[CVE-2026-64132][IMPORTANT] ipv6: ioam: refresh hdr pointer before ioam6_event()

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

CVE: CVE-2026-64132
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv6: ioam: refresh hdr pointer before ioam6_event()
Commit: 769723124b7c3b2bfea4cf68ad292698b87c8d01
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=769723124b7c3b2bfea4cf68ad292698b87c8d01
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64132
Analysis date: Sun, 19 Jul 2026 14:04:28 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A network reachable use-after-free read in IPv6 IOAM processing can occur when skb_ensure_writable reallocates the skb buffer but ipv6_hop_ioam continues using the stale hdr pointer for ioam6_event length handling, causing DoS and possible limited information exposure.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64132 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64132

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
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CVE-2026-64132	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-825;*CWE-20;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free read can occur in the IPv6 IOAM Hop-by-Hop processing path because ipv6_hop_ioam keeps using the old hdr pointer after skb_ensure_writable may reallocate the skb data buffer. The code refreshes the trace pointer but then passes hdr opt_len from the stale header pointer to ioam6_event, so a crafted IOAM packet can make the event path consume a length value read from freed skb memory. For the CVSS the PR:N is used because an attacker does not need local privileges on the target if packets can reach the IOAM enabled IPv6 processing path. AC:H is used because reliable triggering depends on IOAM being enabled and on skb_ensure_writable actually reallocating the skb buffer, for example with a cloned or non writable skb. The issue is network reachable in IOAM deployments, but IOAM is normally used inside controlled operator or data center domains rather than as a public Internet exposed feature. Impact is at least denial of service via kernel crash and may include limited confidentiality or integrity impact in the paranoid case due to the UAF length read influencing event data handling.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-64132 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: f655c78d6225f585ef60a9d93ffb79d507ff3ad3 YES  NO NO unknown 	MAYBE	REMOTE NETWORK SKB IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  The vulnerable path is IPv6 Hop-by-Hop IOAM packet processing. If IOAM is enabled and reachable, a crafted IPv6 packet can reach the bug.

* Generic memory corruption / UAF primitive: +2
  The commit explicitly describes a stale pointer into freed skb linear data after `skb_ensure_writable()` may reallocate and free the old buffer.

* Real lifetime corruption: +1
  This is a real stale-pointer-after-free condition involving skb buffer lifetime.

* Reliable kernel crash / strong DoS concern: +1
  The stale `hdr->opt_len` value is used after reallocation and passed into `ioam6_event()`, making crash or invalid memory access plausible.

* Confidentiality impact plausible: +1
  The stale length may influence how much trace/event data is consumed, so limited unintended memory exposure is plausible, though not proven.

* Integrity impact plausible in paranoid case: +1
  Paranoid only. The stale length can affect downstream event handling, but there is no clear write primitive or object replacement path.

Negative / limiting signals:

* Hard or configuration-dependent trigger: -1
  The issue requires IOAM processing and a case where `skb_ensure_writable()` actually reallocates the skb buffer.

* Rare / difficult-to-reach configuration: -1
  IOAM is normally deployed in controlled operator, data-center, or lab networks, not broadly exposed on the public Internet.

Net result:

* Conservative: 2 + 2 + 1 + 1 + 1 - 1 - 1 = 5 to 6 depending on DoS confidence. I choose 6 due to explicit UAF and network packet path.
* Paranoid: conservative + limited integrity concern = 7.

## 3. Reachability analysis

An attacker can trigger the bug by sending IPv6 packets containing IOAM Hop-by-Hop options to a system that processes IOAM. No local account is required if the packet path is reachable, so the practical privilege level is network-only in affected deployments.

Namespaces and containers do not materially reduce the network-facing risk if the host network stack processes the packet before container isolation matters. However, IOAM is not a default Internet-facing feature in most deployments. It is more likely exposed inside controlled infrastructure networks.

Realistic exploitation depends on two conditions: IOAM must be enabled and `skb_ensure_writable()` must reallocate the skb buffer, for example because the skb is cloned or not writable. This makes exploitation less trivial than a generic packet parser NULL dereference, but the bug is still actionable because it is a network-reachable UAF in IPv6 extension-header handling.

Call-site confidence: high. The provided patch shows the stale pointer use directly in `ipv6_hop_ioam()` and the affected call to `ioam6_event()`.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or validation bug. It is a network-reachable stale-pointer UAF read in packet processing.

The realistic impact is most likely denial of service through invalid memory access or kernel crash. Limited confidentiality impact is plausible because a stale length value is passed to an event path that consumes trace data. Integrity impact is weaker and only included in the paranoid score because the patch does not show an OOB write, arbitrary write, callback hijack, controlled reclaim, or object replacement primitive.

Overall, this should be treated as Actionable Moderate at minimum and a Strong Important candidate for manual triage because network-reachable UAFs in skb processing have historically been underestimated.

## 5. One-sentence report phrase

A network reachable use-after-free read in IPv6 IOAM processing can occur when skb_ensure_writable reallocates the skb buffer but ipv6_hop_ioam continues using the stale hdr pointer for ioam6_event length handling, causing DoS and possible limited information exposure.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a network-reachable UAF in IPv6 packet processing with plausible DoS and limited confidentiality impact. The deployment scope is narrower because IOAM is not commonly Internet-exposed, but the bug class and packet-path reachability make auto-close inappropriate.

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

Patch: ipv6: ioam: refresh hdr pointer before ioam6_event()
Commit: 769723124b7c3b2bfea4cf68ad292698b87c8d01
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=769723124b7c3b2bfea4cf68ad292698b87c8d01

Commit description:

Reported by Sashiko:

In ipv6_hop_ioam(), the hdr pointer is initialized to point into the
skb's linear data buffer. Later, the code calls skb_ensure_writable(),
which might reallocate the buffer:

	if (skb_ensure_writable(skb, optoff + 2 + hdr->opt_len))
		goto drop;

	/* Trace pointer may have changed */
	trace = (struct ioam6_trace_hdr *)(skb_network_header(skb)
					   + optoff + sizeof(*hdr));

	ioam6_fill_trace_data(skb, ns, trace, true);

	ioam6_event(IOAM6_EVENT_TRACE, dev_net(skb->dev),
		    GFP_ATOMIC, (void *)trace, hdr->opt_len - 2);

If the skb is cloned or lacks sufficient linear headroom,
skb_ensure_writable() will invoke pskb_expand_head(), which reallocates
the skb's data buffer and frees the old one, invalidating pointers to
it. While the code recalculates the trace pointer immediately after the
call to skb_ensure_writable(), it fails to recalculate the hdr pointer.

This patch fixes the above by recalculating the hdr pointer before
passing hdr->opt_len to ioam6_event(), so that we avoid any UaF.

Fixes: f655c78 ("net: exthdrs: ioam6: send trace event")
Cc: stable@vger.kernel.org
Signed-off-by: Justin Iurman <justin.iurman@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260520124242.32320-1-justin.iurman@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/exthdrs.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=769723124b7c3b2bfea4cf68ad292698b87c8d01

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

The original Linux kernel CVE announcement for CVE-2026-64132 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64132

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:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/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: 6
  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).