[CVE-2026-98991][MODERATE 7.0] namespace: queue a mount only once for mount notifications

[CVE-2026-98991][MODERATE 7.0] namespace: queue a mount only once for mount notifications

[AL-KERNEL]

CVE: CVE-2026-98991
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: namespace: queue a mount only once for mount notifications
Commit: acbe9e4418e2ef7bef1a12e62e0e7d6145a15190
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=acbe9e4418e2ef7bef1a12e62e0e7d6145a15190
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98991
Analysis date: Mon, 26 Oct 2026 23:24:39 -0400
ActionableScore: 6
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
Namespace-root-controlled repeated unmount processing can corrupt the mount notification list, leave freed mounts linked, and cause later host mount operations to manipulate stale slab objects.

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

The original Linux kernel CVE announcement for CVE-2026-98991 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98991

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
======================================================================

CVE-2026-98991	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98991 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=5 LEGACY=MODERATE_KPANIC RESULT=SAME (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:H/I:H/A:H';*CWE-416;*CWE-672;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'Duplicate insertion of one mount into notify_list corrupts the linked list and leaves notification links referencing mount objects after they are freed following the grace period. Later host mount operations can traverse the freed object and trigger a slab use after free during list manipulation. For the CVSS the PR:L value is selected because an unprivileged local user may obtain the required mount and fanotify capabilities as root inside an enabled user namespace. AC:H reflects the required shrinkable mount source, FAN_MARK_MNTNS setup, mount stacking, and ordered repeated unmount processing. The demonstrated primitive is a kernel use after free with linked list operations. Host denial of service is credible and privilege escalation is plausible but not proven because attacker controlled arbitrary write has not been established. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7 and the production ActionableScore returned 6 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 MODERATE_KPANIC to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because FD false positive review allowed a downgrade. 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_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH. The independent weaker Aegis shadow evaluated CVSS 4.7 and ActionableScore 5 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. 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 6)	MAYBE	READ DANGER DISK INIT UAF IMPROVEONLY DEBUGFS LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=6
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: 6.
* Conservative score: 6; no floor or cap applied.
* Raw paranoid score: 6.
* Paranoid score: 6; no additional unsupported reclaim or write-control bonus.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Real UAF: duplicate `to_notify` insertion corrupts the list, after which mounts are freed while notification links still reference them.
* +1 Reliable kernel crash/strong DoS: KASAN demonstrates slab use-after-free in a later host mount operation.
* +1 Local unprivileged trigger: the commit states that root in an attacker-created user namespace can obtain the required mount and `FAN_MARK_MNTNS` capabilities.
* +1 Attacker-influenced lifetime manipulation: the attacker can construct stacked shrinkable mounts and drive repeated unmount processing that reparents, requeues, and then frees affected mounts.
* +1 Structured stale-object leverage: later list manipulation consumes stale `struct mount` list state, rather than merely detecting it or immediately rejecting it.

No timing penalty: this is an ordered multi-operation sequence, not a proven narrow race. Controlled reclaim, arbitrary target/write, confidentiality, integrity, LPE, and callback control are not shown. The crash receives no duplicate Availability point. No repeated release/refcount signal exists, and no UAF floor changed the arithmetic.

3. Reachability analysis

The supported attacker is a local user who can create and administer a user/mount namespace, obtain namespace-scoped mount and fanotify authority, bind a shrinkable automount-derived mount, and drive the repeated unmount sequence. The commit explicitly says user-namespace root can have both required capabilities, supporting below-host-admin reachability and a namespace-scoped complete setup. No host UID 0, special hardware, or separate host-admin provisioning is shown. User namespaces must be enabled, but that deployment condition does not establish a rare-path penalty.

4. Primitive and severity reading

This is an active structured-object/list UAF: duplicate insertion first corrupts `notify_list`; omitted entries are subsequently freed; later host mount processing traverses and mutates list state through dangling mount links. It is broader than a cleanup-only stale dereference and crosses from namespace-controlled mount activity into host kernel mount processing. However, attacker-selected reclaimed contents or a directed write target is not established, so this remains CLASS_B rather than CLASS_A.

ImportantCandidate is YES because concrete attacker-influenced active mount/list corruption is later consumed by privileged host kernel operations across a user-namespace boundary. Auto-downgrade is disallowed pending exploit-leverage review, although Important severity or LPE is not proven.

5. One-sentence report phrase

Namespace-root-controlled repeated unmount processing can corrupt the mount notification list, leave freed mounts linked, and cause later host mount operations to manipulate stale slab objects.

6. Manual review advice

MANUAL CHECK REQUIRED

The proven active-state UAF and later host list consumer leave material cross-boundary corruption leverage unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: queue a mount only once for mount notifications
Commit: acbe9e4418e2ef7bef1a12e62e0e7d6145a15190
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=acbe9e4418e2ef7bef1a12e62e0e7d6145a15190

Commit description:

mnt_notify_add() puts a mount on notify_list. It doesn't check whether
the mount is on the list already. That's fine as long as every mount is
queued at most once per namespace_sem hold. It isn't.

propagate_umount() moves a surviving overmount off a stack of mounts
that are going away and queues it as moved. shrink_submounts() and
mark_mounts_for_expiry() call umount_tree() for several mounts under a
single namespace_sem hold. So the second umount_tree() can take down
exactly the mount the first one reparented, reparent it once more and so
end up queueing it a second time.

The second list_add_tail() cuts the mounts queued in between out of the
list while the head still points at the last of them. notify_mnt_list()
then only visits that one mount and the others are freed after the grace
period with notify_list still pointing at them. Every later mount
operation on the host walks freed memory:

  BUG: KASAN: slab-use-after-free in __list_add_valid_or_report
  Read of size 8 at addr ffff8881003d57e0 by task notify_dq/162
   __list_add_valid_or_report+0x15c/0x1a0
   mnt_add_to_ns+0x322/0x890
   attach_recursive_mnt.isra.0+0xf2a/0x1a00
   path_mount+0x139c/0x1d40

This needs a mount with MNT_SHRINKABLE to bind from. That's what
finish_automount() creates (submounts of NFS, AFS and CIFS and the
tracefs mount below debugfs). Bind mounts inherit the flag. A mount
namespace with a FAN_MARK_MNTNS mark does the rest and root in a user
namespace can have both.

Initialize to_notify in alloc_vfsmnt() and leave a mount alone that is
queued already. mnt_notify() looks at the state the mount has when the
list is drained so a single entry per mount is enough. The move event
for a mount that is unmounted under the same hold is lost. That's fine.
The detach is what matters.

Fixes: bf630c4 ("vfs: add notifications for mount attach and detach")
Cc: stable@vger.kernel.org # v6.15+
Link: https://patch.msgid.link/20260930-work-mount-fixes-3-v1-1-be34c83956ae@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/mount.h
  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=acbe9e4418e2ef7bef1a12e62e0e7d6145a15190

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

The original Linux kernel CVE announcement for CVE-2026-98991 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98991

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:L/I:L/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: 6

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).