[CVE-2026-80841][IMPORTANT] net/packet: defer vmalloc TX_RING free until skbs finish

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

CVE: CVE-2026-80841
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: net/packet: defer vmalloc TX_RING free until skbs finish
Commit: 0189dce07db2dc059ae0570e06758ec4233c6e11
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0189dce07db2dc059ae0570e06758ec4233c6e11
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80841
Analysis date: Fri, 04 Sep 2026 13:15:24 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
AF_PACKET TX_RING can free vmalloc backed ring storage while outstanding TX skbs still hold raw frame pointers, allowing a local CAP_NET_RAW capable process to trigger a stale kernel write during skb destruction with at least DoS impact and plausible memory corruption risk.

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

The original Linux kernel CVE announcement for CVE-2026-80841 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80841

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-80841	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-80841 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-416;CWE-362;*CWE-787;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 TX_RING skbs keep raw pointers to mmap ring frames. For vmalloc backed ring blocks, page references do not preserve the vmalloc mapping, so socket close or ring replacement can free the mapping while outstanding TX skbs still later write timestamp and TP_STATUS_AVAILABLE to the frame. This is a local race and lifetime bug with a use-after-free style write in skb destruction context. For the CVSS the PR:L value is used because triggering requires local code able to create and use AF_PACKET TX_RING, typically CAP_NET_RAW in a network namespace or an equivalent delegated capability. The issue is not directly network reachable because packet traffic alone does not create or replace the vulnerable ring. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact due to kernel memory corruption, so manual review is needed.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE READ INIT SKB PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

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

Conservative scoring:

* +1 Local low privilege or namespace mediated trigger. AF_PACKET TX_RING is reachable from local code with CAP_NET_RAW, and in some deployments this may be obtainable in user or network namespaces.
* +2 Generic memory corruption. The patch describes outstanding TX skbs keeping raw pointers to mmap ring frames after vmalloc backed ring memory can be freed.
* +1 Real lifetime corruption. This is a lifetime mismatch between skb completion/destructor access and TX_RING/pg_vec teardown.
* +1 Reliable kernel crash / strong DoS. A late skb destructor can write timestamp and TP_STATUS_AVAILABLE through a stale ring frame pointer.
* +1 Weak LPE concern. The bug is a writable stale pointer into freed vmalloc backed memory, but no attacker controlled reclaim or reliable object replacement is demonstrated.
* +1 Broad/default/common subsystem exposure. AF_PACKET is a common Linux networking facility, even if TX_RING usage is specialized.
* -1 Hard or unreliable race / special timing required. The issue depends on outstanding TX skbs racing with close or ring replacement.
* -1 Corruption primitive constrained. The late write contents are mainly timestamp/status data, not an arbitrary attacker chosen payload.

Paranoid delta:

* +1 additional LPE plausibility, upgrading weak concern to stronger concern because the primitive is write after lifetime end in kernel context.
* +1 local high-control kernel data-plane API refinement, only for deployments where AF_PACKET TX_RING is practically reachable by less trusted users through namespaces or delegated CAP_NET_RAW.

## 3. Reachability analysis

The bug is local, not directly network reachable. Packet traffic alone does not create, replace, or close the vulnerable TX_RING. A trigger requires local code that can create an AF_PACKET socket, configure TX_RING, send frames, and race socket close or ring replacement against skb completion.

In typical hardened deployments this requires CAP_NET_RAW, so it is not plain remote unauthenticated exposure. In containerized or namespace-enabled environments, CAP_NET_RAW may be delegated to less trusted workloads, which makes PR:L and manual review more appropriate. The path is not a rare hardware-only path and does not require a special NIC firmware condition.

Call-site confidence: high. The provided patch includes the relevant destructor, release path, ring allocation, ring replacement, and deferred free logic.

## 4. Severity interpretation

This is stronger than an ordinary Moderate DoS because the patch fixes a real lifetime bug where kernel code can write through a stale raw pointer after vmalloc backed ring memory is detached or freed. The realistic demonstrated impact is local kernel crash or DoS. The theoretical higher impact is local privilege escalation through vmalloc reuse or related memory corruption, but the patch does not demonstrate a reliable reclaim or arbitrary write primitive.

Therefore the conservative result is Actionable Moderate. The paranoid result is a Strong Important candidate because AF_PACKET is a historically security sensitive local networking API and the bug class is writable UAF style lifetime corruption.

## 5. One-sentence report phrase

AF_PACKET TX_RING can free vmalloc backed ring storage while outstanding TX skbs still hold raw frame pointers, allowing a local CAP_NET_RAW capable process to trigger a stale kernel write during skb destruction with at least DoS impact and plausible memory corruption risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not a pure resource leak or validation bug. The patch context supports a real lifetime corruption issue with stale writable ring frame pointers in AF_PACKET TX_RING, making it unsuitable for auto-close even though reliable privilege escalation is not demonstrated.

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

Patch: net/packet: defer vmalloc TX_RING free until skbs finish
Commit: 0189dce07db2dc059ae0570e06758ec4233c6e11
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0189dce07db2dc059ae0570e06758ec4233c6e11

Commit description:

AF_PACKET TX_RING skbs keep a raw pointer to their ring frame. The skb
page references preserve page-backed ring blocks after pg_vec is freed,
but they do not preserve a vmalloc mapping.

tpacket_destruct_skb() currently drops the pending reference before
writing the timestamp and TP_STATUS_AVAILABLE to the frame. Move the
decrement after those stores. The smp_wmb() in __packet_set_status()
orders the frame stores before the decrement.

Also recheck pending TX frames under pg_vec_lock before non-closing
ring replacement, so a racing send cannot add a pending skb between
the initial check and the ring swap.

Ring allocation can produce a mixture of page-backed and vmalloc-backed
blocks. Allocate deferred-work storage during TX ring setup when the
first vmalloc-backed block is encountered, and keep its pointer in the
pg_vec allocation header. If allocation fails, return -ENOMEM from ring
setup. On socket close, a non-NULL pointer identifies a vmalloc-backed
vector without a scan. If TX skbs remain, defer the whole vector to
system_long_wq.

After pg_vec is detached, a late destructor can skip the pending
decrement. Use socket write-memory accounting as the deferred lifetime
gate instead: an skb remains charged through its final sock_wfree(),
after all ring-frame accesses. The delayed work retains a socket
reference and reschedules itself until no TX skbs remain.

Move pending_refcnt release to packet_sock_destruct() so late skb
destructors and deferred cleanup can safely use it after
packet_release(). Page-backed teardown remains synchronous, and no lock
is added to the TX completion hot path.

Fixes: b013840 ("packet: use percpu mmap tx frame pending refcount")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/netdev/20260721015824.45829-1-kylebot@openai.com/
Suggested-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Kyle Zeng <kylebot@openai.com>
Link: https://patch.msgid.link/20260816235646.76500-1-kylebot@openai.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
  net/inet_common.h

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=0189dce07db2dc059ae0570e06758ec4233c6e11

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

The original Linux kernel CVE announcement for CVE-2026-80841 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80841

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).