[CVE-2026-64313][MODERATE 7.0] crypto: ecc - Fix carry overflow in vli multiplication

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

CVE: CVE-2026-64313
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: crypto: ecc - Fix carry overflow in vli multiplication
Commit: d11b2bb99bec1f5557c01cac42231e23745f49b8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d11b2bb99bec1f5557c01cac42231e23745f49b8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64313
Analysis date: Sat, 25 Jul 2026 13:45:14 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An ECC carry propagation bug in crypto/ecc.c can produce incorrect ECDH arithmetic under a narrow boundary condition, potentially causing failed key agreement or weakened cryptographic correctness in local or network-facing consumers of the kernel ECC implementation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64313 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64313

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-64313	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:L';CWE-682;CWE-190;CWE-327;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '5.6';DESCR 'An arithmetic carry bug in the kernel ECC implementation can produce an incorrect multi precision multiplication result when the high word is saturated and the low word addition also carries. This can corrupt ECC point arithmetic used by software ECDH and may lead to an incorrect shared secret, failed key agreement, or weakened cryptographic correctness in consumers of crypto/ecc.c. For the CVSS the PR:N is used for the paranoid network interpretation because a remote peer may supply ECDH inputs to an exposed protocol that uses the kernel ECC primitive. The issue is high complexity because the boundary condition requires specific big integer values and there is no evidence of memory corruption, UAF, OOB access, or direct privilege escalation. Impact is mainly cryptographic integrity and availability risk, with limited confidentiality concern only in the conservative interpretation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	SIMPLEFIX IMPROVEONLY LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. Some kernel crypto consumers may be reachable from local user controlled operations, but direct generic unprivileged reachability is not fully established from the patch alone.
* Integrity impact plausible: +1. Incorrect ECC multiplication can corrupt ECDH arithmetic and produce incorrect shared secrets or invalid cryptographic results.
* Availability impact realistic: +1. Failed key agreement or protocol failure is realistic if the boundary condition is hit.
* Broad/default/common subsystem exposure: +1. crypto/ecc.c is a shared kernel crypto primitive used by ECDH software support.
* Hard or unreliable special condition required: -1. The bug requires a narrow arithmetic boundary where the high word is saturated and the low word addition carries.

Paranoid additional signals:

* Remote reachable / network-triggerable: +2. In the paranoid interpretation, a remote peer may influence ECDH public input in a network protocol that uses the kernel ECC implementation.
* Confidentiality impact plausible: +1. Limited confidentiality concern is defensible because incorrect ECDH arithmetic can weaken or invalidate key agreement semantics, although no direct key recovery is demonstrated.

Not applied:

* No generic memory corruption. The patch fixes arithmetic carry propagation, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No privilege escalation signal. The patch does not show a memory corruption primitive or security boundary bypass leading to LPE.
* No reliable kernel crash / strong DoS. The described consequence is incorrect cryptographic computation, not a demonstrated panic or oops.

Call-site confidence: medium. The changed function is a crypto helper used by ECC arithmetic, but the supplied patch does not include concrete higher-level protocol call sites.

## 3. Reachability analysis

The direct bug is in kernel ECC big integer multiplication used by software ECDH. A local trigger is plausible through kernel consumers of ECDH, but the exact user-facing path depends on enabled subsystems and product configuration. A remote trigger is plausible only when a network-facing protocol uses this kernel ECC implementation and accepts peer-controlled ECDH public inputs. This is not a generic packet-processing bug and should not be treated as universally internet-reachable. The arithmetic condition is narrow, so exploitation reliability is likely high complexity.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an Important memory-corruption vulnerability. The theoretical risk is cryptographic correctness failure affecting ECDH shared secret computation. The realistic evidence from the patch supports incorrect arithmetic, failed key agreement, and possible limited cryptographic impact, but not UAF, arbitrary memory access, kernel code execution, or privilege escalation. Manual review is still warranted because crypto primitive bugs can have protocol-dependent impact that is not visible from the helper patch alone.

## 5. One-sentence report phrase

An ECC carry propagation bug in crypto/ecc.c can produce incorrect ECDH arithmetic under a narrow boundary condition, potentially causing failed key agreement or weakened cryptographic correctness in local or network-facing consumers of the kernel ECC implementation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not proven memory corruption, but it affects a shared cryptographic primitive and may have protocol-dependent remote confidentiality or integrity implications that require call-site review.

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

Patch: crypto: ecc - Fix carry overflow in vli multiplication
Commit: d11b2bb99bec1f5557c01cac42231e23745f49b8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d11b2bb99bec1f5557c01cac42231e23745f49b8

Commit description:

The carry flag calculation fails when r01.m_high is saturated
(0xFFFFFFFFFFFFFFFF) and addition of lower bits overflows.

The condition (r01.m_high < product.m_high) doesn't handle the case
where r01.m_high == product.m_high and an additional carry exists
from lower-bit overflow.

When commit 3c4b239 ("crypto: ecdh - Add ECDH software support")
introduced crypto/ecc.c, it split the muladd() function in the
micro-ecc library into separate mul_64_64() and add_128_128() helpers.
It seems the check got lost in translation.

Add proper handling for this boundary by accounting for the carry
from the lower addition.

Fixes: 3c4b239 ("crypto: ecdh - Add ECDH software support")
Signed-off-by: Anastasia Tishchenko <sv3iry@gmail.com>
Cc: stable@vger.kernel.org # v4.8+
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  crypto/ecc.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=d11b2bb99bec1f5557c01cac42231e23745f49b8

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

The original Linux kernel CVE announcement for CVE-2026-64313 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64313

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:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 5.6

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: 5

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 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).