[CVE-2026-68200][IMPORTANT] ALSA: timer: don't re-enter an instance callback that is still running

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

CVE: CVE-2026-68200
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ALSA: timer: don't re-enter an instance callback that is still running
Commit: 996c24377eea4d4506b7c3ccbbf1e490440b5e0b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=996c24377eea4d4506b7c3ccbbf1e490440b5e0b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68200
Analysis date: Mon, 10 Aug 2026 11:56:23 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in the ALSA userspace timer TRIGGER ioctl can re-enter an in-flight timer callback and allow the close path to free the timer instance or callback_data while another callback is still running, causing a local kernel UAF with DoS impact and plausible LPE risk.

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

The original Linux kernel CVE announcement for CVE-2026-68200 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68200

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-2026-68200	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 timer path because the userspace driven timer TRIGGER ioctl can call snd_timer_interrupt concurrently for the same timer instance. Two local threads can re enter an instance callback while the previous callback is still running, and the single SNDRV_TIMER_IFLG_CALLBACK bit cannot represent both in flight callbacks. The close path may then observe the bit as cleared and free the timer instance or callback_data while another callback is still live. For the CVSS the PR:L is used because the attacker needs local user access and permission to open /dev/snd/timer, but root privileges are not required in typical affected configurations. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the primitive is a kernel use after free.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	UAF TIMER HARDWARE LINUS KPANIC 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative scoring:

* Local unprivileged trigger: +1. The bug is reachable by a local user able to open `/dev/snd/timer`.
* Memory corruption, weak race UAF primitive: +1. The patch and commit describe a real use-after-free, but no controlled reclaim or overwrite primitive is demonstrated.
* Real lifetime corruption: +1. The instance and `callback_data` can be freed while another callback is still running.
* Reliable kernel crash / strong DoS: +1. A live callback using freed state can plausibly crash the kernel.
* Broad/common subsystem exposure: +1. ALSA timer support and `/dev/snd/timer` are common on many Linux systems, especially desktop and multimedia-enabled systems.
* Hard or unreliable race / special timing required: -1. Triggering requires concurrent timer trigger and close/drain timing.

Paranoid scoring difference:

* Stronger LPE plausibility: +2 instead of weak concern. This is not just a NULL dereference. It is a kernel UAF involving callback lifetime and freed callback data, so privilege escalation is technically plausible and should not be auto-closed without review.
* Generic memory corruption treated as stronger: +2 instead of +1. The callback-data lifetime violation is more concerning than a simple stale read, although exploitability is still race-dependent.

## 3. Reachability analysis

The trigger is local. An attacker needs local code execution and access to `/dev/snd/timer`, but does not need root in typical affected configurations where sound devices are user-accessible. Containers and sandboxes matter because device-node exposure is the key gate. If `/dev/snd/timer` is not passed into a container, the containerized attacker likely cannot reach this path. If the device is exposed to a reduced-privilege user or service account, PR should remain effectively low rather than high.

The issue is not network reachable. It is triggered through the ALSA timer ioctl path, specifically userspace-driven timer triggering. The path is not universally exposed on headless servers, but it is common enough that it should not be treated as rare hardware-only code.

Call-site confidence: high. The commit message explicitly identifies `snd_timer_interrupt()`, `snd_timer_process_callbacks()`, `snd_timer_close_locked()`, `SNDRV_TIMER_IFLG_CALLBACK`, and the freeing of the instance and `callback_data` while another callback remains live.

## 4. Severity interpretation

This behaves above ordinary Moderate because it is a real local kernel UAF reachable through a user-facing device interface. The conservative interpretation is Actionable Moderate because exploitability depends on a race and no controlled reclaim or write primitive is shown. The paranoid interpretation is a Strong Important candidate because callback lifetime UAFs can sometimes be developed beyond DoS, and the patch context explicitly describes freed callback data still being used by a live callback.

Theoretical memory corruption potential exists and is meaningful. Realistic exploitation evidence is not demonstrated in the patch, so the main practical impact remains local DoS unless further exploit primitives are confirmed.

## 5. One-sentence report phrase

A race in the ALSA userspace timer TRIGGER ioctl can re-enter an in-flight timer callback and allow the close path to free the timer instance or callback_data while another callback is still running, causing a local kernel UAF with DoS impact and plausible LPE risk.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a local race-based kernel UAF reachable from a user-accessible ALSA timer interface. Even though reliable privilege escalation is not demonstrated, the callback lifetime primitive is strong enough that it should not be auto-closed.

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

Patch: ALSA: timer: don't re-enter an instance callback that is still running
Commit: 996c24377eea4d4506b7c3ccbbf1e490440b5e0b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=996c24377eea4d4506b7c3ccbbf1e490440b5e0b

Commit description:

The userspace-driven timer (utimer) TRIGGER ioctl calls
snd_timer_interrupt() directly with no serialization, so two threads
triggering the same utimer can run snd_timer_interrupt() on one
snd_timer concurrently.

snd_timer_process_callbacks() drops timer->lock around each instance
callback and marks the in-flight callback with the single
SNDRV_TIMER_IFLG_CALLBACK bit; snd_timer_close_locked() waits on that
bit to drain an in-flight callback before freeing the instance. The bit
cannot represent two concurrent callbacks: when a second interrupt
re-queues an instance whose callback is still running, both run at once,
the first to finish clears the bit, and the close-path drain then frees
the instance (and its callback_data) while the other callback is still
live - a use-after-free reachable by any user able to open
/dev/snd/timer, both via a user timer instance and via a sequencer queue
timer bound to the utimer.

snd_timer_interrupt() sets IFLG_CALLBACK before dropping timer->lock, so
a concurrent interrupt already observes it under the lock. Skip
re-queuing an instance (and its slaves) to the ack/sack list while its
callback is in flight; the accumulated pticks are delivered on the next
tick, so no event is lost.

Fixes: 3774591 ("ALSA: timer: Introduce virtual userspace-driven timers")
Cc: stable@vger.kernel.org
Suggested-by: Takashi Iwai <tiwai@suse.de>
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Norbert Szetei <norbert@doyensec.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/6F9B6501-8E65-4265-B02C-7EFB240D1664@doyensec.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  sound/core/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=996c24377eea4d4506b7c3ccbbf1e490440b5e0b

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

The original Linux kernel CVE announcement for CVE-2026-68200 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68200

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: 5
  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).