[CVE-2026-46018][LOW] ALSA: usb-audio: stop parsing UAC2 rates at MAX_NR_RATES

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

CVE: CVE-2026-46018
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: ALSA: usb-audio: stop parsing UAC2 rates at MAX_NR_RATES
Commit: ab5ba9fd138758ddc50222264ff246b31e397abf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ab5ba9fd138758ddc50222264ff246b31e397abf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46018
Analysis date: Wed, 27 May 2026 10:39:02 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A malformed UAC2 USB audio RANGE response can keep rate parsing active after `MAX_NR_RATES`, causing probe-time log spam or blocked initialization while holding `register_mutex`, with DoS-only impact.

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

The original Linux kernel CVE announcement for CVE-2026-46018 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46018

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: LOW
Manual review required: NO

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-2026-46018	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-834;**CWE-400;*CWE-20;Other CVSS 'AV:P/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A malformed UAC2 RANGE response can make parse_uac2_sample_rate_range() keep processing additional triplets after MAX_NR_RATES is reached. The previous code only broke out of the current rate loop, so probe could continue parsing invalid data and repeatedly print invalid uac2 rates while register_mutex is held. The realistic exposure is a faulty or malicious USB audio device, or a device emulation path, rather than a network reachable attack. For the CVSS the PR:L is used in the paranoid score because some environments may let a less privileged local user influence USB emulation, USB/IP, VM passthrough, or repeated driver reprobe. Impact is denial of service only through prolonged probe, log spam, or blocked USB audio initialization. The patch context does not support UAF, out of bounds write, arbitrary write, information disclosure, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1)	YES	DANGER USB SIMPLEFIX HARDWARE LINUS SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

Triggered signals:

* Firmware/device-mediated external influence: +1
  A malformed UAC2 RANGE response is supplied by a faulty or malicious USB audio device.

* Availability impact realistic: +1
  Continued parsing and repeated logging while holding `register_mutex` can cause probe delay, log spam, or blocked USB audio initialization.

Subtracting signals:

* Physical-only or device-specific condition: -2
  Realistic triggering requires a malformed USB audio device, USB emulation, USB/IP, or passthrough control.

* No memory corruption / constrained DoS-only behavior: +0
  The patch shows loop termination behavior, not UAF, OOB write, overflow, arbitrary write, or information disclosure.

No signals apply for remote reachability, generic memory corruption, real lifetime corruption, privilege escalation, sensitive-object access, or security-boundary bypass.

## 3. Reachability analysis

The bug is triggered during USB audio probe when a malformed UAC2 device returns a RANGE response with more triplets after `MAX_NR_RATES` is reached. A normal remote network attacker cannot trigger it.

A local user may influence this only in environments exposing USB gadget emulation, USB/IP, VM passthrough, or device reprobe control. In ordinary deployments, it requires physical or emulated malicious USB device access. Namespaces and containers matter only if USB device control is delegated.

## 4. Severity interpretation

This behaves like a Low-like or ordinary Moderate DoS bug. It can cause probe-time delay or logging under a mutex, but the patch does not show memory corruption, UAF, overflow, information disclosure, or privilege escalation.

Theoretical severity is limited to availability. Realistic exploitation evidence supports malformed-device DoS only.

## 5. One-sentence report phrase

A malformed UAC2 USB audio RANGE response can keep rate parsing active after `MAX_NR_RATES`, causing probe-time log spam or blocked initialization while holding `register_mutex`, with DoS-only impact.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a malformed USB descriptor parsing DoS with physical or emulated device preconditions and no supported memory corruption or privilege-escalation primitive.

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

Patch: ALSA: usb-audio: stop parsing UAC2 rates at MAX_NR_RATES
Commit: ab5ba9fd138758ddc50222264ff246b31e397abf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ab5ba9fd138758ddc50222264ff246b31e397abf

Commit description:

parse_uac2_sample_rate_range() caps the number of enumerated
rates at MAX_NR_RATES, but it only breaks out of the current
rate loop. A malformed UAC2 RANGE response with additional
triplets continues parsing the remaining triplets and repeatedly
prints "invalid uac2 rates" while probe still holds
register_mutex.

Stop the whole parse once the cap is reached and return the
number of rates collected so far.

Fixes: 4fa0e81 ("ALSA: usb-audio: fix possible hang and overflow in parse_uac2_sample_rate_range()")
Cc: stable@vger.kernel.org
Reported-by: syzbot+d56178c27a4710960820@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d56178c27a4710960820
Signed-off-by: Cássio Gabriel <cassiogabrielcontato@gmail.com>
Link: https://patch.msgid.link/20260415-usb-audio-uac2-rate-cap-v1-1-5ecbafc120d8@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/format.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=ab5ba9fd138758ddc50222264ff246b31e397abf

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

The original Linux kernel CVE announcement for CVE-2026-46018 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46018

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: unknown
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).