[CVE-2026-98978][IMPORTANT] namespace: never expire a locked mount

[CVE-2026-98978][IMPORTANT] namespace: never expire a locked mount

[AL-KERNEL]

CVE: CVE-2026-98978
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: namespace: never expire a locked mount
Commit: 6b8dc3713a76346d212abab46cbfc8d7070f534c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b8dc3713a76346d212abab46cbfc8d7070f534c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98978
Analysis date: Tue, 27 Oct 2026 03:38:04 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
A local user-namespace owner can bypass locked-mount protection through ancestor expiry and expose readable content that the protected mount was intended to conceal.

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

The original Linux kernel CVE announcement for CVE-2026-98978 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98978

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-2026-98978	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98978 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 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:L/PR:L/UI:N/S:U/C:H/I:H/A:L';CWE-284;CWE-668;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N';BEST CVSS score: '7.3';DESCR 'A mount namespace access control flaw allows shrink_submount to expire a locked mount because it checks whether the mount is busy but does not reject MNT_LOCKED. After MOVE_MOUNT_BENEATH transfers the locked property to the lower mount, unmounting an unlocked ancestor can remove that protected mount and reveal the directory and files it was intended to hide. The demonstrated primitive is unauthorized path exposure and reading of covered content rather than memory corruption. For the CVSS the PR:L value reflects that exploitation requires a local user or process able to own a user namespace and manipulate its mount topology. The base score assigns high confidentiality impact because the example confirms disclosure of a hidden secret. The paranoid score also considers integrity and limited availability impact when the revealed directory contains writable or operationally sensitive files, although normal DAC and LSM checks still apply and privilege escalation is not proven. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7.3 and the production ActionableScore returned 7 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 B4_CONFIRMED_AUTHORIZATION_PRESERVE and MANUAL_REVIEW_REQUIRED and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 4.3 and ActionableScore 4 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 7)	MAYBE	DISK LINUS INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 TEMP_KPANIC_MERGE_IMPORTANT_VETO B4_CONFIRMED_AUTHORIZATION_PRESERVE MANUAL_REVIEW_REQUIRED KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=7
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: 7.
* Conservative score: 7; no floor or cap applied.
* Raw paranoid score: 7.
* Paranoid score: 7; no additional speculative integrity or availability impact.
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

* +2 Concrete authorization decision bypass: `shrink_submount()` could remove an `MNT_LOCKED` mount despite the protection that makes direct `do_umount()` refuse removal.
* +2 Practical protected-resource access: expiring the locked lower mount reveals the covered directory, and the supplied sequence demonstrates reading hidden `secret` content.
* +1 Local unprivileged trigger: the evidenced actor owns an attacker-created user namespace and manipulates its mount namespace without effective host-administrator powers.
* +1 Practical exploit chain: `MOVE_MOUNT_BENEATH` transfers the lock downward, ancestor unmount triggers expiry, and the previously covered path becomes visible.
* +1 Confidentiality impact: actual reading of concealed content is demonstrated.
* +0 Integrity/availability: writes, privilege escalation, persistent damage, and host DoS are not shown.
* +0 Memory-corruption signals: no UAF, OOB access, or kernel-memory write exists.

3. Reachability analysis

The complete demonstrated actor is a local user able to create/own a user namespace and manipulate its associated mount topology. The relevant mount-administration authority is scoped to that attacker-controlled namespace; host UID 0 and effective host-administrator capabilities are not required. No separate privileged provisioning, special hardware, race, or timing condition is shown.

Normal DAC and LSM checks still govern the newly visible inode, but the supplied evidence confirms that at least one concealed resource is readable after the mount-lock protection is bypassed.

4. Primitive and severity reading

The primitive is a mount-namespace authorization/protection bypass, not memory corruption. The missing `IS_MNT_LOCKED()` test lets automatic submount shrinking perform an operation that the locked state is intended to deny. The fix cleanly restores that guard before `umount_tree()`.

B4-PENDING is confirmed through the concrete path: namespace owner invokes ancestor unmount → `shrink_submount()` omits the locked-mount denial → protected lower mount is removed → hidden content is exposed and read. This is CLASS_A and Important-preserving, while integrity impact, arbitrary file access, host escape, and LPE remain unproven.

5. One-sentence report phrase

A local user-namespace owner can bypass locked-mount protection through ancestor expiry and expose readable content that the protected mount was intended to conceal.

6. Manual review advice

MANUAL CHECK REQUIRED

The demonstrated below-host-admin DENY-to-ALLOW path reaches a meaningfully protected resource, so automatic Moderate downgrading would discard a concrete namespace security-boundary bypass.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: never expire a locked mount
Commit: 6b8dc3713a76346d212abab46cbfc8d7070f534c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6b8dc3713a76346d212abab46cbfc8d7070f534c

Commit description:

Locked mounts are special. They protect the underlying files and
directories from being revealed. do_umount() refuses to unmount locked
mounts but shrink_submounts() doesn't.

A shrinkable mount can become locked once the owner of a user namespace
puts it beneath a locked mount with MOVE_MOUNT_BENEATH. The locked
property now moves to the mount at the bottom making it possible to
unmount the top mount.

So umount() of an unlocked ancestor now expires the bottom mount first
and the covered directory is revealed:

  move_mount(c -> x/hidden, MOVE_MOUNT_BENEATH) = 0
  the cover after the lock moved:  umount2(x/hidden) = 0
  the holder of the lock:          umount2(x/hidden) = EINVAL
  the root of the copy, busy:      umount2(x) = EBUSY
  reads x/hidden/secret: "covered-by-root"

So leave a locked mount alone as it dies together with its parent. A
plain umount() of an unlocked mount with a locked one below it is EBUSY
from now on. It's the same for any other locked child. A lazy umount
still takes the whole tree.

mark_mounts_for_expiry() never sees a locked mount. lock_mnt_tree()
leaves a mount on an expiry list alone and nothing else puts a locked
one on such a list. Add an assert for this.

Fixes: 5ff9d8a ("vfs: Lock in place mounts from more privileged users")
Fixes: c62a476 ("move_mount: transfer MNT_LOCKED")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261002-work-mount-fixes-4-v1-8-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=6b8dc3713a76346d212abab46cbfc8d7070f534c

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

The original Linux kernel CVE announcement for CVE-2026-98978 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98978

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:H/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:L
  The Best / paranoid CVSS score: 7.3

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: 7

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).