[CVE-2026-74666][MODERATE 7.0] packet: synchronize pressure clearing with ring reconfiguration

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

CVE: CVE-2026-74666
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: packet: synchronize pressure clearing with ring reconfiguration
Commit: cf8189b82bb93f219ab740e0346c919ad65ada62
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cf8189b82bb93f219ab740e0346c919ad65ada62
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74666
Analysis date: Sat, 22 Aug 2026 15:22:24 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
AF_PACKET ring reconfiguration can race with packet_recvmsg or packet_poll pressure clearing, allowing a local CAP_NET_RAW or namespace delegated user to trigger stale or NULL ring storage dereference and potentially crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-74666 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74666

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-2026-74666	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-362;*CWE-416;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'AF_PACKET ring reconfiguration can race with packet_recvmsg or packet_poll pressure clearing. The socket may still observe tpacket_rcv after the RX ring has been cleared, so __packet_rcv_has_room can dereference stale or NULL ring storage. This is a local control plane race on packet socket ring state rather than a remotely reachable packet input bug. For the CVSS the PR:L is used because a local process with AF_PACKET access, CAP_NET_RAW, or equivalent delegated network namespace capability is needed to create and reconfigure the packet ring. Impact is at least local denial of service via kernel crash. For the paranoid score, stale ring storage makes this a UAF candidate, so confidentiality and integrity impact are kept in scope for manual review while AC:H reflects the race timing requirement.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE READ LOCK INIT KPANIC MEMORY PACKET 	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

Conservative signals:

* Local unprivileged or low-privileged trigger: +1. AF_PACKET ring setup is local and usually needs CAP_NET_RAW or equivalent namespace capability, not full host root.
* Memory corruption, weak or indirect corruption candidate: +1. The patch describes stale or NULL ring storage dereference, but does not show controlled reclaim, overwrite, callback control, or type confusion.
* Real lifetime corruption: +1. The bug is a synchronization issue where ring state can be cleared while another path still observes tpacket_rcv and accesses stale ring storage.
* Reliable kernel crash / strong DoS: +1. NULL or stale ring dereference in __packet_rcv_has_room can plausibly crash the kernel.
* Hard or unreliable race / special timing required: -1. The issue depends on racing packet_set_ring with packet_recvmsg or packet_poll pressure clearing.
* Broad networking local subsystem exposure: +1. AF_PACKET is a common packet socket facility and ring mode is a well-known local networking interface.

Paranoid additional signal:

* Local high-control kernel data-plane API refinement: +1. AF_PACKET TPACKET rings give local users significant control over packet ring configuration and lifetime behavior when CAP_NET_RAW or namespace delegation is available.

Race-UAF cap applied:

* Conservative score is capped at 4 and paranoid score at 5 because this is a race-dependent stale pointer or UAF candidate without demonstrated attacker-controlled reclaim, write-after-free, type confusion, callback dispatch, or refcount takeover.

## 3. Reachability analysis

The bug is locally reachable through AF_PACKET socket ring reconfiguration and receive or poll paths. It is not directly remotely reachable by packet traffic alone, because the attacker needs local control over the packet socket and its ring state.

In typical deployments, AF_PACKET use requires CAP_NET_RAW. However, user namespaces and network namespaces may make this reachable to container root or delegated low-privilege users depending on system policy. This should not be treated as full host-root-only unless the product disables unprivileged user namespaces and tightly restricts CAP_NET_RAW.

The affected path is not a rare hardware path. It is a local networking socket facility. Exploitation still requires timing a race between ring reconfiguration and pressure clearing.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the bug involves stale ring storage and a possible UAF-like lifetime violation in a common local networking subsystem. Realistically demonstrated impact is local kernel crash / DoS.

Theoretical privilege escalation is not proven. The patch does not show attacker-controlled reclaim, object replacement, arbitrary write, callback control, or type confusion. Therefore it should not be scored as a strong Important memory-corruption primitive yet. It is best treated as Actionable Moderate with manual review required.

## 5. One-sentence report phrase

AF_PACKET ring reconfiguration can race with packet_recvmsg or packet_poll pressure clearing, allowing a local CAP_NET_RAW or namespace delegated user to trigger stale or NULL ring storage dereference and potentially crash the kernel.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the demonstrated impact is DoS, but the stale ring storage access makes this a UAF candidate in AF_PACKET ring lifetime handling. The available patch does not prove LPE, but the bug class and subsystem are important enough to avoid auto-close.

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

Patch: packet: synchronize pressure clearing with ring reconfiguration
Commit: cf8189b82bb93f219ab740e0346c919ad65ada62
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=cf8189b82bb93f219ab740e0346c919ad65ada62

Commit description:

packet_set_ring() updates the RX ring state under sk_receive_queue.lock,
but used to publish the tpacket receive mode through po->prot_hook.func
after releasing that lock. packet_poll() and packet_recvmsg() can then
run the pressure clearing path after the ring has been cleared while
still seeing tpacket_rcv, causing __packet_rcv_has_room() to dereference
stale or NULL ring storage.

Move the existing receive hook assignment into the same
sk_receive_queue.lock section as the ring state update. Keep the
assignment otherwise unchanged, including on TX ring reconfiguration, to
avoid adding behavior changes that are not required for the fix.

Serialize packet_recvmsg() pressure clearing with the same queue lock
only after PACKET_SOCK_PRESSURE has been observed. If the flag is clear
and the socket has moved away from tpacket_rcv, packet_set_ring() has
already detached the socket and waited for synchronize_net(), so no new
packet input can set the flag again.

packet_poll() already holds sk_receive_queue.lock, so it uses the new
unlocked helper directly.

Fixes: 2ccdbaa ("packet: rollover lock contention avoidance")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
Link: https://patch.msgid.link/f90b5688311fa278d1361ea8c6be0bf25967d591.1785247446.git.zihanx@nebusec.ai
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
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=cf8189b82bb93f219ab740e0346c919ad65ada62

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

The original Linux kernel CVE announcement for CVE-2026-74666 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74666

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: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: 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).