[CVE-2026-74504][MODERATE REGULAR] ALSA: seq: Fix division by zero in initialize_timer()

[CVE-2026-74504][MODERATE REGULAR] ALSA: seq: Fix division by zero in initialize_timer()

[AL-KERNEL]

CVE: CVE-2026-74504
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ALSA: seq: Fix division by zero in initialize_timer()
Commit: d0e19932875746118e298b4c974f3b2d4aeb16fc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0e19932875746118e298b4c974f3b2d4aeb16fc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74504
Analysis date: Sat, 15 Aug 2026 17:31:10 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline manual review recommended / local unprivileged DoS
Manual review required: YES

Summary:
An unprivileged local user with access to ALSA timer and sequencer devices can set a crafted SND_UTIMER resolution that overflows the timer period denominator and triggers a divide-by-zero in `initialize_timer()`, causing a kernel oops or CPU hang.

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

The original Linux kernel CVE announcement for CVE-2026-74504 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74504

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
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CVE-2026-74504	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-369;CWE-190;**CWE-400;BEST CVSS score: '5.5';DESCR 'A divide by zero can occur in ALSA sequencer timer initialization because a userspace driven SND_UTIMER can provide a very large timer resolution and the r * freq product can overflow to zero before the period division. A local user can trigger this through /dev/snd/timer and /dev/snd/seq when those ALSA device nodes are accessible, causing an oops while the timer lock is held and interrupts are disabled. For the CVSS the PR:L because an attacker needs local code execution and access to the ALSA timer and sequencer device interfaces, but no administrative privileges are required in typical desktop or audio enabled systems. The issue is not network reachable. Impact is denial of service via kernel crash or CPU hang. No confidentiality or integrity impact is assigned because the patch shows an arithmetic overflow leading to divide by zero, not a UAF, OOB access, information leak, or write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / local unprivileged DoS (with actual score 4)	YES	UAF HARDWARE LINUS  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH	YES	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 recommended / local unprivileged DoS**::

## 2. Signal breakdown

Triggered signals:

* Local unprivileged trigger: +1
  The commit explicitly states that an unprivileged user with access to `/dev/snd/timer` and `/dev/snd/seq` can trigger the issue.

* Reliable kernel crash / strong DoS: +1
  A user-controlled timer resolution can make `r * freq` overflow to zero and cause a divide-by-zero fault in `initialize_timer()`.

* Availability impact realistic: +1
  The fault occurs while `tmr->lock` is held with interrupts disabled, so the oops can leave the spinlock held and hang the CPU.

Paranoid-only signal:

* Broad/default/common subsystem exposure: +1
  ALSA timer and sequencer devices are commonly present on desktop or audio-enabled systems, although not universal on servers.

Not awarded:

* No generic memory corruption. The patch shows integer overflow followed by divide-by-zero, not UAF, OOB write, double free, type confusion, or arbitrary write.
* No privilege escalation plausible. There is no reclaim, overwrite, callback control, refcount takeover, or object confusion primitive.
* No confidentiality or integrity impact. No data disclosure or write primitive is supported by the patch.

## 3. Reachability analysis

A local user can trigger the bug if ALSA timer and sequencer device nodes are accessible. This is realistic on many desktop or audio-enabled systems, but less likely on minimal servers or systems without ALSA sequencer access.

Namespaces and containers may affect reachability only if `/dev/snd/timer` and `/dev/snd/seq` are passed through. The issue is not network reachable. No administrative capability is required in the described trigger path, but device-node permissions can block exploitation.

Call-site confidence: high. The commit gives the exact path through `snd_seq_write()`, `snd_seq_control_queue()`, `snd_seq_timer_start()`, and `initialize_timer()`.

## 4. Severity interpretation

This behaves like an actionable local Moderate issue, not an Important-class memory corruption vulnerability. The realistic impact is local denial of service through kernel oops or CPU hang. Theoretical escalation is not supported by the patch because the failure mode is arithmetic overflow into divide-by-zero, with no demonstrated memory corruption primitive.

The conservative score is 3. The paranoid score is 4 because ALSA device access is common enough on some systems to justify manual review rather than automatic closure.

## 5. One-sentence report phrase

An unprivileged local user with access to ALSA timer and sequencer devices can set a crafted SND_UTIMER resolution that overflows the timer period denominator and triggers a divide-by-zero in `initialize_timer()`, causing a kernel oops or CPU hang.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is not memory corruption and should not be treated as Important by default, but it is a local unprivileged, reliable kernel DoS on systems exposing common ALSA device nodes.

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

Patch: ALSA: seq: Fix division by zero in initialize_timer()
Commit: d0e19932875746118e298b4c974f3b2d4aeb16fc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0e19932875746118e298b4c974f3b2d4aeb16fc

Commit description:

A userspace-driven ALSA timer (SND_UTIMER) lets an unprivileged user set
the backing snd_timer's hardware resolution to an arbitrary 64-bit value
via SNDRV_TIMER_IOCTL_CREATE. snd_utimer_create() only rejects zero.

When such a timer is bound to a sequencer queue, initialize_timer()
computes the tick period as

	tmr->ticks = 1000000000 / (r * freq);

where r is that user-controlled resolution and freq is the sequencer
update rate in Hz, clamped to MIN_FREQUENCY..MAX_FREQUENCY (10..6250).
A resolution of 2^63 makes the 64-bit product r * freq wrap to zero for
any even freq, including DEFAULT_FREQUENCY (1000), so the division faults
with a divide-by-zero.

The division runs under tmr->lock with interrupts disabled, so the oops
leaves the spinlock held and hangs the CPU. It is reachable by an
unprivileged user with access to /dev/snd/timer and /dev/snd/seq.

  Oops: divide error: 0000 [#1] SMP KASAN PTI
  CPU: 7 UID: 1000 PID: 456 Comm: alsa_seq_utimer Not tainted 7.2.0-rc4+
  RIP: 0010:initialize_timer.constprop.0+0x20a/0x2d0
   snd_seq_timer_start+0x15e/0x2b0
   snd_seq_control_queue+0x56f/0xba0
   snd_seq_write+0x3e0/0x730

Reject an overflowing product with check_mul_overflow() and fall back to
a single tick, which also avoids feeding a wrapped-but-nonzero divisor
(e.g. 2^63 * 1000 mod 2^64 == 0, or other resolutions wrapping to a small
value) into the period computation.

Fixes: 3774591 ("ALSA: timer: Introduce virtual userspace-driven timers")
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/DF8A3844-AD5E-4B8A-9CFC-BD83C212BA38@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=d0e19932875746118e298b4c974f3b2d4aeb16fc

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

The original Linux kernel CVE announcement for CVE-2026-74504 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74504

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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).