[CVE-2026-63904][MODERATE REGULAR] usb: usbtmc: check URB actual_length for interrupt-IN notifications

[CVE-2026-63904][MODERATE REGULAR] usb: usbtmc: check URB actual_length for interrupt-IN notifications

[AL-KERNEL]

CVE: CVE-2026-63904
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: usb: usbtmc: check URB actual_length for interrupt-IN notifications
Commit: e794bd67b3faf98af46f958897f6b91412c7d2a9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e794bd67b3faf98af46f958897f6b91412c7d2a9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63904
Analysis date: Sun, 19 Jul 2026 13:49:32 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended / Ordinary Moderate otherwise
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-63904 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63904

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
======================================================================

CVE-2026-63904	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:L/A:H';CWE-125;*CWE-20;CWE-908;Other CVSS 'AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '5.7';DESCR 'A USBTMC interrupt-IN notification can be processed even when the URB actual_length is shorter than the two byte notification header expected by the driver. A malicious or malformed USBTMC device can return a short interrupt packet and make the kernel read beyond the received data or consume stale bytes left in the interrupt buffer. For the CVSS the PR:N because the attacker does not need a local user account on the host when the attack is performed through a controlled USB device or physical USB access. The issue is not network reachable and requires access to the USB device path or the ability to attach or emulate a USBTMC device. Impact is mainly local denial of service or limited stale data exposure. In the paranoid interpretation, the kernel out-of-bounds read should be manually reviewed because the exact impact depends on buffer sizing, endpoint descriptors, and whether stale or adjacent kernel data can influence user visible status handling.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Ordinary Moderate otherwise (with actual score 2)	YES	OOB DANGER USB INIT SIMPLEFIX HARDWARE LINUS SYZBOT  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	YES	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

1. ActionableScore
* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended / Ordinary Moderate otherwise**::

2. Signal breakdown
* Firmware or device mediated external influence: +1. A malicious or malformed USBTMC device controls the interrupt-IN transfer length and can return a short URB.
* Limited read-only OOB or stale-buffer read: +0. The patch shows missing `actual_length` validation before reading a two-byte header, but no write primitive, UAF, type confusion, callback control, or controlled overwrite.
* Confidentiality impact plausible: +1. Stale leftover bytes from a previous notification may be consumed, giving a limited stale-data exposure concern.
* Availability impact realistic: +1 in paranoid scoring. Repeated malformed notifications may cause warnings, sanitizer reports, or driver malfunction, but a reliable production kernel crash is not strongly demonstrated.
* Integrity impact plausible: +1 in paranoid scoring. Stale header bytes may influence status notification handling, but this is limited and not a general kernel integrity primitive.
* Physical-only or rare hardware condition: -2. The realistic attacker needs physical USB access, USB device emulation, or control over the attached USBTMC device.

3. Reachability analysis
The bug is triggerable by a USBTMC device using the optional interrupt endpoint and returning less than two bytes for an interrupt-IN notification. It is not network reachable. It does not require a local user account on the host if the attacker controls a physical or emulated USB device, but it does require access to the USB device path or physical attachment. Namespaces and containers do not materially lower the requirement unless they also allow USB device passthrough or device emulation. The affected driver is not a broad default network-facing path and depends on USBTMC hardware or fuzzed USB device exposure. Call-site confidence: high, because the relevant interrupt handler and fixed access are shown directly.

4. Severity interpretation
This behaves more like an ordinary Moderate or Low-like physical-device issue than an Important-class kernel vulnerability. The theoretical concern is a kernel out-of-bounds read or stale-buffer consumption from attacker-controlled USB transfer state. Realistic exploitation evidence supports limited stale data exposure or local DoS, not privilege escalation or arbitrary memory corruption. The paranoid score is kept at 3 only to avoid auto-closing a kernel OOB-read class bug without a human check.

5. One-sentence report phrase
A malformed USBTMC device can send a short interrupt-IN notification and cause the driver to read missing header bytes or consume stale notification data, with practical impact limited to physical-device-triggered DoS or limited stale-data exposure.

6. Manual review recommendation
MANUAL CHECK RECOMMENDED. The issue is physical-device scoped and lacks a write or lifetime corruption primitive, but it is still a kernel OOB/stale-read path from attacker-controlled USB input, so it should not be fully auto-closed without confirming buffer sizing and user-visible disclosure behavior.

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

Patch: usb: usbtmc: check URB actual_length for interrupt-IN notifications
Commit: e794bd67b3faf98af46f958897f6b91412c7d2a9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e794bd67b3faf98af46f958897f6b91412c7d2a9

Commit description:

USBTMC devices can use an optional interrupt endpoint for notification
messages. These typically contain two-byte headers indicating the
payload format, but the driver does not check if these headers are
present before accessing the data buffers. In cases where the URB
actual_length is not enough to fit these headers, the driver will either
cause an out-of-bounds read, or consume stale leftover data from a
previous notification.

Fix by checking if actual_data contains enough bytes for the headers,
otherwise resubmit URB to the interrupt endpoint.

Fixes: dbf3e7f ("Implement an ioctl to support the USMTMC-USB488 READ_STATUS_BYTE operation.")
Reported-by: syzbot+abbfd103085885cf16a2@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=abbfd103085885cf16a2
Cc: stable <stable@kernel.org>
Suggested-by: Michal Pecio <michal.pecio@gmail.com>
Signed-off-by: Heitor Alves de Siqueira <halves@igalia.com>
Link: https://patch.msgid.link/20260505-usbtmc-iin-size-v3-1-a36113f62db7@igalia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/class/usbtmc.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=e794bd67b3faf98af46f958897f6b91412c7d2a9

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

The original Linux kernel CVE announcement for CVE-2026-63904 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63904

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:L/I:N/A:L
  The Best / paranoid CVSS vector: AV:P/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.7

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: 1
  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).