[CVE-2026-45840][MODERATE 7.0] openvswitch: cap upcall PID array size and pre-size vport replies [ Upstream

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

CVE: CVE-2026-45840
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: openvswitch: cap upcall PID array size and pre-size vport replies [ Upstream
Commit: f9ef3db77a383d66847fd082c2b437d8ae4d9c63
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9ef3db77a383d66847fd082c2b437d8ae4d9c63
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45840
Analysis date: Wed, 27 May 2026 08:30:50 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
An oversized Open vSwitch upcall PID array can make vport netlink reply serialization fail and hit `BUG_ON`, allowing a local namespace-scoped `CAP_NET_ADMIN` user to trigger a kernel panic when OVS is available.

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

The original Linux kernel CVE announcement for CVE-2026-45840 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45840

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-2026-45840	MODERATE	CHECK	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-770;CWE-617;*CWE-20;CWE-131;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'Open vSwitch vport control plane can trigger a kernel panic when an oversized upcall PID array is accepted and later serialized into a too small netlink reply buffer. The write does not show a raw heap overflow because nla_put fails with EMSGSIZE, but the failure reaches BUG_ON and becomes a reliable local denial of service. For the CVSS the PR:L is used in the paranoid score because OVS vport mutation uses GENL_UNS_ADMIN_PERM, and on systems with unprivileged user namespaces a local user can create a network namespace and obtain namespace scoped CAP_NET_ADMIN. The issue is not directly network reachable because it is triggered through generic netlink configuration operations rather than packet processing. Impact is denial of service only. No UAF, double free, arbitrary write, or privilege escalation primitive is shown by this patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-45840  SKIP The Fixes patch not applied yet, so unlikely that actual: 5cd667b0a4567048bb555927d6ee564f4e5620a9 YES  NO NO unknown 	MAYBE	KASAN SKB HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

* Local unprivileged trigger: +1
  On systems with unprivileged user namespaces enabled, a local user may use `unshare -Urn` and obtain namespace scoped `CAP_NET_ADMIN`.

* Local high-control netlink control-plane API: +1
  The attacker controls OVS generic netlink configuration, including the upcall PID array size.

* Reliable kernel crash / strong DoS: +1
  The commit shows a concrete path where oversized PID array serialization fails with `-EMSGSIZE` and reaches `BUG_ON(err < 0)`, causing kernel panic.

* Availability impact realistic: +1
  The issue can panic the kernel through a deterministic local control-plane sequence.

* Requires CAP_NET_ADMIN in typical deployments: -2 conservative only
  Without unprivileged user namespaces or delegated network namespace control, OVS vport mutation requires administrative network capability.

* Namespace privilege reduction concern: +1 paranoid interpretation
  Because the operation uses `GENL_UNS_ADMIN_PERM`, namespace-scoped admin can be enough, so this is not equivalent to requiring full host root on systems that allow unprivileged user namespaces.

* No memory corruption: +0
  Although the commit says the reply buffer can be overflowed conceptually, the actual `nla_put()` path detects insufficient skb space and returns `-EMSGSIZE`. The security effect is the subsequent `BUG_ON`, not a demonstrated heap overwrite.

* No remote reachability: +0
  The trigger is OVS generic netlink configuration, not packet processing.

## 3. Reachability analysis

The bug is local. In conservative deployments, the attacker needs `CAP_NET_ADMIN` to mutate OVS vports. In systems with unprivileged user namespaces enabled, especially where OVS generic netlink operations are available with `GENL_UNS_ADMIN_PERM`, a local unprivileged user may create a user and network namespace and trigger the panic with namespace-scoped privileges. The affected path depends on Open vSwitch kernel support being enabled or loadable and reachable in the namespace. It is not directly network-triggerable.

## 4. Severity interpretation

This behaves like an actionable local DoS rather than an Important memory corruption vulnerability. The realistic demonstrated impact is a reliable kernel panic through `BUG_ON`. Theoretical memory corruption is not supported by the patch because netlink attribute insertion fails safely before raw overwrite. It should not be classified as LPE or arbitrary memory corruption, but namespace reachability makes it stronger than an ordinary admin-only crash.

## 5. One-sentence report phrase

An oversized Open vSwitch upcall PID array can make vport netlink reply serialization fail and hit `BUG_ON`, allowing a local namespace-scoped `CAP_NET_ADMIN` user to trigger a kernel panic when OVS is available.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED.

Manual review is recommended because this is a reliable local kernel panic reachable through namespace-scoped OVS generic netlink on some systems, but current evidence supports DoS only and does not show memory corruption or privilege escalation.

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

Patch: openvswitch: cap upcall PID array size and pre-size vport replies [ Upstream
Commit: f9ef3db77a383d66847fd082c2b437d8ae4d9c63
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f9ef3db77a383d66847fd082c2b437d8ae4d9c63

Changed files:
  net/openvswitch/datapath.c
  net/netlink/genetlink.c
  net/netlink/af_netlink.c
  net/socket.c
  arch/x86/entry/syscall_64.c
  arch/x86/entry/entry_64.S
  net/openvswitch/vport.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=f9ef3db77a383d66847fd082c2b437d8ae4d9c63

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

The original Linux kernel CVE announcement for CVE-2026-45840 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45840

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:N/I:N/A:H
  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).