[CVE-2026-98976][IMPORTANT] namespace: keep the lock on a mount that a propagated copy is moved beneath

[CVE-2026-98976][IMPORTANT] namespace: keep the lock on a mount that a propagated copy is moved beneath

[AL-KERNEL]

CVE: CVE-2026-98976
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: namespace: keep the lock on a mount that a propagated copy is moved beneath
Commit: f1f0c24ca701d7d3061c7f69f7450f1734c11745
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f1f0c24ca701d7d3061c7f69f7450f1734c11745
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98976
Analysis date: Mon, 26 Oct 2026 14:24:24 -0400
ActionableScore: 8
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
A propagated mount can strip `MNT_LOCKED` from a namespace cover, allowing a below-host-admin namespace owner to unmount it and expose specifically protected host content.

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

The original Linux kernel CVE announcement for CVE-2026-98976 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98976

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-2026-98976	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98976 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=5 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:L/A:L';CWE-284;CWE-668;CWE-862;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:N/A:N';BEST CVSS score: '7';DESCR 'A mount namespace access control flaw allows a propagated mount placed beneath a locked cover to inherit MNT_LOCKED while the original cover incorrectly becomes unlocked. When the propagated copy is later removed by an unmount in the originating namespace, the user namespace owner can unmount the remaining cover and expose host content that the locked mount was intended to hide. For the CVSS the PR:L value reflects that exploitation requires control of a user namespace and its mount operations but does not require host root privileges. The issue is local and requires a specific shared mount propagation setup and external mount activity, so AC:H is appropriate. The demonstrated impact is cross namespace confidentiality loss. Replacing a locked root or proc cover may also create limited integrity or availability consequences in environment dependent cases. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 8 with recommendation Strong Important candidate. 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. ActionableScore thresholds stage MODERATE_KPANIC to IMPORTANT because the ActionableScore threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because a specific evidence based protection or manual review veto was applied and 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 produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 and TEMP_KPANIC_MERGE_IMPORTANT_VETO and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 3.4 and ActionableScore 5 with primitive class CLASS_A 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 IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 8)	MAYBE	DISK IMPROVEONLY LINUS INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=8
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=YES
ProtectedResourceImpact=CONFIRMED
B4PendingReview=YES
B4PendingConfirmed=YES

1. ActionableScore

* Raw base score: 8.
* Conservative score: 8; no floor or cap applied.
* Raw paranoid score: 8.
* Paranoid score: 8; no additional speculative impact scored.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

* +2 concrete authorization decision bypass: loss of `MNT_LOCKED` changes `umount2(P/d)` from `EINVAL` to success.
* +2 practical security-boundary bypass: a user-namespace owner can remove a cover intended to hide an underlying host-provided mount.
* +1 security-state lifetime mismatch: the cover loses its lock while it remains responsible for hiding the underlying mount after the propagated child disappears.
* +1 local below-host-admin trigger: the demonstrated final unmount and read are performed by the user-namespace owner.
* +1 practical exploit chain: host propagation/unmount causes lock loss, followed by namespace unmount and access to the exposed path.
* +1 confidentiality impact: the supplied sequence specifically reads `P/d/secret`.
* No integrity, availability, memory-corruption, LPE, or arbitrary-access bonus was awarded. The host propagation activity is a substantial prerequisite, but no narrow race or unreliable timing is shown, so no timing penalty applies.

3. Reachability analysis

The attacker is the owner of a user and mount namespace, not host UID 0 or a host administrator. Namespace-scoped mount authority is sufficient for the attacker-controlled unmount stage. A shared host mount and host-side mount/unmount propagation are required, but the patch evidence does not show that the attacker controls those host operations. This limits trigger opportunity without negating the demonstrated below-host-admin boundary bypass.

4. Primitive and severity reading

The primitive is an authorization-state preservation failure, not memory corruption: `attach_recursive_mnt()` transfers `MNT_LOCKED` to a propagated child and incorrectly clears it from the existing cover. Once the child is removed by propagation, the unlocked cover can be unmounted.

B4-PENDING is confirmed through the concrete path: propagated mount lifecycle → lost `MNT_LOCKED` → `umount2()` changes from DENY to ALLOW → protected underlying secret becomes readable. This is CLASS_A because a mandatory mount-namespace isolation guard is bypassed with a demonstrated protected-resource effect. Root escalation and generalized host-file access remain unproven.

5. One-sentence report phrase

A propagated mount can strip `MNT_LOCKED` from a namespace cover, allowing a below-host-admin namespace owner to unmount it and expose specifically protected host content.

6. Manual review advice

MANUAL CHECK REQUIRED

The demonstrated cross-namespace authorization bypass and secret exposure preserve an Important-class candidate despite the host propagation prerequisite.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: keep the lock on a mount that a propagated copy is moved beneath
Commit: f1f0c24ca701d7d3061c7f69f7450f1734c11745
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f1f0c24ca701d7d3061c7f69f7450f1734c11745

Commit description:

attach_recursive_mnt() transfers MNT_LOCKED from the top mount to the
mount that is moved beneath it with MOVE_MOUNT_BENEATH. This allows the
owner of a user namespace to replace its locked /proc or its root. The
mount beneath takes on the job of covering the underlying mountpoint
allowing the top mount to be unmounted.

Consider two mount namespaces:

(H) The host H has a shared mount P with a secret in P/d
(Z) Z is a user namespace made from M. Its copy of P receives
    propagation from the host's P and its copy of M's cover on P/d is
    locked Z's owner must not get to see P/d.

Now (H) mounts X on P/d. This propagates into (Z). The copy of X lands
beneath (Z)'s locked cover. The locked property is transfered from (Z)'s
cover to the copy of X propagated beneath it. The cover is now unlocked.

Now (H) unmounts X again. The copy of X in (Z) gets unmounted and the
covering mount is left unlocked on top of P/d. (Z) can now unmount it:

    Z: umount2(P/d)             = EINVAL            /* the cover is locked */
    H: mount X on P/d, umount X                     /* both propagate into Z /*
    Z: umount2(P/d)             = 0                 /* the cover is now unlocked */
    Z: read P/d/secret          = "covered-by-root" /* secret revealed */

So only transfer the locked property to the mount beneath for mounts the
caller has placed. A propagated copy that lands beneath a locked mount
is locked as well so that the mount at the bottom of the stack carries a
lock the way every check expects. The mount on top of it remains locked
to ensure that it keeps covering even if the propagated mount is
unmounted again.

Fixes: c62a476 ("move_mount: transfer MNT_LOCKED")
Cc: stable@vger.kernel.org # v7.1+
Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-9-dd44b89d44ce@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=f1f0c24ca701d7d3061c7f69f7450f1734c11745

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

The original Linux kernel CVE announcement for CVE-2026-98976 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98976

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:C/C:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:L/A:L
  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: 8

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).