[CVE-2026-74582][IMPORTANT] packet: use consistent hard_header_len in non-ring send paths

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

CVE: CVE-2026-74582
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: packet: use consistent hard_header_len in non-ring send paths
Commit: 91f041451f967cd87ed722a8f43c0b767a64f1a0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=91f041451f967cd87ed722a8f43c0b767a64f1a0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74582
Analysis date: Fri, 21 Aug 2026 12:57:57 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
AF_PACKET non-ring send paths can use inconsistent hard_header_len values during concurrent device reconfiguration, causing skb head underflow and a userspace-driven out-of-bounds kernel write with local DoS and possible privilege escalation impact.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74582 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74582

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
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CVE-2026-74582	IMPORTANT	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:H/I:H/A:H';*CWE-787;CWE-362;CWE-367;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'AF_PACKET non-ring send paths can read dev hard_header_len inconsistently while a network device is being reconfigured. In the SOCK_RAW path, packet_snd can keep a larger reserve value but allocate skb headroom using a smaller later value, so moving skb data back may place it before skb head and the following userspace copy can become an out-of-bounds kernel write. For the CVSS the PR:L is used for the paranoid score because reliable triggering may require local capabilities such as CAP_NET_RAW and CAP_NET_ADMIN in a network namespace, but this is not always equivalent to full host administrator privileges. The issue is not directly network reachable because it is triggered through local AF_PACKET send operations and concurrent local device reconfiguration. Impact is at least local denial of service via kernel memory corruption and in the worst case may allow confidentiality or integrity impact including privilege escalation. For the paranoid score, choose the highest still defensible interpretation supported by the bug class and patch context, with preference for manual review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	WRITE OOB SKB HARDWARE IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	NO	NO	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

Triggered signals:

* Memory corruption, Strong corruption primitive: +2
  The commit explicitly describes an out-of-bounds write caused by moving skb->data before skb->head, followed by a userspace copy.

* Memory layout invariant restoration: +1
  The fix restores consistency between hard_header_len, reserve, headroom allocation, and skb construction.

* Reliable kernel crash / strong DoS: +1
  An skb head underflow followed by copy from userspace is a credible kernel crash condition.

* Weak LPE concern: +1 conservative, Strong LPE plausibility: +2 paranoid
  Conservative view: no demonstrated reclaim or target selection is shown. Paranoid view: attacker-influenced data is copied out of bounds in kernel memory, so privilege escalation is technically plausible and requires manual review.

* Common networking / packet socket path: +1
  AF_PACKET is a common kernel networking interface, although triggering this specific bug depends on local send paths and concurrent device reconfiguration.

Subtractors:

* Hard or unreliable race / special timing required: -1
  The bug requires a race between AF_PACKET send path execution and net_device hard_header_len changes.

* Requires capabilities in typical deployments: -1 conservative
  Reliable triggering likely needs CAP_NET_RAW for AF_PACKET SOCK_RAW and CAP_NET_ADMIN or equivalent control over device reconfiguration. This is not full host root in all namespace or container models, so -1 is used rather than a full -2.

## 3. Reachability analysis

The bug is local, not directly network reachable. Triggering requires local AF_PACKET send operations and concurrent reconfiguration of a network device whose hard_header_len can change, such as bonding device type changes.

In typical deployments, AF_PACKET SOCK_RAW requires CAP_NET_RAW, and the relevant device reconfiguration path usually requires CAP_NET_ADMIN. In containerized or user namespace enabled environments, those capabilities may be delegated in a way that is weaker than full host root, so PR:L style treatment is defensible for triage.

The path is not a remote packet parsing path. Realistic exploitation requires local code execution plus timing control over sendmsg and device reconfiguration.

## 4. Severity interpretation

This should not be treated as an ordinary Moderate auto-close issue. The bug is a race, but the resulting primitive is stronger than a NULL dereference or bookkeeping mismatch because it can cause an out-of-bounds kernel write with userspace-provided data.

Conservative interpretation is Actionable Moderate because exploitation requires local capabilities and a race. Paranoid interpretation is a Strong Important candidate because the patch context explicitly supports kernel memory corruption in a common networking subsystem and the LPE potential cannot be ruled out without manual analysis.

## 5. One-sentence report phrase

AF_PACKET non-ring send paths can use inconsistent hard_header_len values during concurrent device reconfiguration, causing skb head underflow and a userspace-driven out-of-bounds kernel write with local DoS and possible privilege escalation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch fixes a race leading to an explicit out-of-bounds kernel write from userspace data in AF_PACKET skb construction, so privilege escalation feasibility and namespace/capability reachability should be reviewed manually.

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UPSTREAM PATCH SUMMARY
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Patch: packet: use consistent hard_header_len in non-ring send paths
Commit: 91f041451f967cd87ed722a8f43c0b767a64f1a0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=91f041451f967cd87ed722a8f43c0b767a64f1a0

Commit description:

packet_snd() reads dev->hard_header_len multiple times while allocating
and constructing an skb. Device reconfiguration can change this value
concurrently, for example through bonding device type changes.

For SOCK_RAW, packet_snd() can save a larger value in reserve and later
allocate headroom using a smaller value. Moving skb->data back by reserve
then places it before skb->head, and the following copy from userspace can
attempt an out-of-bounds write.

packet_sendmsg_spkt() has the same issue because it calculates its
reservation and header offset from separate reads before dropping the RCU
read lock to allocate the skb.

Add LL_RESERVED_SPACE_EX() for callers that already saved a header length.
Read hard_header_len once in packet_snd() and use it for allocation and
construction. In packet_sendmsg_spkt(), preserve the allocation-time value
through the device lookup retry.

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

Fixes: b84bbaf ("packet: in packet_snd start writing at link layer allocation")
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/20260805125729.19220-3-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:
  include/linux/netdevice.h
  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=91f041451f967cd87ed722a8f43c0b767a64f1a0

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

The original Linux kernel CVE announcement for CVE-2026-74582 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74582

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