[CVE-2026-98994][MODERATE 7.0] statmount: read the parent of an unmounted mount under mount_lock

[CVE-2026-98994][MODERATE 7.0] statmount: read the parent of an unmounted mount under mount_lock

[AL-KERNEL]

CVE: CVE-2026-98994
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: statmount: read the parent of an unmounted mount under mount_lock
Commit: 98fc6c7b6ca676157d2c780bd0a537b2c10fe08f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=98fc6c7b6ca676157d2c780bd0a537b2c10fe08f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98994
Analysis date: Mon, 26 Oct 2026 23:35:09 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A local `STATMOUNT_BY_FD` race can read two identifier fields through a freed parent-mount pointer, plausibly crashing the kernel without a shown write or exploitation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-98994 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98994

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-2026-98994	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98994 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 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:L/I:N/A:H';*CWE-416;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'A race in STATMOUNT_BY_FD allows statmount_mnt_basic to read mnt_parent while final mntput detaches children and frees the parent of an unmounted mount under mount_lock. The primitive is a use-after-free read of two mount identifier fields, which can cause a kernel crash and may expose stale or recycled identifier values, but the shown path provides no write primitive. For the CVSS the PR:L value reflects that a local process must hold a mount file descriptor and invoke statmount. AC:H reflects the required race with final mount teardown. The issue is not directly network reachable and the supported impact is primarily local denial of service with at most limited confidentiality impact. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.3 and the production ActionableScore returned 4 with recommendation Borderline / manual review recommended. 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 2.5 and ActionableScore 4 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 Borderline / manual review recommended (with actual score 4)	MAYBE	DISK LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 4.
* Conservative score: 4; no floor or cap changed the result.
* Raw paranoid score: 4.
* Paranoid score: 4; no additional exploitation primitive is supported.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Real UAF: final `mntput()` can free the unmounted mount’s parent while `statmount_mnt_basic()` dereferences `m->mnt_parent`.
* +1 Reliable kernel crash: dereferencing the freed parent can plausibly fault the kernel.
* +1 Local below-host-admin trigger: supplied evidence grounds a low-privilege local process holding the mount FD and invoking `STATMOUNT_BY_FD`.
* No race penalty: concurrency is required, but the commit says the stale dereference occurs easily; narrow or uncontrollable timing is not shown.
* No confidentiality point: stale/recycled mount identifiers may be returned, but no protected kernel data disclosure is demonstrated.
* No structured-object leverage: only two scalar mount-ID fields are read; no pointer traversal, callback, write, refcount mutation, or controlled reclaim follows.
* No extra availability point: it would duplicate the same crash.
* NEW52/B5-R and NEW53/B3-PENDING do not apply because no write exists.
* No authorization signal: the patch restores lifetime synchronization, not a DENY-to-ALLOW guard.

3. Reachability analysis

The supported attacker is a local, below-host-admin process with a usable FD referring to an intentionally unmounted mount. No host UID 0, capability, namespace-scoped privilege, special hardware, or prior host-administrator setup is shown as necessary. Triggering requires overlap with final mount teardown, but the evidence does not establish a genuinely hard or one-shot race.

4. Primitive and severity reading

This is a real race-driven UAF read from an active structured `struct mount`, not fixed stale storage. The stale use is shallow: it reads `mnt_id_unique` and `mnt_id` from the freed parent. The fix takes `mount_lock` for the unmounted case, preventing parent release and ensuring both IDs come from one live mount.

The primitive is CLASS_B lifetime corruption, with crash as the supported impact. Stale identifier output is possible, but useful disclosure, writes, object replacement, LPE, or authorization bypass are unproven. ImportantCandidate is therefore NO, and constrained crash-oriented downgrade remains allowed.

5. One-sentence report phrase

A local `STATMOUNT_BY_FD` race can read two identifier fields through a freed parent-mount pointer, plausibly crashing the kernel without a shown write or exploitation primitive.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm syscall/FD permission details and whether allocator reuse can expose anything beyond ordinary mount identifiers.

7. Call-site confidence

Call-site confidence: high

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

Patch: statmount: read the parent of an unmounted mount under mount_lock
Commit: 98fc6c7b6ca676157d2c780bd0a537b2c10fe08f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=98fc6c7b6ca676157d2c780bd0a537b2c10fe08f

Commit description:

statmount_mnt_basic() reads the ids of the parent through m->mnt_parent
with namespace_sem held for reading. That's fine for a mount that is
in a namespace. Everything above it is pinned by that namespace.

STATMOUNT_BY_FD also accepts a file descriptor on a mount that has
been unmounted on purpose and that only the file descriptor keeps alive.
If that mount stayed attached to its parent when it was unmounted the
parent is unmounted as well and nothing pins it for the child.

The final mntput() unmounts the children under mount_lock alone and
frees the parent afterwards. Concurrently, statmount_mnt_basic() reads
m->mnt_parent twice while that can happen and can easily dereference a
mount that's already gone.

Read the parent of an unmounted mount under mount_lock. The parent
can't be put while it is held and both ids come from the same mount. A
mounted mount's parent is pinned by the namespace and a mount can't
leave its namespace while namespace_sem is held, so the common case
takes no global lock.

Fixes: 0e50322 ("statmount: accept fd as a parameter")
Cc: stable@vger.kernel.org # v7.0+
Link: https://patch.msgid.link/20260926-work-mount-fixes-2-v1-2-f357abf3d17b@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=98fc6c7b6ca676157d2c780bd0a537b2c10fe08f

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

The original Linux kernel CVE announcement for CVE-2026-98994 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98994

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:L/I:N/A:H
  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: unknown
  Paranoid ActionableScore: 4

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).