allowed, although manual review remains advisable for deployment-specific panic behavior.

5. One-sentence report phrase

A namespace-capable local user can race `pivot_root()` against mount-parent cleanup to cause a four-byte slab UAF read and possible kernel fault, without a demonstrated write, disclosure, or policy bypass.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm unprivileged user-namespace policy and whether KASAN/Oops behavior is fatal in the assessed kernel.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: check the mounts before reading their parents in pivot_root() [ Upstream
Commit: a56d4f8f8f4c5d9ac339d5c03a9d90eecd3697fc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a56d4f8f8f4c5d9ac339d5c03a9d90eecd3697fc

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=a56d4f8f8f4c5d9ac339d5c03a9d90eecd3697fc

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

The original Linux kernel CVE announcement for CVE-2026-98985 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98985

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 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).