[CVE-2026-64046][MODERATE REGULAR] net: tls: prevent chain-after-chain in plain text SG [ Upstream

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

CVE: CVE-2026-64046
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: tls: prevent chain-after-chain in plain text SG [ Upstream
Commit: 49a5faaa471ddcd37b6893970c9916eb836e7c31
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=49a5faaa471ddcd37b6893970c9916eb836e7c31
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64046
Analysis date: Sun, 19 Jul 2026 11:51:58 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended. Not Strong Important based on current patch evidence
Manual review required: YES

Summary:
A local kTLS sender can create an invalid chain-after-chain scatterlist layout in the TLS 1.3 transmit path, causing illegal crypto input and likely denial of service, but no concrete memory corruption or privilege escalation primitive is shown.

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

The original Linux kernel CVE announcement for CVE-2026-64046 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64046

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-2026-64046	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:N/I:N/A:H';*CWE-20;CWE-754;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'A malformed scatterlist can be built in the Linux TLS software transmit path when the plaintext SG ring wraps and TLS 1.3 appends the content type entry after an existing wrap chain link. This creates a chain-after-chain layout which the scatterlist iterator does not recursively resolve and which becomes illegal input to the crypto layer. For the CVSS the PR:L is used because a local user or local process capable of using the kTLS send path is normally needed to shape the outgoing plaintext SG state. The issue is not directly network reachable as a simple packet input bug, although a remote peer might only influence it indirectly through an application that sends attacker controlled TLS responses. Impact is denial of service via warning, BUG, or kernel crash. No concrete UAF, OOB write, arbitrary write, or information leak primitive is visible from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended. Not Strong Important based on current patch evidence (with actual score 4)	YES	REMOTE  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	NO	NO	guess

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review recommended. Not Strong Important based on current patch evidence**

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  A local process using the kTLS send path may be able to shape outgoing TLS plaintext SG state. If the relevant path requires sockmap or privileged BPF setup in the evaluated product, this signal should be reduced.

* Memory layout invariant restoration: +1
  The patch restores consistency between SG ring wrap handling, `start`, `end`, chain entries, and TLS 1.3 content type chaining.

* Reliable kernel crash / strong DoS: +1
  The commit says the old code can create illegal scatterlist input for crypto. The realistic impact is a warning, BUG, crypto failure, or kernel crash.

* Local high-control kernel data-plane API refinement: +1 paranoid only
  kTLS send paths allow relatively fine control over write sizes and SG layout. This is defensible for paranoid scoring, but not strong enough to imply memory corruption or LPE.

Subtracted signals:

* Special trigger conditions: -1 conservative
  Triggering requires TLS 1.3, plaintext SG ring wraparound, and the specific `end = 0` / `start != 0` chain-after-chain layout.

Not triggered:

* Remote reachable: +0
  This is not a direct packet receive parsing bug.

* Generic memory corruption: +0
  The patch shows malformed SG construction, not UAF, OOB write, double free, arbitrary write, or type confusion.

* Privilege escalation plausible: +0
  No reclaim primitive, callback control, writable stale object, or attacker-controlled overwrite is shown.

* Confidentiality / Integrity impact: +0
  No info leak or data corruption primitive is visible.

## 3. Reachability analysis

The likely trigger is a local process exercising the Linux TLS software transmit path and shaping writes so that the plaintext scatterlist ring wraps in the problematic way. A remote peer is not expected to trigger this directly through incoming network packets, though it might influence application behavior indirectly if the local application sends attacker-influenced TLS responses.

Namespace impact is configuration-dependent. If ordinary local users can create sockets and use kTLS in the target product, PR:L-style reachability is reasonable. If the practical path depends on BPF sockmap setup, then CAP_BPF, CAP_NET_ADMIN, or root-controlled configuration may be required, which lowers practical exposure.

The path is not a universal networking core receive path. It is a kTLS / TLS 1.3 / SG construction path with specific runtime conditions.

Call-site confidence: medium. The patch includes the affected construction site in `tls_push_record()`, but not all callers or product configuration details.

## 4. Severity interpretation

This behaves like a Moderate DoS issue with manual-review value, not like a clear Important-class vulnerability. Theoretical concern exists because malformed scatterlists are passed into the crypto layer, but the patch does not show a concrete memory corruption primitive. Realistic exploitation evidence supports denial of service only.

The conservative score is borderline. The paranoid score reaches 4 because kTLS SG layout manipulation is non-trivial kernel data-plane behavior, but there is still no basis to score it as LPE, confidentiality impact, or integrity impact.

## 5. One-sentence report phrase

A local kTLS sender can create an invalid chain-after-chain scatterlist layout in the TLS 1.3 transmit path, causing illegal crypto input and likely denial of service, but no concrete memory corruption or privilege escalation primitive is shown.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is recommended because this involves kTLS and crypto scatterlist handling, where driver-specific SG traversal behavior could affect crash reliability. Current evidence supports DoS only, so it should not be treated as Important without additional proof of memory corruption or LPE.

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

Patch: net: tls: prevent chain-after-chain in plain text SG [ Upstream
Commit: 49a5faaa471ddcd37b6893970c9916eb836e7c31
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=49a5faaa471ddcd37b6893970c9916eb836e7c31

Changed files:
  net/tls/tls_sw.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=49a5faaa471ddcd37b6893970c9916eb836e7c31

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

The original Linux kernel CVE announcement for CVE-2026-64046 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64046

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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).