[CVE-2026-74501][MODERATE 7.0] ALSA: usb-audio: fix use-after-free in ump_to_endpoint()

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

CVE: CVE-2026-74501
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ALSA: usb-audio: fix use-after-free in ump_to_endpoint()
Commit: 49eccef6d6e1c00dac6fb2e7eb6f9206c33e1c37
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=49eccef6d6e1c00dac6fb2e7eb6f9206c33e1c37
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74501
Analysis date: Sat, 15 Aug 2026 19:08:31 -0400
ActionableScore: 6
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malicious USB MIDI 2.0 device can leave an ALSA UMP endpoint pointing to freed `snd_usb_midi2_ump` memory after a failed creation path, causing a use-after-free on later UMP open and at least a kernel crash, with limited but review-worthy memory corruption concerns.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74501 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74501

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-2026-74501	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:P/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;*CWE-672;Other CVSS 'AV:P/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '6.3';DESCR 'A use-after-free in the ALSA USB audio MIDI 2.0 UMP path occurs because a registered snd_ump_endpoint can keep ump private_data pointing to a freed snd_usb_midi2_ump object after a later snd_usb_midi_v2_create failure path. A malicious USB MIDI 2.0 device can force this error sequence after endpoint registration and leave the /dev/snd/umpC*D* node exposed. A later open of the UMP node calls ump_to_endpoint and reads rmidi eps from freed slab memory, which can crash the kernel and may allow confidentiality or integrity impact if the freed object is reclaimed in a controllable way. For the CVSS the PR:N because the attacker does not need an account on the target when the attack is delivered through a malicious USB device. The issue is not remotely network reachable and normally requires physical or equivalent USB device access. Impact is at least denial of service and the UAF nature makes manual review appropriate.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	USB ERRORPATH UAF HARDWARE LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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## 1. ActionableScore

ActionableScore=6
ActionableScoreLower=3

* Conservative score: 3
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Firmware-mediated external influence: +1
  A malicious USB MIDI 2.0 device can influence the failing initialization path after endpoint registration.

* Generic memory corruption, strong corruption primitive: +2
  The patch fixes a real stale pointer use-after-free: `ump->private_data` can point to a freed `snd_usb_midi2_ump`.

* Real lifetime corruption: +1
  The freed object remains reachable through a registered `snd_ump_endpoint` and exposed `/dev/snd/umpC*D*` node.

* Reliable kernel crash / strong DoS: +1
  A later UMP open dereferences freed slab memory and can crash the kernel.

* Physical-only or rare hardware-only condition: -2
  Exploitation normally requires physical or equivalent USB device access and a malicious USB MIDI 2.0 device.

Conservative total: 3

Paranoid additional signals:

* Weak LPE concern: +1
  The stale pointer is used to select `rmidi->eps[dir]`; if freed memory is reclaimed in a useful way, this may become an indirect bogus-object selection primitive, although no controlled reclaim is demonstrated.

* Confidentiality impact plausible: +1
  UAF read from freed slab memory may expose stale kernel object contents or influence object selection in a limited way.

* Integrity impact plausible: +1
  If reclaimed memory can influence the returned endpoint pointer, later open/close/trigger paths may operate on an unintended object.

Paranoid total: 6

## 3. Reachability analysis

The realistic attacker is a malicious USB MIDI 2.0 device or an attacker able to emulate one. The device must make `snd_usb_midi_v2_create()` fail after a `snd_ump_endpoint` has already been registered, leaving the UMP device node exposed with a dangling `private_data` pointer.

No network reachability is present. Namespaces and containers do not materially lower the main requirement, because the key trigger is USB device attachment or emulation rather than ordinary local namespace operations. A local user may help trigger the dereference by opening the exposed `/dev/snd/umpC*D*` node, but the primary attacker model is physical or device-mediated.

The affected path depends on USB audio MIDI 2.0 UMP support and the presence of a malicious or malformed USB MIDI 2.0 device, so it is not a broad default remote exposure. However, the bug is a real kernel lifetime issue, not just a validation warning.

Call-site confidence: high. The provided commit message and diff identify both the stale pointer source in `free_all_midi2_umps()` and the later dereference in `ump_to_endpoint()`.

## 4. Severity interpretation

This is stronger than an ordinary Moderate device quirk because it is an actual kernel use-after-free reachable through a device-created userspace node. Conservative handling is limited by the physical USB requirement and the lack of a demonstrated reclaim, overwrite, type confusion, or callback-control primitive.

The realistic demonstrated impact is kernel crash / DoS. The paranoid score is higher because stale kernel object lifetime bugs can sometimes become exploitable if freed slab memory is reclaimed with attacker-influenced data, but that is not proven by this patch alone. This should be treated as Actionable Moderate and manually reviewed, but not automatically classified as Strong Important without additional exploitability evidence.

## 5. One-sentence report phrase

A malicious USB MIDI 2.0 device can leave an ALSA UMP endpoint pointing to freed `snd_usb_midi2_ump` memory after a failed creation path, causing a use-after-free on later UMP open and at least a kernel crash, with limited but review-worthy memory corruption concerns.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real kernel UAF in a device path with an exposed `/dev/snd/umpC*D*` node after failed initialization. Even though exploitation is physical/device-mediated and no strong LPE primitive is demonstrated, lifetime corruption in kernel object paths should not be auto-closed.

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UPSTREAM PATCH SUMMARY
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Patch: ALSA: usb-audio: fix use-after-free in ump_to_endpoint()
Commit: 49eccef6d6e1c00dac6fb2e7eb6f9206c33e1c37
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=49eccef6d6e1c00dac6fb2e7eb6f9206c33e1c37

Commit description:

create_midi2_ump() registers a card-owned snd_ump_endpoint and stores a
back-pointer to its per-interface snd_usb_midi2_ump object in
ump->private_data, but it never installs an ump->private_free hook and
never clears that pointer.

If a later step of snd_usb_midi_v2_create() fails, its error path calls
free_all_midi2_umps(), which kfree()s the snd_usb_midi2_ump object while
the already-registered endpoint keeps pointing at it.  The created
/dev/snd/umpC*D* node stays exposed, so the first operation of any UMP
open, ump_to_endpoint(), dereferences the dangling ump->private_data and
reads rmidi->eps[dir] out of freed memory.

A malicious USB MIDI 2.0 device that makes creation fail after the
endpoint is registered can thus trigger a slab use-after-free read on a
subsequent open of the UMP node.

Clear the endpoint's back-pointer before freeing the object, and let
ump_to_endpoint() tolerate a NULL private_data so the open/close/trigger
callbacks fail cleanly (their callers already handle a NULL endpoint)
instead of dereferencing a stale pointer.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: ff49d1d ("ALSA: usb-audio: USB MIDI 2.0 UMP support")
Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
Reported-by: Baul Lee <baul.lee@xbow.com>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
Link: https://patch.msgid.link/20260726051337.41124-1-baul.lee@xbow.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/usb/midi2.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=49eccef6d6e1c00dac6fb2e7eb6f9206c33e1c37

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

The original Linux kernel CVE announcement for CVE-2026-74501 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74501

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:R/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:P/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.3

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: 3
  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.
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).