[CVE-2026-99172][LOW] ipc: drop the mqueue mount with kern_unmount() on setup failure

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

CVE: CVE-2026-99172
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ipc: drop the mqueue mount with kern_unmount() on setup failure
Commit: c0620b648321fff654173d238d4a85298ea16d05
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0620b648321fff654173d238d4a85298ea16d05
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99172
Analysis date: Wed, 28 Oct 2026 22:16:15 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
An incorrect failure-path `mntput()` leaks an internal mqueue mount and superblock, but reliable attacker-driven accumulation or host-wide denial of service is not established.

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

The original Linux kernel CVE announcement for CVE-2026-99172 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99172

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: LOW
Manual review required: NO

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-2026-99172	LOW	https://www.kernelcve.org/list/?q=CVE-2026-99172 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=1 LEGACY=LOW RESULT=SAME (final reconciliation agrees; DECISION: both LOW)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-772;CWE-404;*CWE-401;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'An error path after mq_init_ns used mntput for a long term internal mqueue mount. The reference count could reach zero lacking destruction of the mount and its mqueue superblock, causing persistent resource retention after IPC namespace setup failed. For the CVSS the PR:L value reflects that a local user or process must initiate IPC namespace creation, while AC:H reflects the additional need to make a later sysctl or message namespace setup operation fail. No memory corruption or information disclosure primitive is present. Host wide denial of service would require reliable repeated access to the specific failing branch and enough accumulated leaked resources, which is not established by the patch. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 4.7 and the production ActionableScore returned 1 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was LOW. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	MAYBE	INIT SIMPLEFIX LEAK LINUS 	-	-	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 1.
* Conservative score: 1; no floor or cap applied.
* Raw paranoid score: 1.
* Paranoid score: 1; repeated attacker-induced accumulation and host-wide exhaustion are not established.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Concrete persistent kernel resource/ref leak: `fc_mount_longterm()` creates an internal mount, but failure cleanup used `mntput()`, whose internal-mount fast path can reduce the count to zero without destroying the mount or mqueue superblock.
* +0 Reliable kernel crash/strong DoS: neither a crash nor reliably repeatable host-wide exhaustion is shown.
* +0 Local unprivileged trigger: IPC namespace creation may be locally initiated, but the evidence does not establish that a below-host-admin actor can force the later sysctl or message-namespace setup failure.
* No UAF, OOB write, shared-backing write, authorization bypass, disclosure, or protected-state corruption is shown.
* NEW52/B5-R does not apply: there is no retry-accumulated heap OOB write or later consumption of corrupted structured state.
* No negative privilege or race penalty was applied because the complete privilege and failure-induction requirements are unresolved.

3. Reachability analysis

The path is reached only after `mq_init_ns()` succeeds and a subsequent namespace setup stage fails. The evidence does not show who can reliably induce that failure, whether all required operations work within attacker-created user/IPC namespaces, or whether host setup is required. Thus both effective host-admin and namespace-scoped privilege remain unknown. Repeated leak accumulation is plausible but not proven attacker-controllable.

4. Primitive and severity reading

The primitive is a persistent mount and superblock leak caused by using the wrong teardown operation for an internal long-term mount. It is passive resource-retention behavior, classified CLASS_C, not a stale-object use or memory corruption primitive. The fix substitutes `kern_unmount()`, which performs the required mount destruction. A host-wide resource-exhaustion outcome remains theoretical because repeatable access to the necessary failure branch is unshown. There is no Important-preserving boundary or authorization issue, so automatic downgrade is allowed.

5. One-sentence report phrase

An incorrect failure-path `mntput()` leaks an internal mqueue mount and superblock, but reliable attacker-driven accumulation or host-wide denial of service is not established.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch shows a bounded resource-cleanup defect without a demonstrated corruption, authorization bypass, or reliably triggerable strong denial of service.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ipc: drop the mqueue mount with kern_unmount() on setup failure
Commit: c0620b648321fff654173d238d4a85298ea16d05
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c0620b648321fff654173d238d4a85298ea16d05

Commit description:

mq_init_ns() creates the mqueue mount with fc_mount_longterm(), so its
->mnt_ns is MNT_NS_INTERNAL. When setting up the mqueue or ipc sysctls
or msg_init_ns() fails afterwards, create_ipc_ns() drops it with a plain
mntput(). With ->mnt_ns set that takes the fast path of
mntput_no_expire(). The count drops to zero and nothing ever kills the
mount, so the mount and the mqueue superblock are leaked.

Commit dc55e35 ("ipc: Store mqueue sysctls in the ipc namespace")
added the first failure path after mq_init_ns() and never dropped the
mount. Commit 7229d74 ("ipc: create_ipc_ns: drop mqueue mount on
sysctl setup failure") tried to fix that with a mntput() but that
doesn't help for a long-term mount.

Use kern_unmount(). free_ipc() open-codes the same thing for a namespace
that made it.

Fixes: dc55e35 ("ipc: Store mqueue sysctls in the ipc namespace")
Fixes: 7229d74 ("ipc: create_ipc_ns: drop mqueue mount on sysctl setup failure")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20261009-work-fixes-rcu-v1-5-7446d2d335e6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  ipc/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=c0620b648321fff654173d238d4a85298ea16d05

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

The original Linux kernel CVE announcement for CVE-2026-99172 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99172

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).