[CVE-2026-80978][MODERATE 7.0] net: cap advertised IP tunnel headroom

[CVE-2026-80978][MODERATE 7.0] net: cap advertised IP tunnel headroom

[AL-KERNEL]

CVE: CVE-2026-80978
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: cap advertised IP tunnel headroom
Commit: bc4e05ae66c9797a0972ac44326e69c5305e0020
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bc4e05ae66c9797a0972ac44326e69c5305e0020
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80978
Analysis date: Fri, 11 Sep 2026 16:08:12 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-80978 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80978

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-80978	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80978 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-190;*CWE-787;CWE-119;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'IP tunnel devices could advertise a needed_headroom value derived from lower devices without applying the runtime cap used on the transmit path. A stack of locally created tunnel devices can make this value exceed what 16-bit skb header offsets can represent, and later skb head expansion can wrap those offsets during skb_headers_offset_update(). For the CVSS the PR:L because reliable triggering requires a local process able to create or configure tunnel devices, often through CAP_NET_ADMIN or delegated network namespace rights. The issue is not directly network reachable by packet traffic alone. Packets may exercise the corrupted transmit path only after the local tunnel stack has been configured. Impact is at least denial of service via kernel crash, and in the paranoid case may include confidentiality or integrity impact because skb metadata offset wraparound is a memory corruption candidate.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	REMOTE SKB HARDWARE IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Applied conservative floor/cap: 4 -> 5 because a concrete active skb metadata corruption primitive is established and should not be auto closed as ordinary DoS.
* Raw paranoid score: 6
* Paranoid score: 6
* Applied paranoid floor/cap: none
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2: Active kernel metadata corruption candidate. User-created IP tunnel stacking can make advertised needed_headroom exceed what 16-bit skb header offsets can represent, and skb head expansion can wrap those offsets in skb_headers_offset_update().
* +1: Memory layout invariant restoration. The patch restores consistency between published headroom, runtime headroom capping, and the representable skb header-offset range.
* +1: Reliable DoS or strong availability concern. Wrapped skb header offsets in the transmit path can plausibly lead to invalid skb metadata use and kernel crash or packet-path malfunction.
* +1: Common networking skb path. The affected state is consumed by IP output and skb head expansion, not just a passive diagnostic field.

Negative signals:

* -1 conservative: reliable triggering requires local ability to create or configure stacked tunnel devices, normally requiring CAP_NET_ADMIN or equivalent delegated network-management privilege. Host-admin-equivalent privilege is not positively established, so -2 is not used.

Paranoid-only signal:

* +1 paranoid: weak exploitation concern. The corrupted value is an active skb offset used by later packet processing, so impact beyond immediate crash is unresolved enough to require manual review. This does not prove arbitrary write, controlled reclaim, or LPE.

Non-awarded signals:

* RealUAF not awarded. This is not a lifetime or stale-object bug.
* CLASS_A not awarded. The patch establishes skb metadata offset wraparound, but not an arbitrary write, controlled object replacement, controlled reclaim, callback control, or demonstrated privilege escalation.
* Remote reachability not awarded. Packet traffic alone does not create the vulnerable tunnel stack.
* Local unprivileged trigger not awarded. Full below-host-admin reachability through user and network namespaces is plausible in some deployments but not established by the supplied evidence.
* Confidentiality and integrity bonuses not awarded directly. They remain unresolved consequences of metadata corruption, not demonstrated leak or write primitives.
* Extra availability not awarded. The same crash or packet-path failure is not double-counted.

3. Reachability analysis

The realistic trigger requires creating a stack of IP tunnel devices whose derived needed_headroom grows beyond the 16-bit skb header offset range, then causing traffic through the resulting output path so skb head expansion applies the excessive reservation.

This is local control-plane reachability, not direct remote packet reachability. The attacker likely needs CAP_NET_ADMIN or equivalent authority to create and configure IP tunnel devices. In Linux, some tunnel and rtnetlink operations can be namespace scoped, but the supplied evidence does not prove that the complete trigger chain is available to an ordinary unprivileged user in an attacker-created user or network namespace. Therefore HostAdminRequired is UNKNOWN and NamespaceScopedPrivilege is UNKNOWN.

No special physical hardware is required. No firmware mediation is involved. This is not a timing race. The configuration and transmit trigger appear repeatable once the tunnel stack can be created.

4. Primitive and severity interpretation

The exact primitive is integer overflow or wraparound of 16-bit skb header-offset metadata caused by publishing an unbounded tunnel needed_headroom value to upper layers. The affected object is active structured skb metadata, not fixed stale storage and not a cleanup-only path.

This is CLASS_B because it is a concrete kernel corruption or active-state consistency primitive, but exploitation leverage beyond crash is unresolved. It is not CLASS_A because there is no demonstrated arbitrary or directed kernel write, no controlled reclaim, no callback or function-pointer control, and no demonstrated LPE.

The demonstrated consequence is at least DoS or packet-path corruption through invalid skb header offsets. The speculative consequence is stronger memory-corruption impact if later skb processing consumes wrapped offsets in a security-useful way. That possibility is technically plausible enough for manual review, but not enough to claim proven privilege escalation.

ImportantCandidate is YES because active skb metadata corruption in a common packet path should not be automatically downgraded without reviewing downstream offset consumers. AutoDowngradeAllowed is NO because the patch-supported primitive is not a pure warning, resource leak, or ordinary crash-only validation bug.

5. One-sentence report phrase

Stacked IP tunnel devices can publish an excessive needed_headroom value that wraps 16-bit skb header offsets during skb head expansion, creating an active skb metadata corruption primitive with at least DoS impact and unresolved exploitation leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug establishes skb header-offset wraparound in an active networking data path, which is stronger than ordinary DoS even though arbitrary write or privilege escalation is not demonstrated.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net: cap advertised IP tunnel headroom
Commit: bc4e05ae66c9797a0972ac44326e69c5305e0020
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=bc4e05ae66c9797a0972ac44326e69c5305e0020

Commit description:

IP tunnel devices derive their advertised needed_headroom from lower
output devices. A stack of user-created devices can make the derived
value larger than the 16-bit skb header offsets can represent. Once IP
output reserves it, skb head expansion can wrap those offsets.

The runtime transmit path already caps a growing needed_headroom at 512.
Apply the same cap when tunnel configuration publishes needed_headroom
derived from a lower output device.

Capping the advertised value is safe: IP tunnel transmit still expands
the skb when a packet needs more headroom. A nonsensical stacked
configuration can therefore incur an extra reallocation, but it cannot
publish an unbounded reservation to upper layers.

Fixes: 1a37e41 ("net: Use 16bits for *_headers fields of struct skbuff")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/ba04a1fd6bfae2377607fad5d8f80f7eb80fd4c4.1786542637.git.zhilinz@nebusec.ai
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/ip_tunnels.h
  net/ipv4/ip_tunnel.c
  net/ipv6/ip6_gre.c
  net/ipv6/ip6_tunnel.c
  net/ipv6/sit.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=bc4e05ae66c9797a0972ac44326e69c5305e0020

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

The original Linux kernel CVE announcement for CVE-2026-80978 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80978

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:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 6

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).