[CVE-2026-64531][MODERATE 7.0] net: openvswitch: reject oversized nested action attrs

[CVE-2026-64531][MODERATE 7.0] net: openvswitch: reject oversized nested action attrs

[AL-KERNEL]

CVE: CVE-2026-64531
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: openvswitch: reject oversized nested action attrs
Commit: ab855641241387db062a5e41d9ad6b8561542572
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ab855641241387db062a5e41d9ad6b8561542572
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64531
Analysis date: Mon, 27 Jul 2026 02:55:20 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
Open vSwitch can close an oversized generated nested action with a truncated nla_len, causing later dump or teardown code to parse a different action stream than the validated one and creating a DoS and possible memory-safety risk in resource-owning action cleanup paths.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64531 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64531

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-2026-64531	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-197;*CWE-20;*CWE-664;*CWE-416;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'Open vSwitch generated flow actions are stored as nested netlink attributes with a 16 bit nla_len field. An oversized nested action can exceed U16_MAX and then be closed with a truncated length, so later dump or teardown code may parse a different action stream than the one that was validated. Oversized CLONE/CT or CT related containers can make following bytes appear as independent actions, which creates a validation bypass and a plausible memory safety risk in resource owning action cleanup paths. For the CVSS the PR:L is used in the paranoid score because reduced capability root, container root, service accounts, or delegated CAP_NET_ADMIN can sometimes configure Open vSwitch without being full host root. The issue is not directly network reachable through packet traffic and requires access to the local OVS netlink control plane or an equivalent management path. Impact is at least denial of service and malformed datapath behavior, and in the worst case may include confidentiality or integrity impact if the malformed stream reaches resource ownership or stale pointer paths.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	INIT NOMEMCHK LOG IPV6  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Weak/indirect corruption candidate: +1. Oversized nested nlattr can truncate nla_len, causing later dump or teardown to parse a different action stream than the validated one.
* Memory layout invariant restoration: +1. The patch restores consistency between sw_flow_actions actions_len and per-nlattr u16 nla_len boundaries.
* Real lifetime/resource ownership concern: +1. The patch adds reverse-order trimming and explicit resource cleanup for resource-owning generated action tails.
* Reliable malformed-state DoS concern: +1. A structurally inconsistent action stream can plausibly crash or break teardown/dump paths.
* Requires CAP_NET_ADMIN in typical deployments: -2. Direct triggering normally requires OVS control-plane access, not ordinary packet traffic.
* Common networking/virtualization deployment relevance: +1. Open vSwitch is common in virtualization, container, SDN, and cloud networking environments.

Paranoid additional signals:

* Weak LPE concern: +1. Later interpretation of bytes as independent actions plus resource-owning cleanup paths creates a plausible corruption sink, though no direct arbitrary write is demonstrated.
* Integrity impact plausible: +1. A validated action stream may later be interpreted differently, which is a control-plane validation bypass.
* Confidentiality impact plausible: +1. This is weaker than integrity, but resource/lifetime confusion in teardown or dump paths can justify manual review sensitivity.
* Privilege penalty reduced: +1 relative to conservative. CAP_NET_ADMIN may be delegated to container root, service accounts, or reduced-capability management processes, so this is not always equivalent to full host root.

## 3. Reachability analysis

The issue is not directly network-triggerable through normal packet traffic. Triggering requires constructing oversized nested Open vSwitch actions through the local OVS netlink control plane or an equivalent management interface.

In typical host deployments this requires CAP_NET_ADMIN, so the conservative score applies a privilege penalty. In cloud, container, SDN, or OVN deployments, OVS configuration authority may be delegated to a reduced-privilege process, container root, or service account. That makes the paranoid PR:L-style interpretation defensible for triage.

The affected subsystem is not universally enabled on every Linux host, but Open vSwitch is widely deployed in virtualization and container networking environments. The path is configuration/control-plane reachable rather than packet-data-plane reachable.

Call-site confidence: high. The patch shows the helper change, multiple recursive builders, error propagation, and cleanup/trim handling in the visible call paths.

## 4. Severity interpretation

This is stronger than an ordinary Moderate validation bug because the vulnerable condition can make later code walk a structurally different action stream than the one that passed validation. The patch also treats cleanup ordering and resource ownership carefully, which suggests more than simple input rejection.

Realistic demonstrated impact is malformed action parsing and likely DoS or corrupted OVS datapath state. Theoretical impact includes memory-safety or resource-lifetime confusion during dump or teardown, but the patch does not demonstrate a clean arbitrary write, attacker-controlled reclaim, or reliable LPE primitive.

Overall this should be handled as Actionable Moderate at minimum, with Strong Important candidate status for manual review because the bug combines length truncation, parser state mismatch, and resource-owning cleanup paths.

## 5. One-sentence report phrase

Open vSwitch can close an oversized generated nested action with a truncated nla_len, causing later dump or teardown code to parse a different action stream than the validated one and creating a DoS and possible memory-safety risk in resource-owning action cleanup paths.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: length truncation changes parser structure after validation, and the patch explicitly adds resource-owning tail cleanup and reverse-order trimming. This is not enough to claim proven LPE, but it is enough to avoid auto-close and require manual kernel review.

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UPSTREAM PATCH SUMMARY
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Patch: net: openvswitch: reject oversized nested action attrs
Commit: ab855641241387db062a5e41d9ad6b8561542572
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ab855641241387db062a5e41d9ad6b8561542572

Commit description:

Open vSwitch stores generated flow actions as nlattrs, whose nla_len
field is u16. Commit a1e64ad ("net: openvswitch: remove
misbehaving actions length check") allowed the total sw_flow_actions
stream to grow beyond 64 KiB, which is valid, but also removed the last
guard preventing a generated nested action attribute from exceeding
U16_MAX.

An oversized generated container can thus be closed with a truncated
nla_len. A later dump or teardown then walks a structurally different
stream than the one that was validated. In particular, an oversized
nested CLONE/CT action may cause subsequent bytes in the generated
stream to be interpreted as independent actions.

Keep the larger total-action-stream behavior, but make nested action
close reject generated containers that do not fit in nla_len, and return
the error through all callers. For recursive SAMPLE, CLONE, DEC_TTL, and
CHECK_PKT_LEN builders, trim resource-owning action-list tails in reverse
construction order before discarding failed wrappers, so resources copied
into the rejected tails are released before the wrappers are removed.

Most failed outer wrappers are discarded by truncating actions_len after
child resources have been released. CHECK_PKT_LEN also trims its parent
after branch resources are gone. SET/TUNNEL close failures unwind their
known tun_dst ownership directly, and SET_TO_MASKED has no external
ownership and truncates on close failure.

Fixes: a1e64ad ("net: openvswitch: remove misbehaving actions length check")
Cc: stable@vger.kernel.org
Assisted-by: avom-custom-harness:gpt-5.5-qwen3.6-mod-mix
Signed-off-by: Asim Viladi Oglu Manizada <manizada@pm.me>
Reviewed-by: Eelco Chaudron <echaudro@redhat.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>
Link: https://patch.msgid.link/20260706094336.38639-1-manizada@pm.me
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/openvswitch/flow_netlink.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=ab855641241387db062a5e41d9ad6b8561542572

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

The original Linux kernel CVE announcement for CVE-2026-64531 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64531

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:H/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 4
  Paranoid ActionableScore: 7

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).