[CVE-2026-74498][MODERATE 7.0] ALSA: usb-audio: Fix DMA buffer out-of-bounds write when fill_max is set

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

CVE: CVE-2026-74498
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ALSA: usb-audio: Fix DMA buffer out-of-bounds write when fill_max is set
Commit: 3852974608f53e530e27c21d0c6c7d79c17b3f5a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3852974608f53e530e27c21d0c6c7d79c17b3f5a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74498
Analysis date: Sat, 15 Aug 2026 09:41:46 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A crafted USB audio device can trigger a DMA out-of-bounds write in the ALSA usb-audio endpoint setup path when `fill_max` causes the actual packet size to exceed the allocated DMA buffer size, leading to kernel heap corruption with DoS and plausible privilege-escalation impact.

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

The original Linux kernel CVE announcement for CVE-2026-74498 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74498

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
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CVE-2026-74498	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	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-131;CWE-119;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 DMA out of bounds write in the ALSA USB audio path can occur when a USB audio endpoint sets the fill_max flag. data_ep_set_params promotes curpacksize to maxpacksize, but the DMA buffer can still be allocated using the smaller sample rate derived maxsize. A malicious USB audio device can therefore cause the host controller to write past the end of the allocated buffer and corrupt kernel heap memory during streaming. For the CVSS the PR:N because no privileges on the victim system are required beyond the ability to present a crafted USB audio device. The base score uses UI:R because a victim action such as connecting or using the device may be needed in common scenarios. The paranoid score uses UI:N and CIA:HHH because the patch context shows a concrete kernel heap out of bounds write primitive, not only a crash. Impact is at least denial of service and may include privilege escalation or broader confidentiality and integrity impact through memory corruption. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	YES	WRITE OOB USB SIMPLEFIX DMAorINTERRUPT HARDWARE LINUS KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: **7**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* **Firmware-mediated external influence: +1**
  A crafted or malicious USB audio device can influence endpoint descriptors and transfer behavior.

* **Generic memory corruption, strong corruption primitive: +2**
  The commit explicitly describes a DMA buffer out-of-bounds write that corrupts kernel heap memory.

* **Privilege escalation plausible: +1 conservative / +2 paranoid**
  Conservative: kernel heap corruption is present, but target selection and exploit reliability are not demonstrated.
  Paranoid: attacker-influenced DMA OOB writes into kernel heap are a credible LPE class and should not be auto-closed.

* **Memory layout invariant restoration: +1**
  The patch restores consistency between `curpacksize`, `maxsize`, and the allocated DMA buffer size.

* **Reliable kernel crash / strong DoS: +1**
  Kernel heap corruption via DMA is likely to cause crashes or serious kernel instability.

* **Privileged kernel/device-management memory corruption path: +1**
  The corruption occurs in a USB/ALSA DMA-adjacent kernel driver path.

* **Confidentiality impact plausible: +1**
  Kernel heap corruption can plausibly affect confidentiality in worst-case exploitation, although no direct info leak is shown.

* **Integrity impact plausible: +1**
  OOB DMA writes can corrupt adjacent kernel heap objects.

* **Broad/default/common subsystem: +1**
  USB audio support is common and often autoloaded on desktop and workstation systems.

Subtractors:

* **Physical-only or rare hardware-only condition: -2**
  The realistic attacker needs physical access, USB redirection, or control of a USB device exposed to the host.

* **Corruption primitive constrained / no precise overwrite target shown: -1**
  The patch shows a real OOB write, but not a demonstrated controlled target, reclaim strategy, or stable overwrite primitive.

## 3. Reachability analysis

The likely trigger is a crafted USB audio device exposing endpoint parameters where `fill_max` causes `curpacksize` to exceed the originally calculated `maxsize`. The vulnerable write occurs when the device streams data and the host controller writes more data into the DMA buffer than was allocated.

No local OS privileges are required on the victim if the attacker can present the malicious USB audio device. However, this is not network-reachable in the usual sense. It is physical or device-mediated. USB passthrough, malicious peripherals, docking stations, or virtualized USB redirection can make this more practical in some environments.

Namespaces and containers generally do not reduce the privilege requirement because the attack is below normal local process isolation and targets the host kernel device stack. The affected subsystem is common, but the exact trigger depends on USB audio device behavior and whether streaming is started.

## 4. Severity interpretation

This is not an ordinary Moderate cleanup. The patch fixes a concrete DMA out-of-bounds write into kernel heap memory caused by inconsistent transfer-size and allocation-size calculations.

Realistically, exploitation is limited by physical or USB-device-mediated reachability and by uncertainty around adjacent heap target control. That keeps the conservative score below the highest range. However, the primitive is stronger than a NULL dereference or validation-only bug because it is an actual kernel heap overwrite via DMA.

The issue should be treated as **Strong Important candidate** or at least **Actionable Moderate**. It should not be auto-closed as a low-risk DoS-only bug.

## 5. One-sentence report phrase

A crafted USB audio device can trigger a DMA out-of-bounds write in the ALSA usb-audio endpoint setup path when `fill_max` causes the actual packet size to exceed the allocated DMA buffer size, leading to kernel heap corruption with DoS and plausible privilege-escalation impact.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the patch describes a concrete kernel heap DMA out-of-bounds write, not merely a warning or validation issue. Physical reachability limits exposure, but the memory corruption class and possible LPE implications require manual review.

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

Patch: ALSA: usb-audio: Fix DMA buffer out-of-bounds write when fill_max is set
Commit: 3852974608f53e530e27c21d0c6c7d79c17b3f5a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3852974608f53e530e27c21d0c6c7d79c17b3f5a

Commit description:

When a USB audio endpoint requests full packet transfers via the fill_max
descriptor flag, data_ep_set_params() promotes ep->curpacksize to
ep->maxpacksize. However, maxsize is left at the original sample-rate
derived value.

Since u->buffer_size is allocated as maxsize * packets, the resulting
DMA buffer is far too small for the requested transfer length. When the
USB host controller streams up to curpacksize bytes per packet, it writes
past the end of the buffer via DMA, corrupting kernel heap memory.

Update maxsize to curpacksize when fill_max is set so that the allocated
DMA buffer size matches the actual transfer request size.

[ changed to reassign maxsize only when ep->fill_max is set -- tiwai ]

Fixes: 8fdff6a ("ALSA: snd-usb: implement new endpoint streaming model")
Cc: stable@vger.kernel.org
Assisted-by: Jetski:Gemini-3.6-Flash
Signed-off-by: Sonali Pradhan <sonalipradhan@google.com>
Link: https://patch.msgid.link/20260728201716.2347726-1-sonalipradhan@google.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/endpoint.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=3852974608f53e530e27c21d0c6c7d79c17b3f5a

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

The original Linux kernel CVE announcement for CVE-2026-74498 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74498

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: 7
  Paranoid ActionableScore: 8

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).