[CVE-2026-64121][MODERATE REGULAR] net: ifb: report ethtool stats over num_tx_queues

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

CVE: CVE-2026-64121
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: ifb: report ethtool stats over num_tx_queues
Commit: 6afdb8113cb007f9332f59a9b7fd45731b8a9de5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6afdb8113cb007f9332f59a9b7fd45731b8a9de5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64121
Analysis date: Sun, 19 Jul 2026 16:37:41 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / Actionable Moderate candidate
Manual review required: YES

Summary:
IFB ethtool statistics can walk past the `tx_private` allocation when RX and TX queue counts differ, allowing a local actor to read adjacent kernel slab data through `ETHTOOL_GSTATS`, with practical impact primarily as a local information disclosure.

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

The original Linux kernel CVE announcement for CVE-2026-64121 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64121

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 REGULAR
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-2026-64121	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:L';CWE-125;*CWE-200;CWE-129;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '6.1';DESCR 'IFB ethtool stats can read past the dp tx_private allocation because tx_private is allocated for dev num_tx_queues entries but the stats callbacks walked dev real_num_rx_queues and dev real_num_tx_queues. An asymmetric IFB device such as numtxqueues 1 and numrxqueues 8 lets ETHTOOL_GSTATS index beyond the allocated per queue private array and copy adjacent slab data as statistics. For the CVSS the PR:L is used for the paranoid score because a local user may be able to trigger the read on an existing IFB device, or an untrusted container or service with delegated CAP_NET_ADMIN may be able to create the asymmetric IFB device. The issue is not network reachable and is triggered through a local ethtool ioctl path. Impact is primarily confidentiality loss through a kernel slab out-of-bounds read, with possible local DoS only in debugging or faulting configurations.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Actionable Moderate candidate (with actual score 4)	YES	READ KASAN OOB NETWORK SKB HARDWARE LINUS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: **3**
* Paranoid score: **4**
* Final recommended bucket: **Borderline manual review recommended / Actionable Moderate candidate**::

## 2. Signal breakdown

Paranoid score signals:

* **Local trigger: +1**
  The vulnerable path is reached through local `ethtool -S` / `ETHTOOL_GSTATS` on an IFB device. Creating the asymmetric IFB device usually requires `CAP_NET_ADMIN`, but reading stats may be possible by a local user if such a device already exists.

* **Weak memory-safety / unchecked index candidate: +1**
  `dp->tx_private` is allocated using `dev->num_tx_queues`, while the old stats path walked `real_num_rx_queues` and `real_num_tx_queues`. This is an array over-index into adjacent slab memory, but read-only.

* **Confidentiality impact plausible: +1**
  The commit explicitly says adjacent slab data is copied through `ETHTOOL_GSTATS`, so this is a kernel information disclosure candidate.

* **Availability impact in paranoid interpretation: +1**
  KASAN reports a slab-out-of-bounds read. In production this is more likely an info leak than a reliable crash, so this point is paranoid only.

Not applied:

* **Remote reachable: +0**
  This is not packet-triggered and is not remotely reachable by normal network traffic.

* **Privilege escalation plausible: +0**
  No write primitive, UAF reclaim, callback control, refcount takeover, or object replacement is shown.

* **Strong memory corruption: +0**
  The primitive is an OOB read, not an OOB write or object lifetime corruption.

* **Requires admin/CAP_NET_ADMIN: not fully applied in paranoid score**
  Creation of the malformed IFB setup requires `CAP_NET_ADMIN`, but delegated netns/container administration or a pre-existing asymmetric IFB device can lower practical privilege requirements.

## 3. Reachability analysis

The issue is locally reachable through the netdevice ethtool stats ioctl path. A straightforward reproducer creates an IFB device with more RX queues than TX queues and then queries stats. Creating that device normally requires `CAP_NET_ADMIN`, but in containerized or delegated network-administration environments this may be available to a less trusted actor. If an asymmetric IFB device already exists, the stats read path may be reachable with lower privileges depending on system policy.

