[CVE-2026-90033][MODERATE 7.0] ALSA: usb-audio: fix OOB write in snd_usbmidi_us122l_output()

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

CVE: CVE-2026-90033
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ALSA: usb-audio: fix OOB write in snd_usbmidi_us122l_output()
Commit: 5538daf2a5cc80ac3f3e913c0db045d92d995d40
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5538daf2a5cc80ac3f3e913c0db045d92d995d40
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90033
Analysis date: Wed, 16 Sep 2026 11:17:08 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
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-2026-90033 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90033

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

CVE-2026-90033	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90033 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-190;CWE-119;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'An out of bounds write in the ALSA USB MIDI output path can occur because snd_usbmidi_us122l_output() selects a count of 2 for slower than high speed devices without checking that ep max_transfer is at least that large. A device that exposes a one byte bulk endpoint can make the URB transfer buffer smaller than the requested MIDI transmit length, and the later padding calculation can underflow from 1 minus 2 into a very large size. This can corrupt kernel memory or crash the kernel when MIDI output is sent to the device. For the CVSS the PR:L is used for the paranoid score because a local user or process with access to the ALSA rawmidi device can plausibly trigger the transmit path when such a device is present. The issue is not network reachable and normally requires a local USB device or a malformed USB peripheral. Impact is at least denial of service via kernel crash, with a defensible worst case of confidentiality and integrity impact because the primitive is an out of bounds kernel write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	WRITE OOB USB SIMPLEFIX HARDWARE LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Applied conservative floor/cap: 4 -> 5 because a concrete kernel OOB write is established and should not be auto downgraded below actionable Moderate.
* Raw paranoid score: 6
* Paranoid score: 6
* Applied paranoid floor/cap: none.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Strong kernel corruption primitive. The commit explicitly describes an OOB write in snd_usbmidi_us122l_output(), where count can exceed ep->max_transfer and the later padding memset can underflow to a very large size.
* +1 Reliable kernel crash / strong DoS. Kernel memory corruption from an oversized transmit and huge memset can realistically crash the kernel.
* +1 Local below-host-admin trigger. When the malformed USB MIDI device is present, a local user or process with access to the ALSA rawmidi endpoint can plausibly trigger the output path.
* +1 Firmware/device-mediated external influence, paranoid only. The malformed USB endpoint descriptor and maxpacket behavior are controlled by the attached device, so device-mediated attacker input is material.
* +1 Weak exploitation concern, paranoid only. The primitive is not just an invalid read or WARN. It includes at least limited attacker-influenced data written past a fixed URB buffer plus large zero overwrite behavior, leaving memory-corruption leverage unresolved.

Negative or non-awarded signals:

* No RealUAF. This is an OOB write and integer underflow issue, not a lifetime or stale-reference bug.
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Callback or function-pointer overwrite not demonstrated.
* Remote network reachability not established.
* Extra Availability not awarded because the same crash is already counted once.
* Confidentiality and Integrity are not separately scored as demonstrated consequences, although paranoid review is warranted because the primitive is a kernel OOB write.
* Physical or local USB device presence is a real prerequisite, but it is not treated as proof of host-admin requirement.
* Concrete repeated release/refcount: not applicable. No first release, surviving exact reference, or second lifetime mutation is present.

3. Reachability analysis

The complete practical trigger requires a malformed or malicious USB MIDI device that exposes a too-small bulk endpoint, for example one byte, while the driver chooses count 2 for slower than high speed devices. The vulnerable path is reached when MIDI output is sent through ALSA rawmidi to that device.

Host administrator privilege is not positively required. The attacker model is a local user or local process with access to the ALSA rawmidi device, combined with a malicious or malformed USB peripheral. On many systems this may be a logged-in seat user or a user in an audio-related access context rather than host root.

Namespace scoped privilege is not relevant here. This is not a network namespace, CAP_NET_ADMIN, BPF, KVM, or container-scoped operation.

There is no direct network trigger. The attack is local/device-mediated. Prior host setup may include the system accepting and binding the USB audio/MIDI device, but the supplied evidence does not establish a host-admin-only trigger. No race is required. The issue is deterministic once the malformed endpoint size and output operation align.

4. Primitive and severity interpretation

The exact primitive is an out-of-bounds kernel write in the ALSA USB MIDI output path. The URB buffer is sized to ep->max_transfer, but snd_usbmidi_us122l_output() can request count 2 when ep->max_transfer is only 1. After that, the padding calculation can compute 1 minus 2 and wrap to a huge size for memset.

This is fixed stale storage or fixed IO-buffer adjacent corruption rather than an active structured-object UAF. However, it is still a concrete kernel memory corruption primitive, not merely a crash-only invalid state. The overwrite target is not shown to be attacker-directed, and full exploitation is not demonstrated, so the score should not be pushed to high-end Important or Critical from the patch alone.

PrimitiveClass is CLASS_A because a concrete OOB write into kernel memory is directly established. Demonstrated practical consequence is kernel crash or DoS. Speculative consequence is broader kernel memory corruption impact, including possible confidentiality or integrity impact, but no complete LPE or arbitrary R/W chain is supplied.

ImportantCandidate is YES because kernel OOB write primitives require manual exploitation-leverage review even when the initial deployment path is local USB. AutoDowngradeAllowed is NO because automatic closure would discard a concrete kernel memory corruption primitive with unresolved leverage.

5. One-sentence report phrase

A malformed USB MIDI device can make snd_usbmidi_us122l_output() write beyond a too-small URB buffer, causing kernel memory corruption and likely DoS, with manual review required because the primitive is an OOB kernel write rather than a simple warning or resource leak.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes a concrete OOB write in kernel memory, with a huge memset underflow path and at least limited attacker influence through the device and MIDI output path.

7. Call-site confidence

Call-site confidence: high

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

Patch: ALSA: usb-audio: fix OOB write in snd_usbmidi_us122l_output()
Commit: 5538daf2a5cc80ac3f3e913c0db045d92d995d40
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=5538daf2a5cc80ac3f3e913c0db045d92d995d40

Commit description:

The snd_usbmidi_us122l_output() picks a count of 2 on anything slower
than high speed and never relates it to ep->max_transfer.  The URB
buffer holds exactly max_transfer bytes, so a device declaring a one
byte bulk endpoint takes two bytes from snd_rawmidi_transmit(), and the
memset that pads the rest computes 1 - 2 in int and wraps to SIZE_MAX.

Only 0x800e and 0x800f are pinned to nine bytes.  The US-122MKII at
0x0644:0x8021 falls to the default and takes usb_maxpacket(), which the
USB core only clamps downward.

The akai and novation output ops in this file were given the same guard
recently.  Do the same here.

Fixes: 030a07e ("ALSA: Add USB US122L driver")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: HyeongJun An <sammiee5311@gmail.com>
Link: https://patch.msgid.link/20260901090409.1478573-1-sammiee5311@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/midi.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=5538daf2a5cc80ac3f3e913c0db045d92d995d40

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

The original Linux kernel CVE announcement for CVE-2026-90033 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90033

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:R/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 5
  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: YES
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.
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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).