[CVE-2026-98987][MODERATE 7.0] namespace: look at the topmost mount for a mount namespace file

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

CVE: CVE-2026-98987
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: namespace: look at the topmost mount for a mount namespace file
Commit: b9c40553f6ceba50042281eef6f701e5fb719726
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b9c40553f6ceba50042281eef6f701e5fb719726
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98987
Analysis date: Tue, 27 Oct 2026 00:55:54 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A user-namespace-capable local attacker can construct a propagated mount topology that misplaces a live mount beneath an unreachable nsfs dentry, changing later path resolution and blocking synchronous unmount.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98987 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98987

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-98987	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98987 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE_KPANIC, Aegis: MODERATE)	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-670;*CWE-664;CWE-706;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'attach_recursive_mnt did not inspect the topmost source mount when searching for a mount namespace file. If that mount was the only namespace file in the overmount chain, shorter remained NULL and an existing mount from a propagated copy could be attached below an unreachable nsfs dentry. The visible path could then expose S instead of Q, while Q remained hidden and synchronous unmount returned EBUSY because the replacement mount still had a child. For the CVSS the PR:L value reflects that reliable triggering requires local mount namespace operations and CAP_SYS_ADMIN in the governing user namespace, which can be obtained through unprivileged user namespaces on affected configurations. The demonstrated primitive is mount topology corruption and limited availability loss, not memory corruption or a proven privilege escalation path. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.3 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 2.2 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE. The models disagreed but the aggregation rules retained the production result. 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 LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=NO
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: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 5.
* Paranoid score: 5; no floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +1 weak/indirect structured-state corruption: failure to inspect the topmost overmount leaves `shorter == NULL`, causing `mnt_change_mountpoint()` to attach an existing mount beneath an unreachable nsfs dentry.
* +1 local unprivileged trigger: supplied evidence grounds the complete operation in an attacker-created user namespace with namespace-governed `CAP_SYS_ADMIN` on affected configurations.
* +1 high-control local API: the attacker controls the bind/overmount, propagation, and `move_mount()` topology directly relevant to the defect.
* +1 availability effect: the displaced mount becomes hidden and synchronous unmount returns `EBUSY`.
* Paranoid +1 specific trust-boundary risk: propagated copies later consume the corrupted active topology and resolve the path to S instead of Q; authority separation between source and copy namespaces is credible but unresolved.
* No memory OOB, UAF, arbitrary write, controlled reclaim, auth bypass, protected-data modification, crash, or LPE is shown. No separate confidentiality or integrity-impact point is awarded.

3. Reachability analysis

The evidenced attacker is a local user able to create an affected user/mount namespace and obtain `CAP_SYS_ADMIN` in its governing user namespace. This does not require host UID 0 or effective host-administrator power. The attacker can instantiate the necessary mounts and invoke `move_mount()`. No special hardware, boot mode, prior host provisioning, or timing race is shown. Cross-principal ownership of propagated copies is not established.

4. Primitive and severity reading

The primitive is corruption of live mount topology, not kernel-memory corruption. An existing mount is attached under an unreachable nsfs dentry; later path lookup and unmount operations consume that malformed active state, returning the wrong visible object and `EBUSY`. This is CLASS_B structured-state corruption with a specific namespace/delegated-interface boundary requiring review. Stronger cross-namespace integrity impact remains unproven, but the active-state consumer chain prevents automatic downgrade.

5. One-sentence report phrase

A user-namespace-capable local attacker can construct a propagated mount topology that misplaces a live mount beneath an unreachable nsfs dentry, changing later path resolution and blocking synchronous unmount.

6. Manual review advice

MANUAL CHECK REQUIRED

Confirm whether propagated copies can belong to a more privileged mount namespace or principal and whether later VFS consumers produce effects beyond the demonstrated wrong-path and `EBUSY` behavior.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: look at the topmost mount for a mount namespace file
Commit: b9c40553f6ceba50042281eef6f701e5fb719726
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b9c40553f6ceba50042281eef6f701e5fb719726

Commit description:

attach_recursive_mnt() preallocates a mountpoint for the root of the
topmost mount of the source so that a mount that already sits at the
destination can be put on top of the source or on top of one of its
propagated copies.

The copies are made without CL_COPY_MNT_NS_FILE so their chain of
overmounts is shorter than the source's. It ends below the first bind
mount of a mount namespace file. So while the loop walks up the chain of
the source it remembers the mountpoint of that mount in "shorter" and
the existing mount is put there for the copies.

But the loop ends at the topmost mount without ever looking at it. If
the topmost mount is the only mount namespace file in the chain
"shorter" stays NULL and mnt_change_mountpoint() attaches the existing
mount of the copy below the root of the mount namespace file. That's a
dentry of nsfs. No path walk in the copy's mount namespace ever gets
there. The mount is gone until its parent goes and the copy that took
its place can't be unmounted synchronously because it has a child:

  S      bind mount of a file
  N      bind mount of a mount namespace file on top of S
  A      shared mount, B a slave of A, Q mounted on B/file

  move_mount(S, A/file)
  cat B/file          -> the content of S instead of Q
  umount B/file       -> EBUSY

Look at every mount of the chain including the topmost one.

Fixes: 96f5d2e ("attach_recursive_mnt(): unify the mnt_change_mountpoint() logics")
Cc: stable@vger.kernel.org # v6.17+
Link: https://patch.msgid.link/20260930-work-mount-fixes-3-v1-8-be34c83956ae@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=b9c40553f6ceba50042281eef6f701e5fb719726

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

The original Linux kernel CVE announcement for CVE-2026-98987 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98987

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: 4
  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).