[CVE-2026-68296][MODERATE REGULAR] net: gre: fix lltx regression for GRE tunnels with SEQ/CSUM [ Upstream

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

CVE: CVE-2026-68296
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: gre: fix lltx regression for GRE tunnels with SEQ/CSUM [ Upstream
Commit: 9f948e9aede9678f4103457daf2bc9dd54c65a06
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9f948e9aede9678f4103457daf2bc9dd54c65a06
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68296
Analysis date: Mon, 10 Aug 2026 11:36:11 -0400
ActionableScore: 4
ActionableScore lower bound: 1
Actionable bucket: Borderline Moderate. Manual review recommended, not Important
Manual review required: YES

Summary:
GRE and ip6gre tunnels using SEQ or CSUM encapsulation can lose lockless TX and reintroduce qdisc transmit locking, allowing traffic through an affected tunnel to trigger an ABBA deadlock and networking DoS under timing dependent conditions.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68296 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68296

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: 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-2026-68296	MODERATE	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:N/I:N/A:H';CWE-667;*CWE-833;**CWE-400;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'GRE and ip6gre tunnels using SEQ or CSUM encap could miss the lockless TX flag because dev lltx was set after early returns in the tunnel feature setup path. This reintroduces xmit lock acquisition around ndo_start_xmit, while GRE transmit can re enter the stack through ip_tunnel_xmit and then take locks on the underlay device. The resulting lock ordering can create an ABBA deadlock and stall packet transmission or system networking. For the CVSS the PR:N in the paranoid score reflects an already configured vulnerable tunnel where an attacker only needs network adjacency or packet reachability to influence traffic through the tunnel. The issue is not a memory corruption bug and there is no clear confidentiality or integrity impact. Impact is denial of service only, with high attack complexity due to the required tunnel options, traffic pattern, and lock timing.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate. Manual review recommended, not Important (with actual score 3)	YES	REMOTE DEADLOCK NETWORK HARDWARE QDISC IPV6  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 4
* Final recommended bucket: **Borderline Moderate. Manual review recommended, not Important**::

## 2. Signal breakdown

Paranoid score signals:

* Remote reachable / network-triggerable: +2. If a vulnerable GRE/GRETAP/ip6gre tunnel with SEQ or CSUM encapsulation already exists, traffic entering the tunnel can exercise the affected transmit path.
* Reliable kernel crash / strong DoS: +1. The commit describes an ABBA deadlock in qdisc transmit locking, which can stall networking.
* Availability impact realistic: +1. The deadlock can affect packet transmission and system networking beyond a single failed packet.
* Hard or unreliable race / special timing required: -1. Triggering depends on lock ordering, concurrent transmit activity, tunnel options, and timing.
* Rare AND difficult-to-reach subsystem/configuration: -1. The issue requires GRE or ip6gre tunnels with specific SEQ or CSUM related settings, not a default host configuration.
* Common networking packet-processing relevance: +1. Once configured, the bug is in the network transmit path and can be influenced by packet flow.

Conservative score signals:

* Reliable kernel crash / strong DoS: +1.
* Availability impact realistic: +1.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Creating or changing GRE tunnels normally requires CAP_NET_ADMIN.
* Hard or unreliable race / special timing required: -1.
* Common networking packet-processing relevance: +1.

No memory corruption, UAF, OOB write, type confusion, privilege escalation, confidentiality impact, or integrity impact is supported by the patch.

Call-site confidence: high. The commit explicitly describes the relevant GRE transmit re-entry through ip_tunnel_xmit() and the qdisc lock ABBA pattern.

## 3. Reachability analysis

The conservative trigger requires a local administrator or CAP_NET_ADMIN capable process to create or configure the affected GRE/GRETAP/ip6gre tunnel mode. In that interpretation, this is a privileged local configuration bug with DoS consequences.

The paranoid interpretation assumes the vulnerable tunnel is already configured. In that case, an external or adjacent network actor able to send traffic through the tunnel may influence the transmit path without local privileges on the target. This is not general Internet remote reachability by default, because GRE tunnel deployment and the affected SEQ/CSUM configuration are environmental prerequisites.

Namespaces can lower practical privileges if CAP_NET_ADMIN is delegated inside a container or network namespace that can create affected tunnels on the evaluated system. That should be checked per product configuration.

## 4. Severity interpretation

This behaves like a DoS-only networking regression, not an Important-class memory corruption vulnerability. The strongest supported impact is an ABBA deadlock affecting GRE transmit processing and possibly broader networking availability. There is no evidence of attacker-controlled memory corruption, data disclosure, arbitrary write, object lifetime corruption, or privilege escalation.

The issue is still worth manual review because it is network-influenced in already configured tunnel deployments and can cause a kernel networking deadlock.

## 5. One-sentence report phrase

GRE and ip6gre tunnels using SEQ or CSUM encapsulation can lose lockless TX and reintroduce qdisc transmit locking, allowing traffic through an affected tunnel to trigger an ABBA deadlock and networking DoS under timing dependent conditions.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review whether affected products deploy GRE/GRETAP/ip6gre tunnels with SEQ or CSUM encapsulation and whether untrusted traffic can enter those tunnels. This should not be auto-closed solely as a low-risk local admin issue, but it also does not currently justify Important severity without memory corruption or LPE evidence.

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UPSTREAM PATCH SUMMARY
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Patch: net: gre: fix lltx regression for GRE tunnels with SEQ/CSUM [ Upstream
Commit: 9f948e9aede9678f4103457daf2bc9dd54c65a06
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9f948e9aede9678f4103457daf2bc9dd54c65a06

Changed files:
  net/ipv4/ip_gre.c
  net/ipv6/ip6_gre.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=9f948e9aede9678f4103457daf2bc9dd54c65a06

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

The original Linux kernel CVE announcement for CVE-2026-68296 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68296

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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: 1
  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).