[CVE-2026-80590][IMPORTANT] inet: frags: strip GSO state from fragments before reassembly

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

CVE: CVE-2026-80590
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: inet: frags: strip GSO state from fragments before reassembly
Commit: cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80590
Analysis date: Fri, 28 Aug 2026 02:47:28 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
Untrusted GSO metadata on IP fragments can survive reassembly and make the resulting skb reach skb_segment with a non GRO shaped frag_list, allowing a local user or guest controlled tap path to trigger a reliable kernel panic.

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

The original Linux kernel CVE announcement for CVE-2026-80590 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80590

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

CVE-2026-80590	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';*CWE-20;CWE-617;CWE-754;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'IP fragments can incorrectly retain untrusted GSO metadata supplied through virtio_net_hdr on tun tap or AF_PACKET with PACKET_VNET_HDR. After reassembly the resulting skb may still claim to be GSO while its frag_list is not GRO shaped, and a later software segmentation path such as udp_rcv_segment, validate_xmit_skb, or ip_finish_output_gso can reach BUG_ON checks in skb_segment. For the CVSS the PR:L is used because the practical trigger requires a local user or process able to create or use the relevant tun tap, AF_PACKET, netns, or guest networking path. The issue is not public Internet reachable by normal packet traffic, but it may be reachable from a local namespace setup or from a guest when the VMM forwards virtio_net_hdr to a host tap. Impact is at least local denial of service via kernel panic. In the paranoid score, limited confidentiality and integrity impact are kept for manual review because the bug violates skb_segment layout assumptions in core networking code, although the directly demonstrated outcome is DoS.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-80590 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: href YES  NO NO unknown 	MAYBE	REMOTE KASAN NETWORK SKB KERNEL_PANIC_PLUS_UAF HARDWARE IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	-	-	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

Conservative signals:

* Local unprivileged trigger: +1. The description states that two writes to a tap by an unprivileged user in its own userns are enough.
* Memory layout invariant restoration: +1. The patch restores the invariant that an IP fragment queued for reassembly must not carry GSO state that later makes the reassembled skb look like GRO shaped input.
* Reliable kernel crash / strong DoS: +1. The reproducer reaches BUG_ON in skb_segment and causes kernel panic because the path runs with BH disabled.
* Availability impact realistic: +1. The crash is host wide and described as reproducible across IPv4 and IPv6 fragment layouts.
* Common networking core path or packet-processing path: +1. The affected path involves inet fragment reassembly, skb GSO segmentation, local delivery, forwarding, nf_defrag, and shared IPv4/IPv6 reassembly helpers.

Paranoid additional signals:

* Cross-boundary isolation impact: +1. The issue may be reachable from a guest when the VMM forwards virtio_net_hdr to a host tap, making guest-to-host DoS plausible.
* Broad/common exposure: +1. tun/tap, AF_PACKET with PACKET_VNET_HDR, defrag, and GSO are common Linux networking facilities, even though the exact trigger is not normal Internet packet traffic.

Not awarded:

* Remote reachable: +0. Normal external network packets cannot directly attach virtio_net_hdr GSO metadata.
* Strong memory corruption: +0. The demonstrated primitive is BUG_ON/panic from invalid skb layout assumptions, not UAF, OOB write, double free, arbitrary write, or object replacement.
* Privilege escalation plausible: +0 conservative. No concrete LPE primitive is shown.
* Confidentiality / Integrity impact: +0 conservative. The patch and call path show reliable crash, not disclosure or modification of protected data.

## 3. Reachability analysis

The primary trigger is local. A user may reach it through tun/tap with virtio_net_hdr, AF_PACKET with PACKET_VNET_HDR, or CAP_NET_RAW in a netns where defragmentation occurs before a GSO point. Namespaces matter because userns/netns setups can reduce the practical privilege requirement compared with full host root. The path is not public Internet reachable by ordinary fragmented IP traffic because the key condition is attacker supplied GSO metadata, not merely fragmentation. A guest-to-host variant is plausible when a VMM forwards virtio_net_hdr into a host tap.

Call-site confidence: high. The report includes the concrete call trace through tun_get_user, ip_local_deliver, udp_rcv_segment, GSO segmentation, and skb_segment, and the patch modifies the shared fragment queue insertion helper.

## 4. Severity interpretation

This is stronger than an ordinary Moderate DoS because it is a reliable local kernel panic in core networking code and may cross a guest-to-host boundary in common virtualization setups. It does not currently justify confirmed Important on memory-corruption or privilege-escalation grounds because the demonstrated effect is a BUG_ON, not attacker-controlled memory corruption. The conservative score is Actionable Moderate. The paranoid score reaches Strong Important candidate because of local unprivileged reachability, reproducible host-wide panic, common networking paths, and plausible guest-to-host exposure.

## 5. One-sentence report phrase

Untrusted GSO metadata on IP fragments can survive reassembly and make the resulting skb reach skb_segment with a non GRO shaped frag_list, allowing a local user or guest controlled tap path to trigger a reliable kernel panic.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because the bug is locally triggerable without full host root in namespace based setups, affects central skb and fragment reassembly paths, and has plausible guest-to-host DoS exposure, even though confirmed confidentiality, integrity, or LPE impact is not demonstrated.

