[CVE-2026-68202][IMPORTANT] ALSA: seq: close a re-opened queue timer in the destructor

[CVE-2026-68202][IMPORTANT] ALSA: seq: close a re-opened queue timer in the destructor

[AL-KERNEL]

CVE: CVE-2026-68202
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ALSA: seq: close a re-opened queue timer in the destructor
Commit: fb40d03ed792a8a8bf77aa0ee15df57b0ff78b07
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb40d03ed792a8a8bf77aa0ee15df57b0ff78b07
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68202
Analysis date: Mon, 10 Aug 2026 09:02:49 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An unprivileged local user with access to `/dev/snd/seq` can race ALSA sequencer queue destruction to leave a live timer callback pointing at a freed queue, causing a kernel UAF that is at least a reliable local DoS and may require LPE-focused review.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-68202 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68202

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: IMPORTANT
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-2026-68202	IMPORTANT	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-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the ALSA sequencer queue timer destructor when a timer is closed and then re-opened by a borrower that already holds a queue reference. The stale timer instance can remain registered with callback_data pointing to a freed queue, and a later START can arm the timer so the next tick dereferences freed memory in snd_seq_timer_interrupt. For the CVSS the PR:L is used because an unprivileged local user with access to /dev/snd/seq can reach the bug without CAP and without owning the queue. The issue is not network reachable. Impact is at least a local denial of service via kernel crash and in worst case may allow privilege escalation due to a UAF on a kernel object.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	INIT UAF TIMER HARDWARE LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1. The commit explicitly says the bug is reachable by an unprivileged user with access to `/dev/snd/seq`, with no CAP and no queue ownership required.
* Privilege escalation plausible: +1 conservative, +2 paranoid. This is a kernel UAF with a stale timer instance and callback data pointing to a freed queue. There is no demonstrated reclaim or overwrite primitive in the patch, but stale callback based UAF is enough for manual LPE concern.
* Generic memory corruption: +2. The bug is a real use-after-free of an ALSA sequencer queue object.
* Real lifetime corruption: +1. The timer instance outlives the queue and retains callback state referencing freed memory.
* Reliable kernel crash / strong DoS: +1. The next timer tick dereferences the freed queue in `snd_seq_timer_interrupt()`.
* Hard or unreliable race / special timing required: -1. Triggering depends on reopening the timer in the destructor window.
* Broad/default/common subsystem exposure: +1. ALSA sequencer is a long-standing local kernel interface and commonly available on systems with sound support.

Conservative total: 6. Paranoid total: 7 by treating stale callback UAF as stronger LPE plausible.

## 3. Reachability analysis

A local user who can open `/dev/snd/seq` can trigger the vulnerable path. The commit explicitly states that no capability and no queue ownership are required. Containers or sandboxes matter mainly through device access: if `/dev/snd/seq` is exposed into a container, the trigger may be available from that reduced-privilege context. The path is not network reachable. Exploitation requires a race window during queue destruction, but the described sequence is concrete and involves normal ALSA sequencer operations rather than a rare hardware-only path.

Call-site confidence: high. The commit describes the destructor, the reopen window, the lingering timer instance, the global timer callback, and the freed queue dereference.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is a local unprivileged kernel UAF with a stale callback-data reference. Realistic demonstrated impact is kernel crash / local DoS. Theoretical impact includes privilege escalation because the freed object is later accessed asynchronously by a kernel timer callback, although the patch does not demonstrate attacker-controlled reclaim, overwrite, or object replacement. Therefore the conservative interpretation is Actionable Moderate, while the paranoid interpretation is a Strong Important candidate.

## 5. One-sentence report phrase

An unprivileged local user with access to `/dev/snd/seq` can race ALSA sequencer queue destruction to leave a live timer callback pointing at a freed queue, causing a kernel UAF that is at least a reliable local DoS and may require LPE-focused review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is not a pure NULL dereference or resource leak. It is a real local unprivileged UAF with stale asynchronous callback state, so LPE feasibility should be reviewed manually even if the immediate observed impact is a kernel crash.

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

Patch: ALSA: seq: close a re-opened queue timer in the destructor
Commit: fb40d03ed792a8a8bf77aa0ee15df57b0ff78b07
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb40d03ed792a8a8bf77aa0ee15df57b0ff78b07

Commit description:

queue_delete() closes the queue timer, then frees it. snd_seq_timer_close()
clears q->timer->timeri. snd_use_lock_sync() then drains borrowers, and
snd_seq_timer_delete() frees q->timer.

A borrower can re-open the timer inside that window. A SET_QUEUE_CLIENT
that took a queueptr() use_lock reference before the queue was unlinked
runs snd_seq_timer_open() after the close. Open refuses re-open only while
timeri is set, and the close just cleared it, so it re-opens timeri.

snd_seq_timer_delete() does not close that instance. Its snd_seq_timer_stop()
is a no-op, because running was cleared first. So it frees q->timer with the
instance still live. The queue is freed next.

The instance stays on the global timer with callback_data pointing at the
freed queue. A non-owner START on the unlocked queue arms it. The next tick
derefs the freed queue in snd_seq_timer_interrupt().

Reachable by an unprivileged user with access to /dev/snd/seq. No CAP and
no queue ownership required.

Close any lingering instance in the destructor. There, ->timeri can no
longer change: the queue is unlinked and all use_lock borrowers have
drained, so no snd_seq_queue_use() can re-open it. Close it before clearing
q->timer. snd_timer_close() waits for any in-flight snd_seq_timer_interrupt()
to finish, and that callback still reads q->timer (via snd_seq_check_queue()),
so q->timer must stay valid until it drains.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
Link: https://patch.msgid.link/422FDB81-2A68-47C7-A22D-2D3301E2E86D@doyensec.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/core/seq/seq_timer.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=fb40d03ed792a8a8bf77aa0ee15df57b0ff78b07

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

The original Linux kernel CVE announcement for CVE-2026-68202 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68202

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: 6
  Paranoid ActionableScore: 7

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).