[CVE-2026-72057][MODERATE 7.0] net/sched: act_ct: preserve tc_skb_cb across defragmentation

[CVE-2026-72057][MODERATE 7.0] net/sched: act_ct: preserve tc_skb_cb across defragmentation

[AL-KERNEL]

CVE: CVE-2026-72057
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: act_ct: preserve tc_skb_cb across defragmentation
Commit: 2400c4b05d58834b994500a9eec90a37db44187c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2400c4b05d58834b994500a9eec90a37db44187c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72057
Analysis date: Sat, 15 Aug 2026 08:26:30 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-72057 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72057

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.

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

CVE-2026-72057	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-665;CWE-703;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'tc act_ct can lose qdisc metadata for fragmented packets because nf_ct_handle_fragments clears skb control buffer state that aliases tc_skb_cb and qdisc_skb_cb. Fragmented traffic through a configured act_ct path can therefore clear fields such as pkt_segs and later trigger WARN_ON_ONCE() in qdisc_pkt_segs(). For the CVSS the PR:N is used because an attacker only needs network reachability to the affected traffic path and does not need a local account on the target. The issue is network reachable only on systems where tc act_ct and conntrack defragmentation are configured for untrusted fragmented traffic. Impact is denial of service. In the base case this may be limited to warnings or degraded behavior, while with panic_on_warn enabled it can cause a kernel panic. No direct evidence of UAF, OOB access, information disclosure, or privilege escalation is indicated by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE DANGER SIMPLEFIX NETWORK SKB LINUS QDISC KPANIC 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

1. ActionableScore
* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

2. Signal breakdown
* Remote reachable / network-triggerable: +2. Fragmented traffic can reach the affected `tc act_ct` path if the system is configured to process untrusted traffic through this action.
* Common packet-processing path: +1. The bug is in `net/sched` with conntrack defragmentation interaction, not a purely local control-plane-only path.
* Availability impact realistic: +1. Metadata loss can trigger `WARN_ON_ONCE()` and cause operational disruption.
* Reliable kernel crash / strong DoS: +1 only for paranoid score. With `panic_on_warn` enabled, the WARN becomes a kernel panic.
* No memory corruption / LPE signal: +0. The patch shows `skb->cb` metadata loss, not UAF, OOB write, attacker-controlled overwrite, info leak, or privilege escalation.
* No rare-subsystem penalty applied. `act_ct` requires configuration, but this is still a packet-processing path in networking deployments.

3. Reachability analysis
A remote sender can plausibly trigger the condition by sending fragmented traffic through an already configured `tc act_ct` path using conntrack defragmentation. The attacker does not need a local account if the traffic path is exposed. However, the affected path is not enabled by default on all systems and requires traffic-control and conntrack configuration. Namespaces may matter for setup, but triggering the packet path itself can be remote once configured. Call-site confidence: high, because the patch and commit directly identify `tcf_ct_handle_fragments()`, `nf_ct_handle_fragments()`, and the later `qdisc_pkt_segs()` WARN condition.

4. Severity interpretation
This behaves like an actionable Moderate network-triggerable DoS, not an Important-class memory corruption issue. The realistic impact is warning-triggered disruption and, on systems with `panic_on_warn`, kernel panic. Theoretical escalation is not supported by the patch because the affected data is qdisc metadata in `skb->cb`, with no demonstrated stale pointer, object reuse, write primitive, or confidentiality/integrity effect.

5. One-sentence report phrase
Fragmented traffic through a configured `tc act_ct` conntrack path can lose `tc_skb_cb` / `qdisc_skb_cb` metadata during defragmentation and trigger `WARN_ON_ONCE()` in `qdisc_pkt_segs()`, causing a network-reachable DoS on systems where `panic_on_warn` turns the warning into a kernel panic.

6. Manual review recommendation
MANUAL CHECK REQUIRED. The issue is network-triggerable in affected configurations and can become a host-wide panic with `panic_on_warn`, but the manual review should verify actual product exposure to `tc act_ct` fragmented traffic rather than treating it as universal remote DoS.

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

Patch: net/sched: act_ct: preserve tc_skb_cb across defragmentation
Commit: 2400c4b05d58834b994500a9eec90a37db44187c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2400c4b05d58834b994500a9eec90a37db44187c

Commit description:

tcf_ct_handle_fragments() calls nf_ct_handle_fragments() without saving
and restoring skb->cb. The defrag helper clears IPCB/IP6CB, which aliases
the tc_skb_cb/qdisc_skb_cb control buffer. Fragmented traffic through
act_ct therefore loses qdisc metadata such as pkt_segs and can trigger
WARN_ON_ONCE() in qdisc_pkt_segs() when panic_on_warn is enabled.

Save and restore the full tc_skb_cb around nf_ct_handle_fragments(),
matching the pattern used by ovs_ct_handle_fragments().

Fixes: ec624fe ("net/sched: Extend qdisc control block with tc control block")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Zihan Xi <xizh2024@lzu.edu.cn>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Link: https://patch.msgid.link/510c51217fd7aaf29c6dc298bab8d643fe229b1c.1781358692.git.xizh2024@lzu.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/sched/act_ct.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=2400c4b05d58834b994500a9eec90a37db44187c

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

The original Linux kernel CVE announcement for CVE-2026-72057 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72057

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:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 4
  Paranoid ActionableScore: 5

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).