[CVE-2026-98239][MODERATE 7.0] net: lan743x: fix RX checksum use-after-free

[CVE-2026-98239][MODERATE 7.0] net: lan743x: fix RX checksum use-after-free

[AL-KERNEL]

CVE: CVE-2026-98239
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: lan743x: fix RX checksum use-after-free
Commit: 0e52886c4324c9897c2c62f92be3dc8316cee67e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0e52886c4324c9897c2c62f92be3dc8316cee67e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98239
Analysis date: Fri, 09 Oct 2026 03:02:36 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
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-2026-98239 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98239

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-2026-98239	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98239 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;*CWE-787;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.5';DESCR 'A confirmed use-after-free write occurs in the LAN743x receive path when a packet spans multiple receive buffers. lan743x_rx_trim_skb linearizes the head skb and frees fragment skb metadata, but the local skb pointer can still reference the final freed fragment. The checksum success path then writes CHECKSUM_UNNECESSARY to the freed ip_summed field, producing a one-byte kernel heap write. A network sender can potentially reach this path with a suitable multi-buffer packet after a live MTU increase leaves existing receive ring entries with their previous buffer size. For the CVSS the PR:N value is used because the sender does not need an account or privileges on the target system, although an administrator normally creates the required MTU transition state. AV:A and AC:H reflect the need for access through an adjacent LAN743x network path and the additional interface state and timing conditions. Impact is at least denial of service through kernel memory corruption and a possible crash. The confirmed write after free also creates a plausible but unproven path to confidentiality or integrity impact if the freed skb storage is reused for another kernel object. Manual review is required to determine whether the constrained one-byte write can be converted into reliable privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	REMOTE WRITE OOB UAF NETWORK SKB HARDWARE LINUS TEST KPANIC PACKET  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 2
* Conservative score: 4
* Raw paranoid score: 3
* Paranoid score: 5
* Conservative floor applied: 2 -> 4 under the real UAF floor because a freed slab object is actually modified and host-admin-only reachability is not established.
* Paranoid floor applied: 3 -> 5 because the receive path performs a concrete post-free heap write, is conditionally reachable by a below-host-admin packet sender, and leaves meaningful reuse impact unresolved.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF and lifetime violation. `lan743x_rx_trim_skb()` frees fragment skb metadata while the local `skb` pointer survives and is subsequently modified.
* +0 Separate overwrite bonus withheld. The one-byte post-free write is part of the same UAF primitive and is not double-counted as generic corruption.
* +0 Reliable crash not awarded. KASAN confirmed the invalid write, but a production kernel panic was not demonstrated.
* +0 Direct remote-reachability bonus not awarded. Packet input reaches the data path only during a state created by a live MTU increase, which normally involves separate local control-plane activity.
* +0 Hardware penalty not applied. LAN743x hardware is required for the production path, but the supplied evidence does not establish that it is sufficiently rare to justify a physical-only penalty.
* +0 Race penalty not applied. This is an ordered stale-pointer sequence, not a timing race.

Paranoid additional signal:

* +1 Weak exploitation concern. The stale operation writes a fixed byte into freed skb slab storage, and network traffic can influence skb allocation activity. This is stronger than an immediate stale read or poisoned-pointer fault, but controlled reclaim and useful replacement-object corruption are not established.

Important non-awarded signals:

* Controlled reclaim or object substitution is not demonstrated.
* Arbitrary or directed write is not demonstrated. The offset and value are constrained to `ip_summed = CHECKSUM_UNNECESSARY`.
* Structured stale-object leverage is not separately awarded because no later consumption of replacement-object pointers, lengths, callbacks, or ownership fields is shown.
* Strong LPE leverage is not awarded.
* Confidentiality and integrity consequence bonuses are not awarded merely because reclaimed storage could theoretically contain another object.
* Extra Availability is not awarded because neither a distinct persistent consequence nor a demonstrated production crash exists.
* No concrete repeated release or refcount operation exists. The first operation frees fragment metadata, but the later operation is a field write rather than a second put, free, or refcount mutation.
* The B5-R profile does not apply. There is no retry-accumulated index, retry-driven heap OOB write, or later consumption of retry-corrupted descriptor state.

