[CVE-2026-68123][IMPORTANT] openvswitch: fix GSO userspace truncation underflow [ Upstream

[CVE-2026-68123][IMPORTANT] openvswitch: fix GSO userspace truncation underflow [ Upstream

[AL-KERNEL]

CVE: CVE-2026-68123
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: openvswitch: fix GSO userspace truncation underflow [ Upstream
Commit: a16eaaf7c0b0ccdef6166707d90ffbc6eebf6855
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a16eaaf7c0b0ccdef6166707d90ffbc6eebf6855
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68123
Analysis date: Mon, 10 Aug 2026 16:28:10 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
Open vSwitch can underflow the packet length passed to skb_zerocopy() when GSO segmentation reuses a truncation delta on smaller segments, allowing adjacent network traffic through a configured OVS datapath to cause at least DoS and requiring manual review for possible memory exposure.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68123 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68123

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: IMPORTANT
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-2026-68123	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';CWE-191;CWE-125;*CWE-20;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'Open vSwitch can mishandle OVS_ACTION_ATTR_TRUNC when a GSO skb is later segmented for a userspace action. The stored cutlen value was a delta from the original packet length, so each smaller GSO segment could reach queue_userspace_packet() with cutlen larger than the current skb length. This can underflow the length used for skb_zerocopy() and can lead to an oversized copy length, kernel crash, or other memory access side effects in the upcall path. For the CVSS the PR:N is used in the paranoid score because an attacker may only need to send crafted traffic into an already configured OVS datapath with trunc and userspace actions, without privileges on the host. The issue is best treated as adjacent network reachable because practical triggering usually requires access to a VM, container, bridge, tunnel, or local data center network where OVS processes the traffic. Impact is at least denial of service and in the paranoid interpretation may include confidentiality or integrity impact due to the integer underflow reaching skb_zerocopy().';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	OOB DANGER SKB KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  The vulnerable path can be reached by packets processed by an already configured Open vSwitch datapath with truncation and userspace actions. This is not public Internet remote by default, but it is realistically reachable from VM, container, bridge, tunnel, or data-center network traffic.

* Memory corruption, Weak/indirect corruption candidate: +1
  The bug is an integer underflow in a length argument passed to skb_zerocopy(). This is more than pure bookkeeping, but the patch does not demonstrate an arbitrary write, attacker-controlled reclaim, or direct object replacement primitive.

* Memory layout invariant restoration: +1
  The patch restores consistency between skb length, cutlen, skb_len, hlen, truncation size, and zerocopy length calculations.

* Reliable kernel crash / strong DoS: +1
  Passing an underflowed oversized length into skb_zerocopy() is a credible kernel crash or strong DoS condition in the OVS upcall path.

* Common networking packet-processing path: +1
  Open vSwitch is a widely deployed packet-processing component in virtualization, container, and cloud networking environments.

* Confidentiality impact plausible: +1, paranoid only
  An oversized zerocopy length may plausibly create an out-of-bounds read or unintended exposure in the upcall path, although this is not proven by the patch.

* Integrity impact plausible: +1, paranoid only
  Integrity impact is not demonstrated, but the length underflow reaches skb construction and zerocopy logic, so limited memory-access side effects cannot be ruled out without manual review.

Subtracted signals:

* Hard or configuration-dependent trigger: -1
  Exploitation requires a specific combination of GSO skb handling, OVS_ACTION_ATTR_TRUNC, later userspace action processing, and segmentation through queue_gso_packets().

Conservative score calculation: +2 +1 +1 +1 +1 -1 = 5
Paranoid score calculation: +2 +1 +1 +1 +1 +1 +1 -1 = 7

## 3. Reachability analysis

An attacker may be able to trigger the bug by sending traffic through an already configured OVS datapath that applies truncation and then sends segmented GSO packets to userspace. The attacker does not necessarily need privileges on the host if they can inject suitable traffic from a VM, container, bridge peer, tunnel endpoint, or adjacent data-center network.

The configuration itself is typically controlled by a privileged OVS or network administrator. However, the runtime trigger can be packet-driven once the vulnerable datapath and actions exist. Namespaces and containers matter because OVS is often used exactly at VM or container boundaries, making guest-to-host or tenant-to-infrastructure triggering plausible in some deployments.

The path is not default-enabled on every Linux system, but OVS is common in virtualization and cloud networking. Call-site confidence: high, because the relevant consumers in do_output(), execute_psample(), output_userspace(), queue_userspace_packet(), and ovs_vport_receive() are visible in the provided patch.

## 4. Severity interpretation

This is not an ordinary low-risk validation bug. The core issue is an integer underflow in packet length accounting that reaches skb_zerocopy(), which is a sensitive skb data movement path.

Realistically, the most demonstrated impact is DoS through a crash or malformed skb/upcall handling. Theoretical confidentiality or integrity impact is plausible because the underflow can produce an oversized length, but the patch does not prove arbitrary read, arbitrary write, LPE, or controlled memory corruption.

Therefore this should be treated as Actionable Moderate at minimum and a Strong Important candidate under paranoid triage, especially for OVS deployments exposed to untrusted VM, container, bridge, or tenant traffic.

## 5. One-sentence report phrase

Open vSwitch can underflow the packet length passed to skb_zerocopy() when GSO segmentation reuses a truncation delta on smaller segments, allowing adjacent network traffic through a configured OVS datapath to cause at least DoS and requiring manual review for possible memory exposure.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is packet-triggerable in plausible OVS deployments and the underflow reaches skb_zerocopy(), so it should not be auto-closed as a simple DoS without reviewing whether oversized length handling can expose memory or cause stronger corruption.

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

Patch: openvswitch: fix GSO userspace truncation underflow [ Upstream
Commit: a16eaaf7c0b0ccdef6166707d90ffbc6eebf6855
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a16eaaf7c0b0ccdef6166707d90ffbc6eebf6855

Changed files:
  net/openvswitch/actions.c
  net/openvswitch/datapath.c
  net/openvswitch/datapath.h
  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=a16eaaf7c0b0ccdef6166707d90ffbc6eebf6855

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

The original Linux kernel CVE announcement for CVE-2026-68123 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68123

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 5
  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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).