The path is not remotely reachable through normal packet processing. IFB is not as broadly exposed as core TCP/IP or netfilter packet parsing, but it is a standard kernel networking driver and the affected operation is a normal userspace control-plane query.

## 4. Severity interpretation

This behaves more like an **actionable Moderate / borderline manual-review issue**, not a Strong Important candidate. The key security concern is not crash-only DoS, but a user-visible kernel slab out-of-bounds read that can disclose adjacent kernel memory through ethtool statistics.

The theoretical risk is kernel information disclosure useful for bypassing mitigations or leaking sensitive in-kernel data. Realistic exploitation depends on whether an attacker can create or access an asymmetric IFB device and whether useful adjacent slab contents can be exposed. There is no demonstrated integrity impact, LPE primitive, write primitive, UAF reclaim, or remote trigger.

## 5. One-sentence report phrase

IFB ethtool statistics can walk past the `tx_private` allocation when RX and TX queue counts differ, allowing a local actor to read adjacent kernel slab data through `ETHTOOL_GSTATS`, with practical impact primarily as a local information disclosure.

## 6. Manual review recommendation

**MANUAL CHECK RECOMMENDED**

Manual review is recommended because the patch fixes a real kernel slab out-of-bounds read with user-visible data disclosure. It should not be auto-closed as a pure DoS or correctness bug, but the lack of a write primitive or demonstrated privilege escalation keeps it below a strong Important classification.

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

Patch: net: ifb: report ethtool stats over num_tx_queues
Commit: 6afdb8113cb007f9332f59a9b7fd45731b8a9de5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6afdb8113cb007f9332f59a9b7fd45731b8a9de5

Commit description:

ifb_dev_init() allocates dp->tx_private to dev->num_tx_queues
entries via kzalloc_objs(*txp, dev->num_tx_queues). Both IFB
per-queue RX and TX stats live in those entries: ifb_xmit() updates
txp->rx_stats using the skb queue mapping, ifb_ri_tasklet() updates
txp->tx_stats, and ifb_stats64() aggregates both over
dev->num_tx_queues.

The ethtool stats callbacks instead size and walk the per-queue
stats with dev->real_num_rx_queues and dev->real_num_tx_queues. With
an asymmetric device where the RX queue count exceeds the TX queue
count, for example:

    ip link add name ifb10 numtxqueues 1 numrxqueues 8 type ifb
    ethtool -S ifb10

ifb_get_ethtool_stats() indexes past the tx_private allocation and
copies adjacent slab data through ETHTOOL_GSTATS.

Use dev->num_tx_queues consistently for the stats strings, the
stats count, and the stats data walks. This reports one RX stats
group and one TX stats group for each backing ifb_q_private entry,
which is the queue set IFB can actually populate.

Reproduced under UML+KASAN at v7.1-rc2:

  BUG: KASAN: slab-out-of-bounds in ifb_fill_stats_data+0x3c/0xae
  Read of size 8 at addr 0000000062dbd228 by task ethtool/36
  ifb_fill_stats_data+0x3c/0xae
  ifb_get_ethtool_stats+0xc0/0x129
  __dev_ethtool+0x1ca5/0x363c
  dev_ethtool+0x123/0x1b3
  dev_ioctl+0x56c/0x744
  sock_do_ioctl+0x15f/0x1b2
  sock_ioctl+0x4d5/0x50a
  sys_ioctl+0xd8b/0xde9

With the patch applied, the same UML+KASAN repro is silent and
ethtool -S ifb10 reports only the stats backed by the single
allocated tx_private entry.

Fixes: a21ee5b ("net: ifb: support ethtools stats")
Cc: stable@vger.kernel.org
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://patch.msgid.link/20260514013739.3549624-1-michael.bommarito@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/ifb.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=6afdb8113cb007f9332f59a9b7fd45731b8a9de5

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

The original Linux kernel CVE announcement for CVE-2026-64121 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64121

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:L/PR:L/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:L
  The Best / paranoid CVSS score: 6.1

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).