[CVE-2026-64422][MODERATE 7.0] net: ipv4: bound TCP reordering sysctl writes and MTU probe sizes

[CVE-2026-64422][MODERATE 7.0] net: ipv4: bound TCP reordering sysctl writes and MTU probe sizes

[AL-KERNEL]

CVE: CVE-2026-64422
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: ipv4: bound TCP reordering sysctl writes and MTU probe sizes
Commit: f0d88a4cd03affff6c08adf6c63964e235aede43
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f0d88a4cd03affff6c08adf6c63964e235aede43
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64422
Analysis date: Sat, 25 Jul 2026 12:30:37 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A negative net.ipv4.tcp_reordering value can wrap into a large TCP reordering value and overflow tcp_mtu_probe size checks, enabling an OOB read in the TCP output path with local or namespace-mediated privileges.

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

The original Linux kernel CVE announcement for CVE-2026-64422 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64422

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.

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

CVE-2026-64422	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:H';CWE-190;CWE-125;*CWE-20;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7.1';DESCR 'A signed integer handling bug in net.ipv4.tcp_reordering can let a negative sysctl value wrap into a very large tp reordering value for new TCP sockets. With tcp_mtu_probing set to 2, this value can overflow the tcp_mtu_probe size_needed calculation and bypass send queue or window checks, driving the TCP MTU probing path into an OOB read. For the CVSS the PR:L is used because triggering can be possible for a local user or process that can write the per namespace TCP sysctl through CAP_NET_ADMIN or a user controlled network namespace. The issue is not directly network reachable because the attacker must first control local TCP sysctl state, but a controlled TCP connection may be used to exercise the vulnerable output path. Impact is at least local denial of service via kernel crash and in the paranoid case may include high confidentiality impact because the primitive is an OOB read in a packet output path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE OOB INIT NETWORK LINUS  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1 in namespace-enabled or containerized environments where an unprivileged user can obtain CAP_NET_ADMIN inside a user or network namespace and write per-netns TCP sysctls.
* Memory layout invariant restoration: +1 because the patch restores consistency between tcp_reordering, tcp_max_reordering, and the tcp_mtu_probe size calculation.
* Reliable kernel crash / strong DoS: +1 because the commit explicitly says the overflow can drive the MTU probing path into an out-of-bounds read.
* Confidentiality impact plausible: +1 because the primitive is an OOB read in a TCP packet output path, where copied data may theoretically become externally observable.
* Broad/common subsystem: +1 because this is IPv4 TCP core behavior.
* Common networking core path: +1 because tcp_mtu_probe is in TCP output logic.
* Requires CAP_NET_ADMIN in locked-down deployments: -2 for the lower score, because writing these sysctls normally requires administrative network privileges when unprivileged user namespaces are disabled.

## 3. Reachability analysis

The issue is not directly remote-triggerable because an attacker must first set invalid TCP sysctl state locally. In typical locked-down systems this requires CAP_NET_ADMIN or equivalent administrative control, so the lower score is 4. In systems with unprivileged user namespaces, delegated network namespaces, containers, or service accounts with network-admin capabilities, the effective privilege can be closer to PR:L, making the higher score defensible. The vulnerable path also requires tcp_mtu_probing=2 and creation of new TCP sockets inheriting the wrapped tp.reordering value.

Call-site confidence: high, because the patch shows both the sysctl input path and the tcp_mtu_probe arithmetic sink.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the patch describes a concrete integer overflow leading to an OOB read in a common TCP output path. It is not clearly Important by itself because there is no OOB write, UAF, refcount takeover, type confusion, or demonstrated LPE primitive. Theoretical impact includes information disclosure and crash, while realistic exploitation depends on local sysctl control and MTU probing configuration.

## 5. One-sentence report phrase

A negative net.ipv4.tcp_reordering value can wrap into a large TCP reordering value and overflow tcp_mtu_probe size checks, enabling an OOB read in the TCP output path with local or namespace-mediated privileges.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch explicitly describes an OOB read in TCP output logic, and namespace-dependent CAP_NET_ADMIN reachability makes the practical privilege level product-specific.

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

Patch: net: ipv4: bound TCP reordering sysctl writes and MTU probe sizes
Commit: f0d88a4cd03affff6c08adf6c63964e235aede43
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f0d88a4cd03affff6c08adf6c63964e235aede43

Commit description:

Reject invalid `net.ipv4.tcp_reordering` values before they reach TCP
socket state. The sysctl is stored as an `int` but copied into the
`u32` `tp->reordering` field for new sockets, so negative writes wrap
to large values.

With `tcp_mtu_probing=2`, the wrapped value can overflow the
`tcp_mtu_probe()` size calculation and drive the MTU probing path into
an out-of-bounds read. Route `tcp_reordering` writes through
`proc_dointvec_minmax()` and require it to be at least 1. Also require
`tcp_max_reordering` to be at least 1 so the configured maximum cannot
become negative either.

When registering the table for a non-init network namespace, relocate
`extra2` pointers that refer into `init_net.ipv4` so the
`tcp_reordering` upper bound follows that namespace's
`tcp_max_reordering`.

Harden `tcp_mtu_probe()` itself by computing `size_needed` as `u64`.
This keeps the send queue and window checks from being bypassed through
signed integer overflow.

Fixes: 91cc17c ("[TCP]: MTUprobe: receiver window & data available checks fixed")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Wyatt Feng <bronzed_45_vested@icloud.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/1a5b7e1ef4d70fbad8c8ee0b82d8405f3c964a3d.1781395200.git.bronzed_45_vested@icloud.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/sysctl_net_ipv4.c
  net/ipv4/tcp_output.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=f0d88a4cd03affff6c08adf6c63964e235aede43

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

The original Linux kernel CVE announcement for CVE-2026-64422 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64422

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:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 7.1

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: 4
  Paranoid ActionableScore: 6

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).