[CVE-2026-80829][MODERATE 7.0] ALSA: usb-audio: fix OOB write in snd_usbmidi_novation_output()

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

CVE: CVE-2026-80829
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ALSA: usb-audio: fix OOB write in snd_usbmidi_novation_output()
Commit: 558fc4485ecc704edfe7876d6cebae4738ff7ef8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=558fc4485ecc704edfe7876d6cebae4738ff7ef8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80829
Analysis date: Fri, 04 Sep 2026 17:13:28 -0400
ActionableScore: 5
ActionableScore lower bound: 4
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-2026-80829 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80829

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-2026-80829	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-80829 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:H/I:H/A:H';*CWE-787;CWE-195;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '6.8';DESCR 'A kernel OOB write in ALSA USB MIDI occurs because snd_usbmidi_novation_output() subtracts a 2-byte header from ep max_transfer without first checking that the endpoint can hold the header and at least one payload byte. A malformed or malicious USB MIDI device can advertise a bulk OUT endpoint with wMaxPacketSize 1, making the transmit count negative and allowing a later memcpy to treat it as a very large size_t length. For the CVSS the PR:N because the attacker does not need an account on the target if the malicious USB device can be attached or supplied to the victim. The issue is not network reachable and normally requires physical USB device access or victim interaction with an untrusted USB MIDI device. 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.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)  SKIP CVE-2026-80829 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	WRITE OOB USB HARDWARE LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the conservative score. No floor or cap changed the paranoid score.

2. Signal breakdown

Conservative positive signals:

* +2 Strong corruption primitive. The patch establishes a real kernel OOB write caused by negative length propagation into memcpy after ep->max_transfer becomes smaller than the two byte header requirement.
* +1 Firmware or device mediated external influence. A malformed or malicious USB MIDI device can advertise a bulk OUT endpoint with wMaxPacketSize 1.
* +1 Reliable kernel crash or strong DoS. The described memcpy with size_t -1 can write far beyond a usb_alloc_coherent buffer and realistically cause kernel memory corruption or crash.

Conservative negative signals:

* 0 Physical USB access or victim interaction is a major reachability limitation, but I did not subtract a full physical-only penalty because the issue is a concrete kernel OOB write from a malicious peripheral class rather than merely a device-local hang or rare hardware quirk.
* 0 No separate race or timing penalty. This is an ordered validation failure, not a timing race.
* 0 No host-admin penalty. Host administrator privilege is not established as required for the malicious-device attack model.

Paranoid additional signal:

* +1 Higher corruption concern. The OOB write length is extremely large after signed-to-unsigned conversion, and the sink is a kernel memcpy into memory beyond a DMA/coherent transfer buffer. This is still grounded in the established OOB write primitive, not a speculative UAF or arbitrary write claim.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* RealUAF is not present.
* Remote network reachability is not established.
* Confidentiality and integrity are not separately awarded as independent ActionableScore bonuses because no concrete disclosure, credential overwrite, callback control, or protected-object modification is shown.
* Extra Availability is not awarded because the same crash/corruption consequence is already counted once.
* No concrete repeated release/refcount operation exists.

3. Reachability analysis

The most direct attacker model is a malicious or malformed USB MIDI device that presents an invalid endpoint size. The attacker does not need host administrator credentials if they can attach or supply the malicious peripheral and the victim system binds it to ALSA usb-audio.

The complete trigger may require the affected Novation-style MIDI output path to become active and data to be transmitted, so victim interaction or a local audio/MIDI application may be needed in practice. This is not network reachable. It is not namespace-scoped. No sysctl, boot parameter, or special kernel debugging configuration is required by the supplied evidence.

The hardware requirement is USB MIDI class or compatible device behavior. This is physical/peripheral-mediated exposure, but it is not merely a rare internal hardware feature.

4. Primitive and severity interpretation

The exact primitive is an OOB kernel write caused by subtracting a two byte protocol header from an endpoint max transfer size that can be 1. This produces a negative count that later becomes a huge size_t value in memcpy.

This is not a UAF and not a cleanup-only stale dereference. It is an active bounds/integer validation failure leading to kernel memory overwrite. The overwrite target begins from a fixed transfer buffer offset, and controlled target selection is not demonstrated. However, the primitive is still stronger than crash-only invalid dereference because the patch-supported path reaches a real memcpy write far beyond the allocated buffer.

PrimitiveClass is CLASS_A because a concrete attacker-influenced kernel OOB write is established. ImportantCandidate is YES because kernel memory corruption exists and exploitation leverage beyond DoS is unresolved. AutoDowngradeAllowed is NO because automatically closing this as an ordinary low or moderate device bug would discard a real kernel overwrite primitive.

Likely operational severity is Actionable Moderate at minimum, with Strong Important review justified due to the kernel OOB write. The most defensible demonstrated impact is DoS, while privilege escalation or broader C/I impact remains possible but not proven.

5. One-sentence report phrase

A malformed USB MIDI device can advertise an endpoint with too small wMaxPacketSize, causing snd_usbmidi_novation_output() to pass a negative transmit length that becomes a huge memcpy size and triggers a kernel OOB write.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a concrete kernel OOB write with a malicious-device trigger and unresolved exploitation leverage beyond crash.

7. Call-site confidence

Call-site confidence: high

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

Patch: ALSA: usb-audio: fix OOB write in snd_usbmidi_novation_output()
Commit: 558fc4485ecc704edfe7876d6cebae4738ff7ef8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=558fc4485ecc704edfe7876d6cebae4738ff7ef8

Commit description:

snd_usbmidi_novation_output() lays out a two-byte header at
transfer_buffer[0..1] and passes &transfer_buffer[2] together with a
length of ep->max_transfer - 2 to snd_rawmidi_transmit():

	count = snd_rawmidi_transmit(ep->ports[0].substream,
				     &transfer_buffer[2],
				     ep->max_transfer - 2);

ep->max_transfer comes from the output endpoint's wMaxPacketSize via
usb_maxpacket(). A malformed or malicious device can advertise a bulk
OUT endpoint with a wMaxPacketSize of 1 - the USB core only clamps this
value downwards - so ep->max_transfer becomes 1 and the count argument
becomes -1.

snd_rawmidi_transmit() passes the negative count on to
__snd_rawmidi_transmit_peek(), where "if (count1 > count) count1 = count"
leaves count1 negative; get_aligned_size() keeps it negative for a
byte-stream substream, so the following memcpy(buffer, ..., count1) runs
with a (size_t)-1 length and writes far past the transfer buffer, which
was allocated with usb_alloc_coherent(ep->max_transfer).

This is the same class of bug that was fixed for snd_usbmidi_akai_output()
in commit 0970274 ("ALSA: usb-audio: fix OOB write in
snd_usbmidi_akai_output()"); the novation output routine was left
unguarded. Bail out when the endpoint cannot hold the two-byte header
plus at least one payload byte.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Marouane El Moufid <eun0us@espilon.net>
Link: https://patch.msgid.link/178749334830.543645.13722252148340572274@espilon.net
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=558fc4485ecc704edfe7876d6cebae4738ff7ef8

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

The original Linux kernel CVE announcement for CVE-2026-80829 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-80829

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:P/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.8

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: 4
  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: 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.
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).