3. Reachability analysis

A network sender can supply a packet that spans multiple receive buffers, but the documented production condition arises after a live MTU increase leaves existing RX ring entries with their previous buffer size. The sender does not need a local account once that transient state exists, but the supplied evidence does not show that the sender can initiate the MTU transition.

The MTU operation on a host-owned physical LAN743x interface normally involves network-administration authority. It is unresolved whether the complete sequence must be controlled by one host administrator or can arise from legitimate prior administration followed by an unprivileged adjacent sender. Therefore `HostAdminRequired=UNKNOWN`.

No evidence establishes that the complete physical-device and MTU sequence is available through an attacker-created user or network namespace, so `NamespaceScopedPrivilege=UNKNOWN`.

The real production path requires LAN743x hardware, RX checksum offload, a live MTU transition, remaining old-sized RX buffers, and a multi-buffer packet. The bug was reproduced with KUnit and KASAN but was not tested on physical hardware. There is no race, although the old-buffer transition creates a limited trigger window.

4. Primitive and severity interpretation

The exact primitive is a one-byte write through a pointer to freed fragment skb metadata. This is a real slab use-after-free write and therefore CLASS_B, not a NULL dereference or warning-only defect.

The stale storage is skb metadata from an active receive operation, not a fixed asynchronous DMA buffer. The stale use is shallow and constrained to `ip_summed`, but it occurs in the normal RX data path rather than cleanup or destruction. If the slab slot has been reused, the fixed byte can modify a different kernel object at the corresponding offset. The patch does not establish attacker-controlled reclaim, a selected victim type, callback influence, arbitrary read or write, disclosure, or privilege escalation.

The demonstrated consequence is kernel heap corruption detected by KASAN. A crash or stronger effect is plausible but not demonstrated on production hardware. Important-grade exploitation is unresolved rather than proven.

`ImportantCandidate=YES` because an externally influenced receive path performs an actual post-free heap write, and automatic Moderate closure could discard meaningful reuse analysis. `AutoDowngradeAllowed=NO` because the unresolved primitive is concrete memory corruption rather than crash-only stale dereference. The likely operational severity is Actionable Moderate unless manual analysis establishes reliable reclaim or a security-sensitive adjacent victim.

5. One-sentence report phrase

A multi-buffer LAN743x receive operation can write a fixed checksum value through a freed fragment skb pointer after linearization, creating a confirmed one-byte kernel heap use-after-free write that requires manual reclaim and victim-object analysis.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The write is constrained, but it targets freed slab storage in an attacker-influenced packet path, and the behavior of reused storage on physical hardware remains unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: lan743x: fix RX checksum use-after-free
Commit: 0e52886c4324c9897c2c62f92be3dc8316cee67e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0e52886c4324c9897c2c62f92be3dc8316cee67e

Commit description:

lan743x_rx_process_buffer() adds each non-first receive buffer to the
head skb's frag_list.  On the last descriptor, lan743x_rx_trim_skb()
linearizes the head and frees the fragment skb metadata.

The checksum-success path then writes ip_summed through the local skb
pointer, which still points to the final fragment.  This causes a
use-after-free write when a packet spans more than one receive buffer.

Set ip_summed on the surviving head skb instead.  Multi-buffer receive
can occur after a live MTU increase because existing ring entries keep
their old buffer size until they are replenished.

A KUnit test invoking lan743x_rx_process_buffer() with a two-buffer
packet produced a one-byte KASAN use-after-free write before this change.
The same test passed after the change.  The driver object also builds
with W=1.  This was not tested on physical LAN743x hardware.

Fixes: cd69105 ("net: lan743x: Add support for Rx IP & TCP checksum offload")
Cc: stable@vger.kernel.org
Signed-off-by: Mark Amirkan <markdamirkan@gmail.com>
Reviewed-by: Chenguang Zhao <zhaochenguang@kylinos.cn>
Link: https://patch.msgid.link/20260913-b4-send-lan743x-uaf-v1-1-73d563d08ba9@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/ethernet/microchip/lan743x_main.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=0e52886c4324c9897c2c62f92be3dc8316cee67e

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

The original Linux kernel CVE announcement for CVE-2026-98239 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98239

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:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 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).