[CVE-2026-74678][LOW] net: usb: ax88179_178a: fix skb leak in ax88179_tx_fixup()

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

CVE: CVE-2026-74678
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net: usb: ax88179_178a: fix skb leak in ax88179_tx_fixup()
Commit: 83a765cbd7b4d11b0b9fa1bb9d941ae911a2159b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83a765cbd7b4d11b0b9fa1bb9d941ae911a2159b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74678
Analysis date: Sat, 22 Aug 2026 13:26:39 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A local skb leak in the ax88179_178a USB Ethernet TX path can occur when skb_linearize() fails under NETIF_F_SG, allowing sustained local traffic under memory pressure to cause resource exhaustion and DoS.

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

The original Linux kernel CVE announcement for CVE-2026-74678 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74678

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: LOW
Manual review required: NO

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
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CVE-2026-74678	LOW	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A memory leak in the ax88179_178a USB Ethernet transmit path can occur when NETIF_F_SG is enabled and skb_linearize() fails in ax88179_tx_fixup(). The function can return NULL without freeing the original skb, so repeated TX failures under memory pressure may leak kernel network buffers and eventually cause resource exhaustion. For the CVSS the PR:L is used because reliable triggering requires a local process that can generate outbound traffic through the affected interface, but no administrative privilege is normally required just to send traffic. The issue is not directly network reachable because it is triggered in the local TX path rather than by received packets. Impact is denial of service through memory exhaustion, with no evidence of information disclosure, integrity impact, UAF, OOB access, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	USB LEAK ERRORPATH UAF NETWORK SKB LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=2
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process can generate outbound traffic through the affected AX88179 USB Ethernet interface. No administrative privilege is normally required just to transmit packets.

* Availability impact realistic: +1
  Repeated leaked skbs may contribute to memory exhaustion and DoS, especially under memory pressure.

Negative or limiting signals:

* Hard or unreliable condition required: -1
  The leak occurs only when NETIF_F_SG is enabled and skb_linearize() fails, which normally requires memory pressure or allocation failure conditions.

* Rare AND difficult-to-reach subsystem/configuration: -1 for conservative scoring
  This is limited to a specific USB Ethernet driver and TX path behavior. It is not a common networking core receive path.

Not triggered:

* No remote reachable / network-triggerable signal.
  The vulnerable path is local TX fixup, not packet receive parsing.

* No generic memory corruption signal.
  The bug is an skb leak, not UAF, double free, OOB write, type confusion, or arbitrary write.

* No privilege escalation plausible signal.
  There is no reclaim, stale callback, object replacement, or write primitive.

* No confidentiality or integrity impact.
  The patch shows resource leakage only.

Call-site confidence: high.
The commit explicitly explains the caller behavior in usbnet_start_xmit() and why returning NULL loses the original skb reference.

## 3. Reachability analysis

A local user or process can potentially trigger the path by sending outbound traffic through the affected AX88179 USB Ethernet device. The practical trigger also requires NETIF_F_SG and a failure of skb_linearize(), so exploitation depends on memory pressure or allocation failure rather than a simple packet pattern.

Namespaces or containers may allow unprivileged workloads to generate traffic through an assigned interface, but they do not make this remotely reachable. The affected driver is hardware and configuration dependent. It is not enabled on systems without this USB Ethernet adapter or driver in use.

## 4. Severity interpretation

This behaves like a Low-like or ordinary Moderate resource-exhaustion issue. The realistic impact is local DoS through gradual memory leakage, not immediate kernel memory corruption.

The theoretical worst case is system-wide memory pressure from sustained leaking skbs, but the patch does not support UAF, OOB access, attacker-controlled overwrite, information disclosure, or privilege escalation. Therefore it should not be treated as Actionable Moderate or Important.

## 5. One-sentence report phrase

A local skb leak in the ax88179_178a USB Ethernet TX path can occur when skb_linearize() fails under NETIF_F_SG, allowing sustained local traffic under memory pressure to cause resource exhaustion and DoS.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a constrained resource leak with no supported memory corruption or privilege escalation primitive, and the practical trigger requires a specific driver plus memory-pressure conditions.

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

Patch: net: usb: ax88179_178a: fix skb leak in ax88179_tx_fixup()
Commit: 83a765cbd7b4d11b0b9fa1bb9d941ae911a2159b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=83a765cbd7b4d11b0b9fa1bb9d941ae911a2159b

Commit description:

When the interface has NETIF_F_SG enabled and skb_linearize() fails in
ax88179_tx_fixup(), the function returns NULL without freeing the skb.

usbnet_start_xmit() treats a NULL return from tx_fixup() as a drop
(info->flags does not set FLAG_MULTI_PACKET for this driver), jumping
to the "drop" label where it does `if (skb) dev_kfree_skb_any(skb)`.
Because tx_fixup() returned NULL, the local skb variable in
usbnet_start_xmit() is NULL, so the original skb is never freed — a
memory leak on every TX frame whose linearization fails (i.e. under
memory pressure).

Free the skb before returning, matching the error handling already used
for the pskb_expand_head() failure path in the same function.

Fixes: 16b1c4e ("net: usb: ax88179_178a: add TSO feature")
Cc: stable@vger.kernel.org
Signed-off-by: Yi Cong <yicong@kylinos.cn>
Link: https://patch.msgid.link/20260729030436.3420477-1-cong.yi@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/usb/ax88179_178a.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=83a765cbd7b4d11b0b9fa1bb9d941ae911a2159b

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

The original Linux kernel CVE announcement for CVE-2026-74678 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74678

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 1
  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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).