[CVE-2026-74668][MODERATE 7.0] packet: use consistent hard_header_len in TX_RING send path

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

CVE: CVE-2026-74668
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: packet: use consistent hard_header_len in TX_RING send path
Commit: d85d2fd54e901637c81d847811e03c662aee13cd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d85d2fd54e901637c81d847811e03c662aee13cd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74668
Analysis date: Sat, 22 Aug 2026 14:31:54 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
AF_PACKET TX_RING can race with netdevice reconfiguration and use inconsistent hard_header_len values during skb allocation and construction, allowing a local capability-bearing attacker to trigger skb bounds corruption and likely kernel crash, with limited memory-corruption concern requiring manual review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74668 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74668

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: 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-2026-74668	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;*CWE-787;CWE-191;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'AF_PACKET TX_RING send can use inconsistent hard_header_len values when tpacket_snd() races with netdevice reconfiguration. The skb may be allocated with smaller reserved headroom than the amount later pushed in tpacket_fill_skb(), or copylen minus hard_header_len may become negative, which can trigger skb bounds checks and a kernel crash. For the CVSS the PR:L is used in the paranoid score because reduced capability root, container root, or delegated CAP_NET_RAW and CAP_NET_ADMIN access may be enough to reach the packet socket and reconfiguration path. The issue is local and is not directly network reachable. Impact is at least denial of service. Because the race touches skb headroom and length calculations, limited confidentiality or integrity impact is kept in the paranoid score and the issue should be manually reviewed.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE READ SIMPLEFIX SKB PACKET  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

Conservative signals:

* **Memory layout invariant restoration: +1**. The patch restores consistency between `hard_header_len`, headroom allocation, `copylen`, and skb construction in the AF_PACKET TX_RING send path.
* **Constrained kernel buffer overwrite / weak corruption candidate: +1**. A changed `hard_header_len` can make reserved skb headroom smaller than the later `skb_push()` amount, or make `copylen - hard_header_len` negative. This is a plausible skb bounds or memory-layout corruption sink, but not a demonstrated arbitrary write.
* **Reliable kernel crash / strong DoS: +1**. The most direct consequence is skb bounds failure, warning, BUG, or kernel crash.
* **Broad local networking subsystem exposure: +1**. AF_PACKET is a long-standing local networking interface and TX_RING is a known high-control packet buffer path.
* **Hard or unreliable race / special timing required: -1**. Triggering requires a race between TX_RING send processing and netdevice reconfiguration.
* **Requires CAP_NET_RAW plus CAP_NET_ADMIN in typical deployments: -1**. I use only `-1` rather than `-2` because delegated capabilities, container root, or reduced-capability service contexts may be realistic and are not equivalent to full host root.

Paranoid additional interpretation:

* **Local privileged but reduced-capability trigger: +1**. In environments where an attacker has delegated `CAP_NET_RAW` and `CAP_NET_ADMIN` in a network namespace or container-like context, the trigger is closer to PR:L than full PR:H.
* **Weak LPE concern: +1**. The affected state involves skb headroom and length arithmetic, which is more security-relevant than a pure warning or leak. However, there is no demonstrated reclaim, type confusion, callback control, or arbitrary write, so this remains weak and does not justify Important by itself.

## 3. Reachability analysis

The issue is local, not directly network reachable. A realistic trigger requires access to AF_PACKET TX_RING send functionality and concurrent netdevice reconfiguration that changes `dev->hard_header_len`. In typical host deployments this implies privileged capabilities such as `CAP_NET_RAW` and `CAP_NET_ADMIN`, so ordinary unprivileged users usually cannot trigger it. Namespaces and containers matter because reduced-capability root or delegated network administration may provide enough control without full host-root equivalence. The path is in a broadly available networking subsystem, but the race condition and capability requirements reduce practical exploitability.

## 4. Severity interpretation

This is stronger than an ordinary low-priority Moderate because it affects skb allocation and construction invariants in a local networking data path. The realistic impact is local DoS through skb bounds failure or kernel crash. Theoretical memory corruption concern exists because the race can desynchronize allocation size, pushed header length, and copy length, but the patch does not show a controlled overwrite, UAF reclaim, type confusion, or reliable privilege-escalation primitive. Therefore this is best treated as **Actionable Moderate**, not a clear Important-class issue.

## 5. One-sentence report phrase

AF_PACKET TX_RING can race with netdevice reconfiguration and use inconsistent hard_header_len values during skb allocation and construction, allowing a local capability-bearing attacker to trigger skb bounds corruption and likely kernel crash, with limited memory-corruption concern requiring manual review.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

The bug touches skb headroom and length calculations in a networking transmit path, so it should not be auto-closed as a simple DoS. However, exploitation beyond crash is not demonstrated, and the race plus capability requirements keep it below a strong Important candidate.

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

Patch: packet: use consistent hard_header_len in TX_RING send path
Commit: d85d2fd54e901637c81d847811e03c662aee13cd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d85d2fd54e901637c81d847811e03c662aee13cd

Commit description:

tpacket_snd() reads dev->hard_header_len independently for skb
allocation and header construction in tpacket_fill_skb(). Concurrent
netdevice reconfiguration can therefore make the reserved headroom
smaller than the amount later pushed, or make copylen - hard_header_len
negative.

Snapshot hard_header_len once before processing ring frames and use it
for the frame limit, headroom allocation, copy length, and skb
construction. Pass the snapshot to tpacket_fill_skb().

The separate SOCK_DGRAM consistency problem between hard_header_len and
header_ops->create is not addressed here.

Fixes: 69e3c75 ("net: TX_RING and packet mmap")
Cc: stable@vger.kernel.org
Signed-off-by: Qihang Tang <q.h.hack.winter@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260805125729.19220-4-q.h.hack.winter@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/packet/af_packet.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=d85d2fd54e901637c81d847811e03c662aee13cd

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

The original Linux kernel CVE announcement for CVE-2026-74668 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74668

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:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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).