[CVE-2026-98993][IMPORTANT] namespace: don't inherit MNT_UMOUNT in clone_mnt() [ Upstream

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

CVE: CVE-2026-98993
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: namespace: don't inherit MNT_UMOUNT in clone_mnt() [ Upstream
Commit: 99698cfa34e493fc091219056aae446fe9442a96
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=99698cfa34e493fc091219056aae446fe9442a96
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98993
Analysis date: Mon, 26 Oct 2026 17:04:07 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A namespaced local attacker can clone a lazily unmounted mount with `MNT_UMOUNT`, causing premature namespace-reference loss and later host-kernel consumption of a stale `mnt_namespace` pointer.

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

The original Linux kernel CVE announcement for CVE-2026-98993 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98993

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: IMPORTANT
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
======================================================================

CVE-2026-98993	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98993 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT IMPORTANT AEGIS_AS=6 LEGACY=IMPORTANT RESULT=SAME INDEPENDENT_LEGACY=MODERATE_KPANIC INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=MODERATE_KPANIC (final reconciliation agrees; DECISION: both 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:H/I:H/A:H';*CWE-416;*CWE-672;CWE-911;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'clone_mnt could copy MNT_UMOUNT from a lazily unmounted source and attach the clone to a new mount namespace with an invalid internal state. The inherited flag can make __detach_mounts drop a namespace reference too early and leave a mount pointing to a freed struct mnt_namespace. It can also corrupt propagation bookkeeping and cause repeated unmount traversal. For the CVSS the PR:L value reflects a local user able to create and administer a user and mount namespace when unprivileged user namespaces are enabled. The base impact is local denial of service. The paranoid HHH impact accounts for the concrete stale pointer and reference lifetime failure while arbitrary write or reliable privilege escalation is not established. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 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 ActionableScore threshold rule was applied. final reconciliation stage MODERATE_KPANIC to IMPORTANT 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_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 4.6 and ActionableScore 6 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was IMPORTANT. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	DISK LINUS INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 5.
* Conservative score: 5; no floor or cap changed the result.
* Raw paranoid score: 5.
* Paranoid score: 5; stronger reclaim or structured-reuse leverage is not established.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF/lifetime violation: inherited `MNT_UMOUNT` makes `__detach_mounts()` drop the namespace-held reference prematurely, leaving the live mount referring to a freed `struct mnt_namespace`.
* +1 Local unprivileged trigger: the supplied reproducer is available to a user able to create and administer a user/mount namespace where unprivileged user namespaces are enabled.
* +1 Attacker-influenced active-state/lifetime manipulation: the actor drives bind, lazy unmount, cloning through the retained fd, detach, and subsequent propagation or `mount_setattr()` consumption.
* +1 Plausible kernel crash/strong DoS: later reads through the stale namespace pointer and repeated unmount traversal can fault or corrupt teardown processing.
* No extra Availability point: it would duplicate the same crash/DoS.
* No repeated-release bonus: the description does not establish two specific lifetime-mutating operations on the exact same dangling reference.
* No structured-reuse or strong-leverage bonus: controlled reclaim, attacker-shaped replacement contents, arbitrary write, callback control, disclosure, and LPE are not shown.
* No C/I bonus: propagation corruption and stale reads do not prove unauthorized modification of protected host state.
* B5-R, B3-PENDING, and B4-PENDING do not apply: there is no retry-accumulated OOB write, shared-backing write, or authorization DENY-to-ALLOW change.

3. Reachability analysis

A normal local user can complete the demonstrated sequence inside an attacker-created user and mount namespace when unprivileged user namespaces are enabled, obtaining the namespace-scoped mount capability without host-administrator powers. No host UID 0, special hardware, prior host provisioning, or timing race is shown. Systems disabling unprivileged user namespaces restrict exposure, but that deployment condition does not negate the evidenced below-host-admin path in affected configurations.

4. Primitive and severity reading

This is active structured lifetime corruption, not fixed stale storage: an invalid inherited internal flag causes premature reference loss, self-parenting mount state, and a stale `mnt_namespace` pointer later consumed by privileged VFS operations. The shown effects are host-kernel UAF reads, propagation/bookkeeping corruption, leaks, and plausible host DoS. Controlled post-free contents or a useful write primitive remain unproven, so this is CLASS_B rather than CLASS_A.

ImportantCandidate is YES because attacker-created namespace state corrupts active mount state that is subsequently trusted by the host kernel across a specific namespace/kernel trust boundary. Auto-downgrade is unsafe until the later `may_change_propagation()` and `anon_ns_root()` consumers and reclaim behavior are reviewed.

5. One-sentence report phrase

A namespaced local attacker can clone a lazily unmounted mount with `MNT_UMOUNT`, causing premature namespace-reference loss and later host-kernel consumption of a stale `mnt_namespace` pointer.

6. Manual review advice

MANUAL CHECK REQUIRED

Review stale namespace-object consumption and reclaim behavior; a concrete active-state UAF crosses the namespace-to-host-kernel boundary even though arbitrary write or LPE is not shown.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: don't inherit MNT_UMOUNT in clone_mnt() [ Upstream
Commit: 99698cfa34e493fc091219056aae446fe9442a96
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=99698cfa34e493fc091219056aae446fe9442a96

Changed files:
  fs/namespace.c
  include/linux/mount.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=99698cfa34e493fc091219056aae446fe9442a96

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

The original Linux kernel CVE announcement for CVE-2026-98993 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98993

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:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 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: 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: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT

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).