[CVE-2026-64579][MODERATE 7.0] xfrm: policy: preallocate inexact bins before xfrm_hash_rebuild reinsert [ Upstream

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

CVE: CVE-2026-64579
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: xfrm: policy: preallocate inexact bins before xfrm_hash_rebuild reinsert [ Upstream
Commit: d9d9cc21cc90014724a14c447e3d587be9447107
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d9d9cc21cc90014724a14c447e3d587be9447107
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64579
Analysis date: Wed, 05 Aug 2026 05:17:43 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An inverted preallocation guard in `xfrm_hash_rebuild()` can leave a poisoned XFRM policy list node after allocation failure, causing a later rebuild to dereference `LIST_POISON2` and panic the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-64579 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64579

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
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CVE-2026-64579	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:L/A:H';CWE-670;CWE-703;**CWE-476;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'An inverted guard in xfrm_hash_rebuild causes preallocation to cover exact XFRM policies while skipping inexact policies that may need a bin during the reinsert phase. If allocation later fails under memory pressure after hlist_del_rcu, the error path can leave a poisoned bydst list node and a later rebuild may dereference LIST_POISON2, causing a general protection fault and kernel panic. For the CVSS the PR:L is used for the paranoid score because reliable triggering requires local ability to create or modify XFRM policies, often CAP_NET_ADMIN or delegated network administration in a namespace. The issue is not directly network reachable through packet traffic and is triggered through the XFRM control plane. Impact is at least local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact due to corrupted list state, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	KASAN OOB ERRORPATH KERNEL_PANIC_PLUS_UAF  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Weak/indirect corruption candidate: +1. The failed reinsert can leave a poisoned `bydst` list node, later dereferenced through `hlist_del_rcu()`.
* Real lifetime/list corruption: +1. The bug creates stale or invalid list state across rebuilds after an object was removed from the hash/list path.
* Reliable kernel crash / strong DoS: +1. The commit provides a concrete GPF and kernel panic trace.
* Availability impact realistic: +1. The failure is host-wide once triggered, not merely a local process crash.
* Broad networking subsystem exposure: +1. XFRM/IPsec policy handling is a core networking subsystem, although not ordinary packet-path parsing.
* Requires CAP_NET_ADMIN or equivalent in typical deployments: -1. Full -2 is softened because CAP_NET_ADMIN can be delegated to containers or network namespaces.
* Memory pressure / special allocation failure condition: -1. The crash requires allocation failure during rebuild, although it is deterministic with failslab.

Paranoid additions:

* Local delegated low-privilege trigger concern: +1. In environments with unprivileged user namespaces or delegated net administration, an attacker may obtain practical control over XFRM policy creation without full host root.
* Confidentiality impact plausible: +1. Limited C impact is defensible only as a manual-review concern from corrupted kernel list state, not as a proven leak.
* Integrity impact plausible: +1. Limited I impact is defensible only as a corruption concern, not as a demonstrated write primitive.
* No strong LPE bonus. There is no demonstrated controlled reclaim, arbitrary write, callback control, or type confusion.

## 3. Reachability analysis

The bug is triggered through the XFRM policy control plane, not directly by network packets. In normal host deployments, creating or modifying relevant XFRM policies requires `CAP_NET_ADMIN`, so a fully unprivileged local user usually cannot trigger it directly.

Namespaces matter. If a container, service, or user namespace setup grants effective network administration rights, the practical privilege level can drop from host-admin to lower-privileged delegated control. The issue is not remotely reachable over the network by packet traffic alone.

Realistic exploitation needs inexact XFRM policies, a hash rebuild, and allocation failure under memory pressure. The commit states that the crash is deterministic with failslab, but real-world reliability depends on the attacker’s ability to shape memory pressure and policy state.

## 4. Severity interpretation

This is stronger than an ordinary Moderate cleanup because the patch fixes a concrete corrupted list-state path ending in a kernel panic. It is not a Strong Important candidate on the available evidence because the primitive is a poisoned-list dereference and crash, not a controlled UAF reclaim, overwrite, type confusion, or demonstrated privilege escalation path.

The conservative interpretation is local privileged DoS. The paranoid interpretation is Actionable Moderate because delegated `CAP_NET_ADMIN`, corrupted kernel list state, and a concrete panic path justify manual review.

## 5. One-sentence report phrase

An inverted preallocation guard in `xfrm_hash_rebuild()` can leave a poisoned XFRM policy list node after allocation failure, causing a later rebuild to dereference `LIST_POISON2` and panic the kernel.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue involves corrupted kernel list state and a concrete panic path in XFRM policy rebuild logic. No proven LPE primitive is shown, but the combination of list corruption, delegated namespace reachability, and kernel panic makes auto-closure inappropriate.

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

Patch: xfrm: policy: preallocate inexact bins before xfrm_hash_rebuild reinsert [ Upstream
Commit: d9d9cc21cc90014724a14c447e3d587be9447107
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d9d9cc21cc90014724a14c447e3d587be9447107

Changed files:
  include/linux/rculist.h
  net/xfrm/xfrm_policy.c
  kernel/workqueue.c
  kernel/kthread.c
  arch/x86/kernel/process.c
  arch/x86/entry/entry_64.S

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=d9d9cc21cc90014724a14c447e3d587be9447107

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

The original Linux kernel CVE announcement for CVE-2026-64579 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64579

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:H/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:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 3
  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: NO
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).