[CVE-2026-80837][IMPORTANT] netfilter: nf_tables: don't queue packet path object notifications

[CVE-2026-80837][IMPORTANT] netfilter: nf_tables: don't queue packet path object notifications

[AL-KERNEL]

CVE: CVE-2026-80837
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: nf_tables: don't queue packet path object notifications
Commit: df86c0e84025be8b6dd572a20852698927aa666b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df86c0e84025be8b6dd572a20852698927aa666b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80837
Analysis date: Fri, 04 Sep 2026 14:58:38 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
nf_tables can enqueue object notifications from the packet path without commit_mutex, allowing list_add_tail() to race with nf_tables_commit() list deletion and kfree_skb(), causing a sk_buff use-after-free write and at least a local kernel crash risk.

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

The original Linux kernel CVE announcement for CVE-2026-80837 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80837

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
======================================================================

CVE-2026-80837	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-80837 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-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'nf_tables object notifications can be reached from the packet path through nft_quota_obj_eval() while no commit_mutex is held. Since object notifications were queued on notify_list, an unlocked list_add_tail() from packet processing can race with list_del() and kfree_skb() in nf_tables_commit on another CPU. This creates a sk_buff use-after-free write in __list_add() and can crash the kernel. For the CVSS the PR:L is used for the paranoid score because reliable triggering generally needs local code execution plus access to an existing nftables quota path or delegated net administration capability, but not necessarily full host root in containerized or namespace based environments. The issue is not directly network reachable as a remote service vulnerability. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to memory corruption and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE OOB DANGER NETFILTER RACE UAF SKB HARDWARE KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**

## 2. Signal breakdown

* Local unprivileged or reduced privilege trigger: +1
  The packet path caller is reached from sendto() through nft_quota_obj_eval(). Reliable setup may require an existing quota rule or delegated net administration, but the crashing path itself is packet driven.

* Memory corruption, strong corruption primitive: +2
  The KASAN trace shows slab-use-after-free with a write in __list_add(), specifically WRITE_ONCE(prev->next, new) through a freed sk_buff.

* Real lifetime corruption: +1
  The bug is a race between unlocked enqueue on notify_list and list_del() plus kfree_skb() during nf_tables_commit().

* Reliable kernel crash / strong DoS: +1
  The report includes a concrete KASAN UAF write trace in the kernel packet path.

* Common networking core or packet-processing path: +1
  The affected object notification is reachable from nf_tables packet evaluation through nft_quota.

* Hard or unreliable race / special timing required: -1
  Triggering requires overlap between packet-path notification enqueue and transaction commit draining on another CPU.

* Weak LPE concern, paranoid only: +1
  The primitive is a write-after-free into sk_buff list metadata. No controlled reclaim or object replacement is demonstrated, so this is not strong LPE, but it is enough for manual review.

* Confidentiality impact plausible, paranoid only: +1
  Not proven, but UAF write corruption in shared kernel allocator objects can have secondary effects worth review.

* Integrity impact plausible, paranoid only: +1
  Not proven, but the write-after-free affects kernel heap/list state and should not be treated as DoS-only for paranoid triage.

## 3. Reachability analysis

The bug is not a direct remote network service vulnerability. A remote packet alone is not enough unless the vulnerable nftables quota path and concurrent commit activity are already present. In realistic local scenarios, a user or process can send packets through an existing chain that references a depleted quota object, while another CPU drains queued notifications during nf_tables_commit().

Typical rule creation and commit operations require CAP_NET_ADMIN. However, CAP_NET_ADMIN may be delegated inside network namespaces or containers, and reduced-capability root should not always be treated as full host-root. Therefore PR:L is defensible for paranoid triage, while a stricter host-only deployment could score lower if only trusted administrators can modify nftables state.

Call-site confidence: high. The provided text identifies the packet-path caller, the transaction-path drain, the unlocked list operation, and the KASAN UAF write.

## 4. Severity interpretation

This is above ordinary Moderate because the patch fixes a real packet-path race producing a slab use-after-free write, not just a warning, leak, or validation issue. Realistic exploitation evidence currently supports kernel crash / DoS most strongly. Privilege escalation is not demonstrated, but the UAF write into sk_buff/list metadata is a memory corruption primitive that should not be auto-closed without manual review.

Conservative handling: Actionable Moderate.
Paranoid handling: Strong Important candidate.

## 5. One-sentence report phrase

nf_tables can enqueue object notifications from the packet path without commit_mutex, allowing list_add_tail() to race with nf_tables_commit() list deletion and kfree_skb(), causing a sk_buff use-after-free write and at least a local kernel crash risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: the issue includes a concrete KASAN slab-use-after-free write in a common netfilter packet path, with possible but unproven impact beyond DoS.

