[CVE-2026-74670][MODERATE 7.0] ipvs: stop estimator after disabled calc phase

[CVE-2026-74670][MODERATE 7.0] ipvs: stop estimator after disabled calc phase

[AL-KERNEL]

CVE: CVE-2026-74670
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipvs: stop estimator after disabled calc phase
Commit: d5122a2b2601145975d387006e517c56896e310e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d5122a2b2601145975d387006e517c56896e310e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74670
Analysis date: Sat, 22 Aug 2026 18:02:28 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
IPVS estimator teardown can leave zero estimator limits after a disabled calculation phase, allowing `ip_vs_enqueue_estimator()` to write past internal tick arrays and potentially cause a local kernel crash or memory corruption.

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

The original Linux kernel CVE announcement for CVE-2026-74670 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74670

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-2026-74670	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-787;CWE-362;*CWE-416;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'IPVS estimator kthread 0 can continue after the initial calculation phase even when IPVS has been disabled during network namespace teardown. In that state the estimator limits may remain zero, and draining est_temp_list can make ip_vs_enqueue_estimator write past the ticks and tick_len arrays. This is a local control plane trigger rather than a packet processing network trigger. For the CVSS the PR:L is used because a local user with access to an IPVS capable network namespace or delegated CAP_NET_ADMIN may be able to reach the vulnerable path. AC:H is used because the condition depends on a teardown timing window during the estimator calculation phase. Impact is at least local denial of service via kernel crash. In the paranoid score, the out of bounds write is treated as a plausible kernel memory corruption path with possible confidentiality, integrity, and availability impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-74670 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE NETFILTER KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged or low-privilege trigger: +1
  The likely trigger is local IPVS control-plane activity combined with network namespace teardown. In deployments with delegated network namespaces or CAP_NET_ADMIN inside a namespace, this is closer to PR:L than full host-root.

* Generic memory corruption, strong corruption primitive: +2
  The commit explicitly states that `ip_vs_enqueue_estimator()` can write past the `ticks` and `tick_len` arrays. This is a concrete kernel OOB write, not just a warning or bookkeeping issue.

* Real lifetime or teardown corruption: +1
  The bug depends on estimator kthread lifetime and netns teardown state. The patch also adds an extra task reference and switches to `kthread_stop_put()`, indicating a real task lifetime window.

* Reliable kernel crash / strong DoS: +1
  OOB writes in this kernel path can plausibly crash the kernel. Availability impact is realistic.

* Privilege escalation plausible, weak to moderate: +1 conservative, +2 paranoid
  Conservative scoring treats the OOB write as potentially dangerous but not proven attacker-controlled. Paranoid scoring treats the concrete kernel OOB write as a plausible LPE candidate requiring manual analysis.

Subtracted signals:

* Hard or timing-dependent race: -1
  The issue depends on network namespace teardown disabling IPVS during the estimator calculation phase.

Not applied:

* Remote/network-triggerable: +0
  This is IPVS control-plane and teardown behavior, not direct packet-triggered remote input.

* Common packet-processing path: +0
  Although IPVS is networking-related, the vulnerable path is estimator kthread initialization/teardown, not ordinary packet receive/transmit processing.

* CAP_SYS_ADMIN/CAP_NET_ADMIN penalty: not fully applied
  Typical host-level IPVS control requires privileges, but namespace delegation can make the practical attacker model weaker than full host root.

## 3. Reachability analysis

The bug is locally reachable through IPVS estimator setup and network namespace teardown. A fully unprivileged user may not reach it on hardened systems where unprivileged user namespaces are disabled and IPVS control is restricted. However, in containerized or delegated network namespace environments, a user with CAP_NET_ADMIN in a namespace or equivalent service-level network control may be able to exercise the affected path.

The path is not directly network reachable. Packet traffic alone is not enough based on the patch. Realistic exploitation requires creating or manipulating IPVS estimator state and racing teardown or disablement during the calculation phase.

Call-site confidence: high. The commit message directly identifies the downstream write past `ticks` and `tick_len`, and the patch modifies the exact kthread lifecycle path that enables the bad state.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or cleanup-only bug. The patch context describes a concrete OOB write in kernel memory after estimator limits remain zero. Realistic impact is at least local DoS via kernel crash.

Theoretical privilege escalation is plausible because the primitive is an OOB write in kernel memory, but the provided patch does not prove attacker-controlled target selection or a reliable overwrite payload. Therefore the conservative result is Actionable Moderate, while the paranoid result is a Strong Important candidate for manual review.

## 5. One-sentence report phrase

IPVS estimator teardown can leave zero estimator limits after a disabled calculation phase, allowing `ip_vs_enqueue_estimator()` to write past internal tick arrays and potentially cause a local kernel crash or memory corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is a race with an explicitly described kernel OOB write, and namespace-delegated IPVS control may reduce practical privileges below full host-root.

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

Patch: ipvs: stop estimator after disabled calc phase
Commit: d5122a2b2601145975d387006e517c56896e310e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d5122a2b2601145975d387006e517c56896e310e

Commit description:

IPVS estimator kthread 0 starts with zeroed chain and tick limits until
its initial calculation phase completes. If network namespace teardown
clears ipvs->enable during that phase, ip_vs_est_calc_phase() can return
without installing positive limits.

The kthread can then continue into its main loop and drain
est_temp_list with zero chain_max, tick_max and est_max_count values.
Each enqueue consumes one available tick row, but est_count never
reaches the zero est_max_count value. After all rows are consumed, the
row lookup returns IPVS_EST_NTICKS and ip_vs_enqueue_estimator() writes
past the ticks and tick_len arrays.

Exit kthread 0 after the calculation phase if the kthread is stopping or
IPVS has been disabled. That keeps temporary estimators from being
drained after the limits failed to initialize.

Estimator kthreads can now self-exit before teardown or reload stops
kd->task. Keep an extra task reference after creation and release it
with kthread_stop_put(), so kd->task remains valid until the stop paths
consume that reference.

Fixes: 705dd34 ("ipvs: use kthreads for stats estimation")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/ipvs/ip_vs_est.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=d5122a2b2601145975d387006e517c56896e310e

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

The original Linux kernel CVE announcement for CVE-2026-74670 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74670

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: 5
  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).