[CVE-2026-74681][MODERATE 7.0] usb: misc: usbio: check ibuf_len against rxbuf_len in bulk msg

[CVE-2026-74681][MODERATE 7.0] usb: misc: usbio: check ibuf_len against rxbuf_len in bulk msg

[AL-KERNEL]

CVE: CVE-2026-74681
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: usb: misc: usbio: check ibuf_len against rxbuf_len in bulk msg
Commit: ebfd1e82ab0a6d26efd9bdd89de899215851f5bf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebfd1e82ab0a6d26efd9bdd89de899215851f5bf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74681
Analysis date: Sat, 22 Aug 2026 17:10:07 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
usbio_bulk_msg() validated the bulk IN response length against the OUT endpoint buffer size instead of rxbuf_len, allowing a malformed USBIO device or reachable local USBIO operation to exceed the receive buffer and potentially cause kernel memory corruption or DoS.

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

The original Linux kernel CVE announcement for CVE-2026-74681 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74681

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-74681	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-131;*CWE-20;Other CVSS 'AV:P/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'usbio_bulk_msg() checks the requested IN buffer length against txbuf_len instead of rxbuf_len, even though the receive buffer size is taken independently from the bulk IN endpoint. If rxbuf_len is smaller than txbuf_len, a request that is valid for the OUT endpoint can exceed the actual receive buffer and may lead to an out of bounds write or memory corruption in the USBIO receive path. For the CVSS the PR:L is used for the paranoid score because a local process may be able to trigger affected USBIO operations once the device is present, while the base case is a malicious or malformed physical USB device. The issue is not network reachable. Impact is at least denial of service via kernel crash and in the worst case may allow confidentiality or integrity impact due to kernel memory corruption, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-74681 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	USB SIMPLEFIX IMPROVEONLY  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Paranoid scoring signals:

* Firmware-mediated external influence: +1. Endpoint sizes come from the USB device, and a malformed or malicious USBIO device can make txbuf_len and rxbuf_len differ.
* Local unprivileged trigger: +1. If a local process can reach a USBIO client operation after the device is present, it may trigger the affected bulk IN path.
* Constrained kernel buffer overwrite: +1. The wrong length check can allow an IN transfer length larger than the receive buffer. The target is still a driver-local transfer buffer, so this is not scored as a fully strong arbitrary overwrite.
* Weak LPE concern: +1. Kernel memory corruption is plausible, but no reclaim, object replacement, callback control, or arbitrary write primitive is shown.
* Reliable kernel crash / strong DoS: +1. An oversized receive transfer can plausibly corrupt adjacent kernel memory or crash the driver/kernel.
* Privileged kernel/device-management memory corruption path: +1. The corruption occurs in a trusted USB kernel driver path fed by device endpoint metadata and device response data.
* Integrity impact plausible: +1. Kernel heap corruption can plausibly corrupt driver or kernel state.
* Hard or special preconditions: -1. Triggering depends on endpoint size mismatch and a caller requesting an ibuf_len accepted by the old txbuf_len check but larger than rxbuf_len.

Conservative score omits the local-trigger and weak-LPE points, giving 4.

Call-site confidence: medium. The changed function is visible, but full callers and userspace exposure are not included in the supplied patch.

## 3. Reachability analysis

This is not network reachable. The direct external influence is a USB device or firmware/descriptor-controlled endpoint layout where the bulk OUT size is larger than the bulk IN receive buffer size. A local process may also be relevant if it can cause a USBIO client driver to issue bulk messages with a large ibuf_len, but that depends on device-node exposure and the specific USBIO client stack.

Namespaces and containers do not automatically make this reachable. If the USB device or relevant character/device interface is delegated into a container, the practical PR may drop to a local low-privilege context. In a typical host setup, physical/device access and driver availability are significant limiting factors.

The path is not a broad default network surface. It requires the Intel USBIO bridge driver and affected hardware or a malicious compatible USB device.

## 4. Severity interpretation

This is stronger than an ordinary Moderate correctness bug because the patch fixes a wrong receive-buffer bounds check and the failure mode is plausible kernel memory corruption. It is not a clear Important-class issue by default because the overwrite appears constrained to a driver-local transfer buffer, and the patch does not show attacker-controlled target selection, UAF reclaim, object replacement, callback control, or arbitrary write.

Realistic impact is likely DoS or driver/kernel crash. Paranoid impact includes limited LPE concern due to kernel heap corruption, so it should not be auto-closed without manual review.

## 5. One-sentence report phrase

usbio_bulk_msg() validated the bulk IN response length against the OUT endpoint buffer size instead of rxbuf_len, allowing a malformed USBIO device or reachable local USBIO operation to exceed the receive buffer and potentially cause kernel memory corruption or DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: this is a receive-buffer bounds bug with plausible OOB write behavior in kernel USB driver code, even though the primitive is constrained and not proven to provide reliable privilege escalation.

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

Patch: usb: misc: usbio: check ibuf_len against rxbuf_len in bulk msg
Commit: ebfd1e82ab0a6d26efd9bdd89de899215851f5bf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ebfd1e82ab0a6d26efd9bdd89de899215851f5bf

Commit description:

ibuf_len is the bulk IN (receive) buffer size, but the EMSGSIZE check
in usbio_bulk_msg() compares it against txbuf_len — the bulk OUT
endpoint size.  Both are taken independently from different endpoints
in usbio_probe(), so the check is wrong when they differ.

Use rxbuf_len for the IN direction.  This matches the buffer that
actually holds the response data.

Fixes: 121a0f8 ("usb: misc: Add Intel USBIO bridge driver")
Cc: stable <stable@kernel.org>
Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
Tested-by: Antti Laakso <antti.laakso@linux.intel.com>
Link: https://patch.msgid.link/20260722101810.458634-1-yijiangshan@kylinos.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/usb/misc/usbio.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=ebfd1e82ab0a6d26efd9bdd89de899215851f5bf

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

The original Linux kernel CVE announcement for CVE-2026-74681 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74681

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:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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: 4
  Paranoid ActionableScore: 6

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).