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

Patch: inet: frags: strip GSO state from fragments before reassembly
Commit: cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee

Commit description:

A virtio_net_hdr (tun/tap, or AF_PACKET with PACKET_VNET_HDR) can mark
an IPv4 or IPv6 fragment as GSO; nothing relates gso_type to frag_off.
inet_frag_reasm_prepare()/inet_frag_reasm_finish() keep the first
fragment's skb as the head of the reassembled datagram, including its
shinfo->gso_size/gso_type/gso_segs, and chain the remaining fragments
on frag_list with whatever linear/paged layout they arrived with.

After ip_defrag() (ip_local_deliver(), nf_defrag_ipv4, ...) the
reassembled skb therefore still claims to be GSO (SKB_GSO_DODGY), and
the next software segmentation point - udp_rcv_segment() on local
delivery, validate_xmit_skb(), or the ip_finish_output_gso() slow
path - hands it to skb_segment(). skb_segment()'s frag_list walk
assumes GRO-shaped input and hits one of its BUG_ON()s. Two writes to
a tap by an unprivileged user in its own userns are enough:

  kernel BUG at net/core/skbuff.c:4899!
  Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
  CPU: 0 UID: 1000 PID: 82 Comm: poc Not tainted 7.2.0-pentest+ #2
  RIP: 0010:skb_segment+0x20ca/0x48b0
  Call Trace:
   <TASK>
   __udp_gso_segment+0x29a/0x27d0
   udp4_ufo_fragment+0x458/0x6c0
   inet_gso_segment+0x429/0x1340
   skb_mac_gso_segment+0x233/0x4f0
   __skb_gso_segment+0x308/0x660
   udp_queue_rcv_skb+0x440/0xad0
   udp_unicast_rcv_skb+0xc7/0x2c0
   udp_rcv+0x16ce/0x2260
   ip_protocol_deliver_rcu+0x197/0x2d0
   ip_local_deliver+0x430/0x690
   ip_rcv+0x16f/0x1f0
   __netif_receive_skb_one_core+0x15e/0x1c0
   __netif_receive_skb+0x1e/0x110
   netif_receive_skb+0xf6/0x5c0
   tun_rx_batched.isra.0+0x3ab/0x790
   tun_get_user+0x17c3/0x3550
   tun_chr_write_iter+0xba/0x1b0
   vfs_write+0x646/0x1130
   </TASK>
  Kernel panic - not syncing: Fatal exception in interrupt

This runs with BH disabled, so it is a panic rather than an oops. The
same is reachable with CAP_NET_RAW in a netns where a defrag point
precedes a GSO point, and from a guest whose VMM forwards
virtio_net_hdr to a tap. The SKB_GSO_DODGY frag_list checks added by
commit 3dcbdb1 ("net: gso: Fix skb_segment splat when splitting
gso_size mangled skb having linear-headed frag_list") and by
commit 9e4b7a9 ("net: gso: fix panic on frag_list with mixed head
alloc types") do not cover it: page-backed heads skip them, and kmalloc
heads skip them when gso_size == skb_headlen(head), which the sender
controls.

An skb entering a frag queue is an IP fragment by definition and
cannot legitimately carry GSO state: GRO does not merge fragments and
the stack segments before it fragments, so only untrusted sources are
affected. This has been reachable since
commit f43798c ("tun: Allow GSO using virtio_net_hdr"), the first
path that let userspace attach GSO metadata to an IP fragment. Reset
the GSO fields of every fragment as it is queued, in
inet_frag_queue_insert(), which IPv4, IPv6, nf_conntrack_reasm and
6lowpan reassembly share; then neither the head nor the frag_list
members of the reassembled skb carry them (the members matter too:
the ip_do_fragment()/ip6_fragment() fast paths send them out as they
are). The head may remain CHECKSUM_PARTIAL; that is already accepted
on receive and resolved by skb_checksum_help() in
ip_do_fragment()/ip6_fragment() on forward.

Tested on top of net.git ( dc4b95b ), x86_64: the tap reproducer
above, two further IPv4 frag_list geometries that reach
BUG_ON(i >= nfrags) and BUG_ON(!list_skb->head_frag), and an IPv6
fragment-header variant (udp6_ufo_fragment()) each panic the unpatched
kernel; with this patch all four datagrams are delivered intact and
nothing is logged.

Fixes: f43798c ("tun: Allow GSO using virtio_net_hdr")
Cc: stable@kernel.org
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Xinyang Ge <xinyang@anthropic.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

[Commit description truncated; see the upstream URL below]

Changed files:
  net/core/skbuff.c
  net/ipv4/inet_fragment.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=cfdbc8c2e6f9ef5d8b8e54859da03dfe682b0bee

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

The original Linux kernel CVE announcement for CVE-2026-80590 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80590

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:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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).