[CVE-2026-98989][LOW] namespace: check a recursive bind mount for mount namespace loops

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

CVE: CVE-2026-98989
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: namespace: check a recursive bind mount for mount namespace loops
Commit: a0ba48256f5dff6f75a45f8ce39fe57f21df1022
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a0ba48256f5dff6f75a45f8ce39fe57f21df1022
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98989
Analysis date: Mon, 26 Oct 2026 22:01:16 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A namespace-capable local process can repeatedly create recursive bind-mount cycles that retain mount namespaces and consume unbounded Shmem.

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

The original Linux kernel CVE announcement for CVE-2026-98989 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98989

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-2026-98989	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98989 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT LOW AEGIS_AS=2 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:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-400;*CWE-772;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A recursive bind mount with MS_REC could copy nested nsfs or pidfs mount namespace files from another mount namespace without validating the complete copied tree. This can create a reference cycle that permanently retains the affected namespace and its mounts. Repeating the deterministic operation can grow Shmem without limit and may eventually exhaust host resources. For the CVSS the PR:L value reflects that local code execution and mount capability inside an available user namespace are required. The issue is not directly network reachable. No confidentiality or integrity impact is supported because the demonstrated primitive is resource retention rather than invalid memory access or memory corruption. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. 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 CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE_KPANIC to LOW because the ActionableScore threshold rule was applied. 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. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE and DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3. The independent weaker Aegis shadow evaluated CVSS 4.4 and ActionableScore 2 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 2)	MAYBE	DISK LEAK IMPROVEONLY LINUS  INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 2.
* Conservative score: 2; no floor or cap applied.
* Raw paranoid score: 2.
* Paranoid score: 2; no additional technically supported impact.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Concrete persistent kernel resource/reference leak: the recursive copy can create a mount-namespace reference cycle, permanently retaining the namespace and its mounts.
* +1 Material resource exhaustion: the demonstrated operation is repeatable and grows Shmem without limit, supporting eventual host resource pressure beyond a one-time bookkeeping leak.
* +0 Namespaced privilege: the supplied evidence places the required mount capability in an available user namespace, so no host-administrator penalty applies.
* No memory corruption, UAF, OOB access, protected-state modification, confidentiality impact, or integrity impact is shown.
* No separate crash point is awarded; eventual exhaustion is already represented by the resource-exhaustion signal.
* No authorization-bypass score: `check_for_nsfs_mounts()` enforces a cycle/retention invariant, not a mandatory access-control decision changing DENY to ALLOW for a protected resource.
* NEW52/B5-R and NEW53/B3-PENDING do not apply: there is no heap OOB write or shared-backing write.

3. Reachability analysis

A local actor with mount capability in an available user namespace can construct the bind-mount stack and repeatedly invoke `MS_BIND|MS_REC`. The supplied evidence supports a namespace-scoped complete trigger below host-administrator power and does not require host UID 0, networking, special hardware, or a timing race. Availability of user namespaces remains a deployment prerequisite, but the described trigger is deterministic once available.

4. Primitive and severity reading

The primitive is a persistent namespace/mount lifetime-retention cycle causing repeatable kernel resource consumption. It is CLASS_C because the demonstrated security effect is resource exhaustion, without memory corruption or another useful exploitation primitive. The patch validates the copied recursive tree before grafting it. ImportantCandidate is NO and auto-downgrade is allowed because no unresolved corruption, authorization, ownership, or cross-authority integrity primitive remains.

5. One-sentence report phrase

A namespace-capable local process can repeatedly create recursive bind-mount cycles that retain mount namespaces and consume unbounded Shmem.

6. Manual review advice

NO MANUAL CHECK NEEDED

The deterministic resource-retention impact is adequately established and no Important-preserving primitive is shown.

7. Call-site confidence

Call-site confidence: high

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

Patch: namespace: check a recursive bind mount for mount namespace loops
Commit: a0ba48256f5dff6f75a45f8ce39fe57f21df1022
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a0ba48256f5dff6f75a45f8ce39fe57f21df1022

Commit description:

do_loopback() refuses to bind mount the file of a mount namespace that
is as old as the caller's or older because a mount namespace that holds
a mount of its own file, or of an ancestor's, would cause a cycle. But
it only checks the dentry the bind mount starts from. With MS_REC
everything below it is copied and do_loopback() passes
CL_COPY_MNT_NS_FILE so mount namespace files below the source are
copied.

That's fine for a source in the caller's own mount namespace. Every
mount namespace file in there passed the same check when it was
bind-mounted. It isn't fine for a source in another mount namespace.
may_copy_tree() accepts a bind mount of any nsfs or pidfs file no matter
what mount namespace it lives in so that /proc/<pid>/ns/<ns> and pidfds
can be bind mounted. If we create a bind-mount stack of mount namespaces
file descriptors the kernel will copy them irrespective of their
ancestoral relationship to the mount namespace in question:

  151 146 0:7 net:[4026531833] /tmp/nrc/y rw - nsfs nsfs rw
  152 151 0:7 mnt:[4026532293] /tmp/nrc/y rw - nsfs nsfs rw

Mount 152 is a mount of the mount namespace file of mount namespace
4026532293 inside mount namespace 4026532293. That namespace and every
mount in it are leaked... All it takes is a process in an older mount
namespace that stacks the file and repeating it leaks without limit:

  after control: Shmem:               380 kB
  child: mount(/proc/self/fd/6, MS_BIND|MS_REC): ok
  after cycle: Shmem:             65916 kB

do_move_mount() runs check_for_nsfs_mounts() over a detached tree for
exactly that reason. Do the same for the copy before it is grafted.

Fixes: e149ed2 ("take the targets of /proc/*/ns/* symlinks to separate fs")
Fixes: ef4144a ("pidfs: allow bind-mounts")
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260930-work-mount-fixes-3-v1-4-be34c83956ae@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=a0ba48256f5dff6f75a45f8ce39fe57f21df1022

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

The original Linux kernel CVE announcement for CVE-2026-98989 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98989

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 2

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).