[CVE-2026-46273][MODERATE REGULAR] ibmveth: Disable GSO for packets with small MSS

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

CVE: CVE-2026-46273
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ibmveth: Disable GSO for packets with small MSS
Commit: 86fc64584811d43c9ccd74447de58620189d8b77
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86fc64584811d43c9ccd74447de58620189d8b77
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46273
Analysis date: Wed, 03 Jun 2026 16:17:32 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline / Actionable Moderate if exposed on affected IBM Power networking
Manual review required: NO

Summary:
A remote TCP peer may be able to trigger an ibmveth adapter freeze by causing transmission of GSO packets with MSS below 224 bytes on affected IBM Power adapters, resulting in persistent network DoS until manual reset.

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

The original Linux kernel CVE announcement for CVE-2026-46273 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46273

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 REGULAR
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-2026-46273	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-20;CWE-754;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'ibmveth can pass GSO packets with MSS below 224 bytes to the hardware segmentation path on affected IBM Power adapters. Some adapters cannot handle this LSO case and may freeze, stopping network traffic until the adapter is manually reset. A remote TCP peer may be able to influence the effective MSS and then cause the vulnerable system to transmit multi segment GSO traffic with a small gso_size. For the CVSS the PR:N because no local account on the victim is required if the attacker can reach a service that sends data over the affected interface. The base score keeps AC:H because exploitation depends on affected hardware, enabled GSO, small MSS, and transmitted multi segment traffic. Impact is denial of service only through adapter freeze and loss of network availability, with no evidence of memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / Actionable Moderate if exposed on affected IBM Power networking (with actual score 4) 	MAYBE	REMOTE NETWORK SKB HARDWARE LINUS TEST PACKET  INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline / Actionable Moderate if exposed on affected IBM Power networking**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  A remote TCP peer may be able to negotiate or influence a small MSS and cause the host to transmit multi-segment GSO traffic over the affected ibmveth interface.
* Reliable kernel crash / strong DoS: +1
  The described impact is adapter freeze, stopping all traffic until manual reset.
* Rare hardware / configuration dependency: -1
  The issue depends on IBM Power ibmveth deployments, affected physical adapters, enabled GSO/TSO, MSS below 224 bytes, and `gso_segs > 1`.

Paranoid additional signal:

* Availability impact realistic and persistent: +1
  The availability loss is not just a dropped packet or transient warning. The adapter can remain frozen until manual reset, so operational impact may persist.

Not counted:

* No generic memory corruption.
  The patch prevents unsupported hardware segmentation, not UAF, OOB write, double-free, or object lifetime corruption.
* No privilege escalation plausible from the patch.
  The evidence supports network availability loss only.
* No confidentiality or integrity impact.
  There is no data disclosure, overwrite primitive, or permission bypass.

## 3. Reachability analysis

The likely trigger is a remote TCP peer that can cause the vulnerable system to send traffic with MSS below 224 bytes, producing multi-segment GSO packets that reach the hardware LSO path. No local account on the victim is required if an exposed service sends enough data back over the affected ibmveth path.

Namespaces and containers are not central to exploitation. The relevant condition is whether traffic exits through an affected ibmveth interface with GSO enabled.

The path is not broad default Linux networking. It is specific to IBM Power ibmveth and affected physical adapters. However, in deployments using this adapter, the trigger can be network-originated and may cause persistent interface-level DoS.

## 4. Severity interpretation

This behaves more like an actionable Moderate availability issue than an Important memory-safety vulnerability. The strongest realistic impact is persistent network DoS through adapter freeze. There is no evidence of memory corruption, privilege escalation, or sensitive data exposure.

The paranoid score is higher only because the DoS may be remotely triggerable and may require manual adapter reset. It should not be escalated to Important unless additional evidence shows broader host impact, common exposure, or a corruption primitive.

## 5. One-sentence report phrase

A remote TCP peer may be able to trigger an ibmveth adapter freeze by causing transmission of GSO packets with MSS below 224 bytes on affected IBM Power adapters, resulting in persistent network DoS until manual reset.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended to confirm whether the environment uses affected IBM Power ibmveth hardware, whether GSO/TSO is enabled, and whether untrusted remote peers can influence MSS and receive enough traffic to trigger multi-segment GSO.

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

Patch: ibmveth: Disable GSO for packets with small MSS
Commit: 86fc64584811d43c9ccd74447de58620189d8b77
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=86fc64584811d43c9ccd74447de58620189d8b77

Commit description:

Some physical adapters on Power systems do not support segmentation
offload when the MSS is less than 224 bytes. Attempting to send such
packets causes the adapter to freeze, stopping all traffic until
manually reset.

Implement ndo_features_check to disable GSO for packets with small MSS
values. The network stack will perform software segmentation instead.

The 224-byte minimum matches ibmvnic
commit <f10b09ef687f> ("ibmvnic: Enforce stronger sanity checks
on GSO packets")
which uses the same physical adapters in SEA configurations.

The issue occurs specifically when the hardware attempts to perform
segmentation (gso_segs > 1) with a small MSS. Single-segment GSO packets
(gso_segs == 1) do not trigger the problematic LSO code path and are
transmitted normally without segmentation.

Add an ndo_features_check callback to disable GSO when MSS < 224 bytes.
Also call vlan_features_check() to ensure proper handling of VLAN packets,
particularly QinQ (802.1ad) configurations where the hardware parser may
not support certain offload features.

Validated using iptables to force small MSS values. Without the fix,
the adapter freezes. With the fix, packets are segmented in software
and transmission succeeds. Comprehensive regression testing completedd
(MSS tests, performance, stability).

Fixes: 8641dd8 ("ibmveth: Add support for TSO")
Cc: stable@vger.kernel.org
Reviewed-by: Brian King <bjking1@linux.ibm.com>
Tested-by: Shaik Abdulla <shaik.abdulla1@ibm.com>
Tested-by: Naveed Ahmed <naveedaus@in.ibm.com>
Signed-off-by: Mingming Cao <mmc@linux.ibm.com>
Link: https://patch.msgid.link/20260424162917.65725-1-mmc@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/ethernet/ibm/ibmveth.c
  drivers/net/ethernet/ibm/ibmveth.h

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=86fc64584811d43c9ccd74447de58620189d8b77

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

The original Linux kernel CVE announcement for CVE-2026-46273 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46273

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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: 3
  Paranoid ActionableScore: 4

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 REGULAR

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).