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

Patch: netfilter: nf_tables: don't queue packet path object notifications
Commit: df86c0e84025be8b6dd572a20852698927aa666b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df86c0e84025be8b6dd572a20852698927aa666b

Commit description:

All file:line references below are against v7.2-rc4 (ac5b0e5651b1). The
trace was captured on 7.2.0-rc6-kasan72rc6 ( 075b748 ), where the same
lines apply.

nft_obj_notify() is exported and reached from the packet path. Its only
in-tree caller is nft_quota_obj_eval() (net/netfilter/nft_quota.c:68),
which notifies with GFP_ATOMIC while evaluating a rule for a transiting
packet, holding no mutex.

Since commit 67cc570 ("netfilter: nf_tables: coalesce multiple
notifications into one skbuff") that notification is no longer sent
immediately. __nft_obj_notify() queues it onto nft_net->notify_list via
nft_notify_enqueue() (net/netfilter/nf_tables_api.c:1211), which is a bare
list_add_tail(). notify_list has no lock of its own
(include/net/netfilter/nf_tables.h:1951), it is serialised by commit_mutex:
the six other enqueue sites all run inside a netlink transaction, and the
drain in nft_commit_notify() (net/netfilter/nf_tables_api.c:10746) does
list_del() + kfree_skb() from nf_tables_commit() with commit_mutex held.

Sending packets through a chain that references a depleted quota object
therefore races an unlocked list_add_tail() against list_del() +
kfree_skb() on another CPU. The WRITE_ONCE(prev->next, new) in __list_add()
then stores through an sk_buff that has already been freed:

  BUG: KASAN: slab-use-after-free in __nft_obj_notify+0x2c5/0x2d0
  Write of size 8 at addr ff110001047183c0 by task poc/76
  CPU: 0 UID: 1000 PID: 76 Comm: poc Tainted: G  W  7.2.0-rc6-kasan72rc6 #4
  Call Trace:
   <IRQ>
   __nft_obj_notify (include/linux/list.h:164 include/linux/list.h:191
                     net/netfilter/nf_tables_api.c:1211
                     net/netfilter/nf_tables_api.c:8743)
   nft_quota_obj_eval (net/netfilter/nft_quota.c:68)
   nft_do_chain_inet
   nf_hook_slow
   __ip_local_out
   ip_push_pending_frames
   udp_send_skb
   udp_sendmsg
   __x64_sys_sendto

  Allocated by task 77:
   __alloc_skb (net/core/skbuff.c:704)
   __nft_obj_notify (include/net/netlink.h:1055
                     net/netfilter/nf_tables_api.c:8731)
   nft_quota_obj_eval (net/netfilter/nft_quota.c:68)
   nft_do_chain

  Freed by task 79:
   nf_tables_commit (include/linux/skbuff.h:1332
                     net/netfilter/nf_tables_api.c:10759
                     net/netfilter/nf_tables_api.c:11185)
   nfnetlink_rcv_batch (net/netfilter/nfnetlink.c:574)
   netlink_unicast
   netlink_sendmsg

  The buggy address belongs to the cache skbuff_head_cache of size 232

Queueing from the packet path is wrong even leaving the race aside:
notify_list is only drained by nft_commit_notify() from nf_tables_commit()
(:11185), so a notification enqueued outside a transaction is not sent
until some later netlink batch commits, if one ever does.

The gfp argument that nft_obj_notify() still takes is a leftover of the
pre-67cc570edaa0 behaviour, where this path called nfnetlink_send()
directly. Restore that: split the message construction out into
nft_obj_notify_alloc() and let each caller decide what to do with the skb.
nft_obj_notify(), the exported one reached from the packet path, sends it
straight away; nf_tables_obj_notify(), which runs under commit_mutex, keeps
queueing it, so transaction notifications are still coalesced.

Fixes: 67cc570 ("netfilter: nf_tables: coalesce multiple notifications into one skbuff")
Cc: stable@kernel.org
Reported-by: TencentOS Corvus AI <corvus@tencent.com>
Assisted-by: tencentos-corvus-ai:kimi-k3
Signed-off-by: Fourie Zhang <fouriezhang@tencent.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/nf_tables_api.c:1211
  net/netfilter/nf_tables_api.c:8743)
  net/netfilter/nf_tables_api.c:8731)
  net/netfilter/nf_tables_api.c:10759
  net/netfilter/nf_tables_api.c:11185)

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=df86c0e84025be8b6dd572a20852698927aa666b

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

The original Linux kernel CVE announcement for CVE-2026-80837 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80837

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: 8

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).