[CVE-2026-64351][MODERATE REGULAR] net: usb: kalmia: bound RX frame length in kalmia_rx_fixup()

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

CVE: CVE-2026-64351
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: usb: kalmia: bound RX frame length in kalmia_rx_fixup()
Commit: 391706889a5112feafdc0c68db3ecc7ed325d09c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=391706889a5112feafdc0c68db3ecc7ed325d09c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64351
Analysis date: Sat, 25 Jul 2026 12:04:42 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline Moderate, manual review required
Manual review required: YES

Summary:
A malformed USB Kalmia RX frame can underflow an internal packet length calculation and drive an out-of-bounds read in `kalmia_rx_fixup()`, allowing a malicious device or firmware to crash the kernel and possibly expose limited adjacent memory state.

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

The original Linux kernel CVE announcement for CVE-2026-64351 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64351

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-2026-64351	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H';CWE-125;CWE-191;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.2';DESCR 'A malformed RX frame in the USB Kalmia network driver can make kalmia_rx_fixup compute usb_packet_length from a buffer that contains only one framing header or a short trailing remainder. The subtraction can underflow a u16 length and lets a device supplied ether_packet_length drive an out-of-bounds read in the receive path. For the CVSS the PR:N because the attacker does not need an account on the host when controlling a malicious or compromised USB modem. The issue is not directly reachable over IP network traffic and normally requires a malicious attached USB device or compromised modem firmware. Impact is primarily denial of service via kernel crash. For the paranoid score, limited confidentiality impact is kept because the primitive is an out-of-bounds read, but there is no clear write primitive or supported privilege escalation path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline Moderate, manual review required (with actual score 3)	YES	WRITE OOB USB SIMPLEFIX NETWORK SKB HARDWARE 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=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline Moderate, manual review required**

## 2. Signal breakdown

Triggered signals:

* +1 Firmware-mediated external influence: the malformed RX frame is supplied by a USB Kalmia device or compromised modem firmware.
* +1 Reliable kernel crash / strong DoS: the device-controlled length underflow can drive OOB access in RX processing and plausibly crash the kernel.
* +1 Availability impact realistic: impact can be host-wide if the OOB access faults in kernel context.
* +1 Privileged kernel/device-management memory access path: parsing occurs inside a USB network driver RX path using device-supplied framing fields.
* +1 Confidentiality impact plausible, paranoid only: the primitive is an out-of-bounds read, but no clear user-visible disclosure channel is shown.

Negative signals:

* -2 Physical-only or rare hardware-only condition: exploitation normally requires a malicious attached Samsung Kalmia USB modem or compromised device firmware, not a remote network packet.

Not awarded:

* No remote/network-triggerable +2, because this is not reachable through ordinary IP traffic.
* No privilege escalation +1/+2, because the patch supports OOB read and crash risk, not UAF reclaim, OOB write, arbitrary write, type confusion, or callback control.
* No strong generic memory corruption +2, because the described primitive is read-only OOB, not an overwrite.

Call-site confidence: high. The patch and commit message directly describe `kalmia_rx_fixup()` RX handling and the affected length calculations.

## 3. Reachability analysis

The likely attacker is a malicious USB device, malicious modem firmware, or someone with physical/control access to the affected USB modem. No local user account is needed on the host if the attacker controls the device, but physical or firmware-mediated access is required. Namespaces and containers do not materially reduce the privilege requirement because the trigger is in the host USB network receive path. The path is not broadly default-exposed and depends on the Kalmia USB network driver and compatible hardware being present.

## 4. Severity interpretation

This behaves more like a low-to-borderline Moderate issue than an Important-class vulnerability. The theoretical memory-safety concern is real because a device-controlled length can cause an out-of-bounds read in kernel RX processing. Realistic exploitation evidence points mainly to denial of service, with only limited paranoid confidentiality concern and no supported LPE path.

## 5. One-sentence report phrase

A malformed USB Kalmia RX frame can underflow an internal packet length calculation and drive an out-of-bounds read in `kalmia_rx_fixup()`, allowing a malicious device or firmware to crash the kernel and possibly expose limited adjacent memory state.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a device-controlled kernel out-of-bounds read in RX handling, so it should not be auto-closed despite the physical or firmware-mediated attack model.

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

Patch: net: usb: kalmia: bound RX frame length in kalmia_rx_fixup()
Commit: 391706889a5112feafdc0c68db3ecc7ed325d09c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=391706889a5112feafdc0c68db3ecc7ed325d09c

Commit description:

kalmia_rx_fixup() computes usb_packet_length = skb->len - (2 *
KALMIA_HEADER_LENGTH) as a u16, guarded only by a pre-loop check that
skb->len is at least KALMIA_HEADER_LENGTH, which is 6. A device can
deliver a short bulk-IN frame with skb->len in the 6 to 11 range, or
leave a short trailing remainder on a later loop iteration. Either case
underflows usb_packet_length to about 65530.

That bypasses the usb_packet_length < ether_packet_length truncation path.
The device-supplied ether_packet_length, a le16 up to 65535 read from
header_start[2], then drives a memcmp() and the following skb_trim() and
skb_pull() past the end of the rx buffer. The rx buffer is hard_mtu * 10,
which is 14000 bytes. That is an out of bounds read.

Require both the start and end framing headers to be present before
subtracting them, on every loop iteration.

Fixes: d402612 ("net/usb: Add Samsung Kalmia driver for Samsung GT-B3730")
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/178211531778.2216480.12637613349790980750@maoyixie.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/usb/kalmia.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=391706889a5112feafdc0c68db3ecc7ed325d09c

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

The original Linux kernel CVE announcement for CVE-2026-64351 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64351

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:P/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 5.2

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: 2
  Paranoid ActionableScore: 3

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).