[CVE-2026-64525][MODERATE 7.0] xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit [ Upstream

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

CVE: CVE-2026-64525
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit [ Upstream
Commit: bca6386dc08750fc7cdcbc7683473748ba3114b9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bca6386dc08750fc7cdcbc7683473748ba3114b9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64525
Analysis date: Sat, 25 Jul 2026 14:40:03 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A local process able to create network namespaces can trigger inefficient XFRM policy cleanup with one RCU grace period per namespace, causing cleanup_net backlog, large Percpu memory growth, and possible OOM-based denial of service.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64525 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64525

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: 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
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CVE-2026-64525	MODERATE	CHECK WITH IMPACT FROM ORIG NN 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-770;CWE-407;BEST CVSS score: '5.5';DESCR 'xfrm policy cleanup can cause resource exhaustion because xfrm_policy_fini used a blocking RCU grace period in the per netns exit path. A local process that can repeatedly create and destroy network namespaces can make cleanup_net spend one RCU grace period per namespace, causing a large cleanup backlog, high percpu memory growth, and possible OOM. For the CVSS the PR:L is used because reliable triggering requires local code execution and the ability to create network namespaces, which may be available to unprivileged users through user namespaces or delegated container privileges. The issue is not network reachable and does not provide a direct memory corruption primitive. Impact is denial of service only through memory pressure and stalled namespace cleanup.';NO MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64525 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 069daad4f2ae9c5c108131995529d5f02392c446 YES  NO NO unknown 	MAYBE	LOCK INIT WARNONLY LEAK HARDWARE LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_HIGHCVSSSCORE	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process may trigger this by repeatedly creating and destroying network namespaces if unprivileged user namespaces or delegated container privileges allow CLONE_NEWNET.
* Reliable kernel crash / strong DoS: +1. The commit reports cleanup_net and netns_wq rescuer stalls, more than 300k struct net objects queued, 130 GB plus Percpu growth, and memcg OOMs.
* Availability impact realistic: +1. The impact is not just a transient warning. It can create sustained cleanup backlog and system-wide memory pressure.
* Broad/default/common subsystem exposure: +1. Network namespace teardown and cleanup_net are common kernel infrastructure. XFRM itself may be configuration dependent, but this is not a rare hardware-only path.

Paranoid additional signal:

* Cross-boundary isolation/resource impact: +1. The resource buildup occurs in global namespace cleanup and Percpu memory paths and can affect workloads beyond the triggering process or container.

Not applied:

* Remote reachable: +0. No packet-triggered or network-origin trigger is shown.
* Generic memory corruption: +0. The patch moves RCU synchronization between pernet phases and does not show UAF, OOB write, type confusion, stale callback, or arbitrary write.
* Privilege escalation plausible: +0. No credible LPE primitive is supported.
* Requires admin/root penalty: not applied for the paranoid view because userns/container delegation can make netns churn available to non-root users. If unprivileged user namespaces are disabled and only host root can create netns, score should be lower.

## 3. Reachability analysis

The realistic trigger is local namespace churn using unshare(CLONE_NEWNET) or an equivalent container/runtime path. On systems where unprivileged user namespaces are enabled, an unprivileged local user may be able to create network namespaces and trigger the issue, so PR:L is a defensible practical interpretation. On hardened systems where network namespace creation requires host root or CAP_SYS_ADMIN, exploitability is much lower.

The path is not remotely reachable through normal network traffic. It is a control-plane resource exhaustion issue in per-netns XFRM cleanup. The exploit condition is realistic on hosts that allow frequent namespace creation, especially large CPU-count systems where Percpu memory growth is amplified.

Call-site confidence: high. The patch and commit directly show the pernet .pre_exit/.exit split and explain the cleanup_net batching behavior.

## 4. Severity interpretation

This behaves like an actionable Moderate resource-exhaustion DoS, not an Important memory-corruption vulnerability. The demonstrated impact is strong availability loss through cleanup backlog and OOM, but there is no evidence of confidentiality impact, integrity impact, UAF exploitation, or privilege escalation.

The paranoid score reaches 5 because the DoS can be host-wide and namespace/container reachable in common configurations, but it should not be treated as a Strong Important candidate without a memory corruption or security-boundary bypass primitive.

## 5. One-sentence report phrase

A local process able to create network namespaces can trigger inefficient XFRM policy cleanup with one RCU grace period per namespace, causing cleanup_net backlog, large Percpu memory growth, and possible OOM-based denial of service.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a well-supported resource exhaustion DoS with no indicated UAF, OOB access, information disclosure, integrity corruption, or privilege escalation primitive.

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

Patch: xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit [ Upstream
Commit: bca6386dc08750fc7cdcbc7683473748ba3114b9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bca6386dc08750fc7cdcbc7683473748ba3114b9

Changed files:
  include/net/net_namespace.h
  net/xfrm/xfrm_policy.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=bca6386dc08750fc7cdcbc7683473748ba3114b9

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

The original Linux kernel CVE announcement for CVE-2026-64525 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64525

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: Not available
  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: 4
  Paranoid ActionableScore: 5

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).