[CVE-2026-90207][MODERATE 7.0] ALSA: seq: midi: Serialize input teardown with event_input [ Upstream

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

CVE: CVE-2026-90207
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ALSA: seq: midi: Serialize input teardown with event_input [ Upstream
Commit: de76a2b47582cd6dba2e15f024681bf377ecc94a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=de76a2b47582cd6dba2e15f024681bf377ecc94a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90207
Analysis date: Thu, 17 Sep 2026 17:52:10 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90207 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90207

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-2026-90207	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90207 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-362;CWE-667;**CWE-476;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in ALSA sequencer MIDI input teardown can allow snd_midi_input_event to run while the rawmidi input substream is being closed. The callback can reenable the input trigger after teardown has started, leaving input work active after rawmidi_release_priv clears the associated rmidi pointer. A later workqueue execution can dereference the cleared state and crash the kernel. For the CVSS the PR:L value is used because a local process must have access to the ALSA sequencer and rawmidi interfaces, but full administrative privileges are not normally required when the device permissions grant audio access. AC:H is used because successful triggering depends on a timing window between input event processing, trigger cancellation, and substream release. The issue is not network reachable. Impact is at least a local denial of service via kernel crash. The demonstrated primitive is a NULL pointer dereference, while limited confidentiality or integrity impact remains a conservative worst case because teardown overlaps asynchronous access to release managed state.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)  SKIP CVE-2026-90207 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	OOB NULLPTR INIT HARDWARE LINUS KPANIC 	-	-	checked

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

ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* REAL UAF FLOOR 4 was checked but did not change the score.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +2 Real lifetime violation. An input work callback can continue using the rawmidi substream after its open-file lifetime has been terminated by `snd_rawmidi_kernel_release()`.
* +1 Reliable kernel crash. The commit supplies a concrete NULL-pointer dereference trace from `snd_midi_input_event()`.
* +1 Local unprivileged trigger. A process with ordinary ALSA sequencer and rawmidi device access can subscribe, unsubscribe, and close the stream without host-admin-equivalent privilege.

Negative signals:

* No negative points. The race alone does not justify a timing penalty, and MIDI being a specialized interface does not establish rare or difficult reachability.

Material non-awarded signals:

* Controlled reclaim or replacement is not demonstrated.
* No heap storage reuse, attacker-controlled post-release contents, arbitrary write, disclosure, callback hijack, or privilege escalation is established.
* Structured stale-object leverage is not awarded because the observed stale operation reaches cleared release state and faults rather than consuming attacker-shaped replacement fields.
* Attacker-influenced lifetime manipulation is not separately awarded. Userspace controls subscribe and unsubscribe, but meaningful control over stale storage or its replacement is absent.
* No extra Availability point is awarded because it would duplicate the same NULL-dereference crash.
* No C/I or weak-LPE point is awarded merely because asynchronous activity survives teardown.
* No concrete repeated release or refcount operation exists. The patch does not show the same reference receiving a second `put`, `free`, or lifetime mutation.
* The paranoid score receives no additional point because stronger consequences would require an unproven reclaim or post-release corruption primitive.

3. Reachability analysis

The relevant attacker is a local process with permission to access the ALSA sequencer and the affected rawmidi input endpoint. Such access can be granted through normal device ACLs or audio-group membership and does not require host root or an administrative capability, so `HostAdminRequired=NO`.

No namespace-scoped capability is involved in the demonstrated path. Device-node access controls reachability, so `NamespaceScopedPrivilege=NO`.

An ALSA MIDI endpoint and permission to open it are prerequisites. The attacker can control subscription and teardown operations, while concurrent incoming MIDI activity drives `snd_midi_input_event()`. The race requires input processing to overlap stream closure, but the evidence does not establish a sufficiently narrow or one-shot window to justify a timing penalty. The path is local and is not network reachable.

4. Primitive and severity interpretation

The exact primitive is asynchronous use of a rawmidi substream after its open-file lifetime has entered release. The callback can re-enable input while teardown is cancelling work, allowing another workqueue invocation after `rawmidi_release_priv()` has cleared `rfile->rmidi`.

This is a real logical post-lifetime access and therefore constrained CLASS_B rather than a mere warning. The affected substream is fixed, driver-owned storage rather than a demonstrated reclaimed heap object. The supplied evidence establishes a NULL dereference and kernel DoS, not use of attacker-controlled replacement contents.

There is no concrete repeated release/refcount mutation. The first release ends the open-file lifetime, but the stale callback performs input processing rather than a second lifetime mutation on the same released reference.

Important-grade leverage beyond the crash is not established, so `ImportantCandidate=NO`. `AutoDowngradeAllowed=YES` because this is a constrained asynchronous teardown lifetime bug with a demonstrated crash, no useful reclaim mechanism, no security-boundary bypass, and no unresolved repeated lifetime mutation. Operational severity is Borderline Moderate with manual confirmation recommended.

5. One-sentence report phrase

A local user with access to an ALSA rawmidi endpoint can race MIDI input processing with stream teardown, leaving asynchronous work active after release and causing a kernel NULL-pointer dereference and denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm from the affected kernel version that rawmidi release cannot free or repurpose the substream or runtime before the late callback, since such behavior would strengthen this from a constrained logical lifetime violation to a heap UAF.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ALSA: seq: midi: Serialize input teardown with event_input [ Upstream
Commit: de76a2b47582cd6dba2e15f024681bf377ecc94a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=de76a2b47582cd6dba2e15f024681bf377ecc94a

Changed files:
  sound/20260813144224.753399-1-tiwai
  sound/core/seq/seq_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=de76a2b47582cd6dba2e15f024681bf377ecc94a

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

The original Linux kernel CVE announcement for CVE-2026-90207 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90207

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: unknown
  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: 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).