[CVE-2026-98853][MODERATE REGULAR] net: gso: limit recursive IP-in-IP segmentation

[CVE-2026-98853][MODERATE REGULAR] net: gso: limit recursive IP-in-IP segmentation

[AL-KERNEL]

CVE: CVE-2026-98853
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: gso: limit recursive IP-in-IP segmentation
Commit: c949646c900e879afce47df1b607fa320eb63bf2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c949646c900e879afce47df1b607fa320eb63bf2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98853
Analysis date: Mon, 19 Oct 2026 04:41:42 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: YES

Summary:
Unbounded recursive IPv4 and IPv6 GSO segmentation of deeply nested IP in IP packets can exhaust the kernel stack and crash the system, while direct remote and guest to host reachability remain unresolved.

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

The original Linux kernel CVE announcement for CVE-2026-98853 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98853

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
======================================================================

CVE-2026-98853	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98853 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-674;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.5';DESCR 'Recursive IPv4 and IPv6 GSO processing can repeatedly enter inet_gso_segment or ipv6_gso_segment for nested IP in IP headers because the previous encap_level guard tracks header bytes rather than callback depth. A sufficiently deep encapsulation chain can exhaust the kernel stack and cause a kernel panic or system stall. The realistic trigger is local and requires control over a GSO capable tunnel or virtual networking path. Ordinary packets received from a physical network do not normally carry SKB GSO metadata, so direct Internet reachability is not established. For the CVSS the PR:N value in the paranoid vector assumes that a guest or adjacent virtual endpoint can submit attacker controlled GSO metadata to a vulnerable host backend without holding privileges on the host. Impact is denial of service through kernel stack exhaustion. No UAF, OOB write, information disclosure, or controlled write primitive is demonstrated, so confidentiality and integrity remain unaffected. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 6.5 and the production ActionableScore returned 2 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.7 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2)	MAYBE	REMOTE OOB SIMPLEFIX SKB IPV6 PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED	-	-	checked

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

ActionableScore=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Reliable kernel crash or strong DoS. Unbounded recursive entry into the IPv4 and IPv6 GSO handlers can exhaust the kernel stack.
* +1 Common networking packet-processing path. The defect is in core IPv4 and IPv6 GSO segmentation rather than passive or diagnostic code.
* No additional Availability point. Kernel panic or system stall is the same stack-exhaustion consequence.
* No remote-reachability points. Ordinary network packets do not directly control internal SKB GSO metadata.
* No local-unprivileged point. The complete below-host-admin trigger through tunnels, namespaces, TUN, virtio, or vhost is not established.
* No corruption point. The patch establishes excessive callback recursion, not heap OOB write, UAF, controlled overwrite, or active structured-state corruption.
* No privilege penalty. Host-admin-equivalent privilege is not positively established.
* B5-R does not apply. There is no retry-accumulated heap OOB write or later consumption of corrupted heap state.
* B3-PENDING, V32, V33, and V34 do not apply. No checksum-placement write or shared-backing modification is involved.
* B4-PENDING does not apply. No authorization decision changes from DENY to ALLOW.

3. Reachability analysis

A local actor able to submit a deeply nested GSO packet through a suitable tunnel or virtual-networking path can potentially trigger the recursion. Creating the complete topology may require CAP_NET_ADMIN, but the supplied evidence does not establish whether the relevant checks require host capabilities or can be completed inside an attacker-created user and network namespace.

A malicious guest or adjacent virtual endpoint might be able to provide GSO metadata through virtio, vhost, or TAP, but the supplied call chain does not confirm that such input reaches the recursive handlers with attacker-controlled nested headers. This guest-to-host interpretation therefore remains unresolved and receives no reachability points. Direct Internet reachability is not established.

No race or special hardware is required. Once a reachable GSO path and sufficiently deep encapsulation are available, the recursion is deterministic.

4. Primitive and severity interpretation

The exact primitive is uncontrolled recursive IPv4 and IPv6 GSO callback dispatch leading to kernel-stack exhaustion. It is not a lifetime violation, stale-object use, repeated release, authorization bypass, or heap corruption primitive.

PrimitiveClass is CLASS_C because the demonstrated consequence is a kernel panic or system stall without independently useful corruption. No leverage remains beyond denial of service after excluding the crash. ImportantCandidate is NO because neither active-state corruption nor a concrete cross-boundary integrity primitive is established.

AutoDowngradeAllowed is YES because the supplied evidence supports a DoS-only classification. Manual review is still required to resolve whether a guest, delegated TUN endpoint, or attacker-controlled namespace can reach the host GSO path without host-administrator privilege.

5. One-sentence report phrase

Unbounded recursive IPv4 and IPv6 GSO segmentation of deeply nested IP in IP packets can exhaust the kernel stack and crash the system, while direct remote and guest to host reachability remain unresolved.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Verify virtio net, vhost net, TAP, tunnel offload, and user namespace call paths to determine whether a below-host-admin actor can supply the required GSO metadata and nested packet structure.

7. Call-site confidence

Call-site confidence: medium

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

Patch: net: gso: limit recursive IP-in-IP segmentation
Commit: c949646c900e879afce47df1b607fa320eb63bf2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c949646c900e879afce47df1b607fa320eb63bf2

Commit description:

IP-in-IP GSO can re-enter inet_gso_segment() or ipv6_gso_segment()
for each nested IP header. encap_level tracks header bytes, not callback
depth, so a deep chain can exhaust the kernel stack. Making
inet_gso_segment() stackable introduced unbounded IPv4 nesting; IPIP
GSO/TSO later made the path reachable. The IPv6 stackable path was
introduced separately and uses the same guard.

Count IPv4 and IPv6 GSO handler entries in skb_gso_cb, initialized once
per top-level GSO operation and preserved across GRE/UDP context changes.
Use the existing IP_TUNNEL_RECURSION_LIMIT for both handlers. The first
five entries pass, and the sixth returns -EINVAL before dispatching
another GSO callback.

Fixes: 3347c96 ("ipv4: gso: make inet_gso_segment() stackable")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Closes: https://lore.kernel.org/all/cover.1790157745.git.zihanx@nebusec.ai/
Assisted-by: LLM
Co-developed-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260924051521.32568-2-zihanx@nebusec.ai
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/gso.h
  net/core/gso.c
  net/ipv4/af_inet.c
  net/ipv6/ip6_offload.c
  net/udp.h
  net/gro.h
  net/gso.h
  net/ip_tunnels.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=c949646c900e879afce47df1b607fa320eb63bf2

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

The original Linux kernel CVE announcement for CVE-2026-98853 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98853

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:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: unknown
  Paranoid ActionableScore: 2

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).