[CVE-2026-74387][MODERATE REGULAR] ALSA: seq: midi: Serialize output teardown with event_input [ Upstream

[CVE-2026-74387][MODERATE REGULAR] ALSA: seq: midi: Serialize output teardown with event_input [ Upstream

[AL-KERNEL]

CVE: CVE-2026-74387
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ALSA: seq: midi: Serialize output teardown with event_input [ Upstream
Commit: f5d470b808bc01f70978e22e595c6f7768313406
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5d470b808bc01f70978e22e595c6f7768313406
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74387
Analysis date: Sun, 16 Aug 2026 07:22:38 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review required / Actionable Moderate if local ALSA access is exposed
Manual review required: YES

Summary:
A race in ALSA seq MIDI output teardown can let event_input use a stale rawmidi substream or runtime after close, causing a local kernel crash and a UAF candidate that requires manual review for possible privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-74387 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74387

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 REGULAR
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-2026-74387	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;*CWE-416;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'ALSA sequencer MIDI output teardown has a race with event_input processing. event_process_midi snapshots msynth output_rfile output and then calls dump_midi and snd_rawmidi_kernel_write while midisynth_unuse can release the rawmidi file and free substream runtime. This can leave the event path using a stale substream or runtime and can cause a NULL pointer dereference or use-after-free. For the CVSS the PR:L is used because a local user or local process with access to ALSA sequencer and rawmidi interfaces is required to create the use and unuse race. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may allow confidentiality or integrity impact because the bug includes a UAF candidate in kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review required / Actionable Moderate if local ALSA access is exposed (with actual score 4)	YES	WRITE OOB LOCK NULLPTR INIT UAF HARDWARE LINUS  LOWERED_FROM_HIGH_BASED_ON_NULLPTR DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline manual review required / Actionable Moderate if local ALSA access is exposed**

## 2. Signal breakdown

* Local unprivileged trigger: +1
  Local users or local processes with access to ALSA sequencer and rawmidi interfaces may be able to exercise the subscribe, event input, and teardown paths.

* Memory corruption, weak or indirect corruption candidate: +1
  The patch describes a stale substream or stale runtime after rawmidi teardown. This is a UAF candidate, but the patch does not show attacker controlled reclaim, type confusion, callback control, or write after free.

* Real lifetime corruption: +1
  The bug is a lifetime race where midisynth_unuse can release the rawmidi file and free substream runtime while event_process_midi still uses the borrowed substream.

* Reliable kernel crash / strong DoS: +1
  The report includes KASAN null-ptr-deref in snd_rawmidi_kernel_write1, so local kernel crash is realistic.

* Hard or unreliable race / special timing required: -1 in conservative score
  Triggering requires a race between event_input processing and the last output unuse teardown.

* Paranoid adjustment: +1 relative to conservative
  The race was reproduced and the stale runtime/substream path is concrete. Paranoid score avoids subtracting the race penalty, but still does not upgrade to strong LPE because no reclaim or controlled reuse primitive is shown.

## 3. Reachability analysis

The bug is local, not network reachable. A realistic trigger requires access to ALSA sequencer and rawmidi functionality and the ability to race MIDI event processing against output close or unsubscribe. On desktop or multimedia systems this may be reachable by an unprivileged local user depending on device permissions, audio group policy, udev rules, and whether ALSA sequencer support is enabled. In containers, reachability depends on whether sound devices or ALSA interfaces are passed through. Namespaces do not inherently make this remote or PR:N, but delegated device access can make it practical for a reduced privilege user.

Call-site confidence: high. The patch includes the borrowing site, the open/publish path, and the teardown/release path.

## 4. Severity interpretation

This behaves more like a borderline to actionable Moderate kernel lifetime bug, not a clear Important-class issue by itself. The theoretical memory corruption potential is real because the patch explicitly describes stale substream/runtime use and possible UAF. The realistic evidence currently supports local DoS more strongly than privilege escalation, because the shown failure is a NULL dereference and the patch does not demonstrate attacker controlled reclaim, object replacement, write primitive, callback dispatch, or type confusion.

## 5. One-sentence report phrase

A race in ALSA seq MIDI output teardown can let event_input use a stale rawmidi substream or runtime after close, causing a local kernel crash and a UAF candidate that requires manual review for possible privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a local race with a concrete lifetime violation and UAF candidate in kernel memory. Even though the demonstrated impact is DoS and the ActionableScore is capped below Important without a stronger primitive, it should not be auto-closed without checking local reachability and exploitability.

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

Patch: ALSA: seq: midi: Serialize output teardown with event_input [ Upstream
Commit: f5d470b808bc01f70978e22e595c6f7768313406
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f5d470b808bc01f70978e22e595c6f7768313406

Changed files:
  sound/core/seq/seq_midi.c
  sound/seq_device.h
  sound/seq_midi_event.h
  sound/initval.h

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=f5d470b808bc01f70978e22e595c6f7768313406

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

The original Linux kernel CVE announcement for CVE-2026-74387 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74387

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).