[CVE-2025-38352][MODERATE 7.0] posix-cpu-timers: fix race between handle_posix_cpu_timers() and posix_cpu_timer_del()

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

CVE: CVE-2025-38352
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: posix-cpu-timers: fix race between handle_posix_cpu_timers() and posix_cpu_timer_del()
Commit: 78a4b8e3795b31dae58762bc091bb0f4f74a2200
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=78a4b8e3795b31dae58762bc091bb0f4f74a2200
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38352
Analysis date: Wed, 16 Sep 2026 13:50:50 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A race in POSIX CPU timer handling can allow timer processing to continue on an exiting task while concurrent timer deletion misses the firing state, creating a local kernel UAF candidate that can crash the system and requires manual review for possible privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2025-38352 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38352

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2025-38352	MODERATE	https://www.kernelcve.org/list/?q=CVE-2025-38352 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:H/I:H/A:H';CWE-362;*CWE-416;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A race in POSIX CPU timer handling can allow handle_posix_cpu_timers() to run on an exiting task while posix_cpu_timer_del() concurrently misses the active firing state. The problematic window appears after exit_notify() and unlock_task_sighand(), where the task may be reaped by a parent or debugger while timer handling is still in progress. This is a local race and UAF candidate rather than a pure warning because it involves task lifetime and concurrent timer deletion around release_task(). For the CVSS the PR:L value is used because a local user or local process is needed to create the relevant task and timer activity, but full administrative privileges are not required. AC:H is used because reliable triggering depends on a narrow timing window. The issue is not network reachable. Impact is at least local denial of service via kernel crash and in the worst case may allow confidentiality and integrity impact due to kernel use after free, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) 	MAYBE	DANGER INIT SIMPLEFIX RACE TIMER HARDWARE LINUS KPANIC 	-	-	checked

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

ActionableScore=6
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 3
* Conservative score: 4
* Applied conservative floor/cap: Real UAF floor 4 applied because below-host-admin reachability is established and the issue is a real task/timer lifetime violation.
* Raw paranoid score: 6
* Paranoid score: 6
* Applied paranoid floor/cap: none.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

## 2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation. The patch comment explicitly says `release_task(tsk)` must not happen while `handle_posix_cpu_timers()` is running. This directly establishes a task/timer lifetime race.
* +1 Reliable kernel crash / strong DoS. A stale task or timer state in POSIX CPU timer IRQ handling can realistically crash the kernel.
* +1 Local unprivileged trigger. A local user can create processes and POSIX CPU timer activity without host-admin privileges.
* -1 Hard timing window. The conservative path treats the window after `exit_notify()` and `unlock_task_sighand()` with concurrent `posix_cpu_timer_del()` as narrow and timing-sensitive.

Paranoid additional signals:

* +1 Attacker-influenced repeatable lifetime manipulation. A local attacker can repeatedly create task and timer lifecycles and attempt the exit/delete timing sequence.
* +1 Weak LPE concern. This is more than a generic UAF label because the stale lifetime involves active task and POSIX timer state around `release_task()` and concurrent timer deletion. Supplied context also indicates possible crash or privilege escalation, but no controlled reclaim or arbitrary R/W is established.

Non-awarded signals:

* Strong exploitation leverage +2 not awarded. Controlled reclaim, attacker-shaped replacement object, arbitrary write, callback control, or credential manipulation are not demonstrated.
* Concrete repeated release/refcount +1 not awarded. The patch does not establish a first put/free followed by a second concrete lifetime/refcount mutation on the exact same dangling reference.
* Structured stale-object leverage +1 not separately awarded. The stale object is active and structured, but no distinct post-reuse field-consumption primitive beyond the lifetime race is shown.
* Extra availability point not awarded. The same kernel crash/DoS is already counted once.
* Confidentiality and integrity points are not awarded directly. Possible LPE remains an unresolved concern, not a demonstrated C/I primitive.

Repeated-release/refcount checklist:

* First release can end lifetime: `release_task(tsk)` can end task lifetime, but not as a visible first release in a double-release sequence.
* Exact same reference survives to a second lifetime mutation: not established.
* Second operation mutates lifetime/refcount: not established.
* Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 from repeated-release rule applied: no.

## 3. Reachability analysis

The evaluated attacker is a local user or local process. Host-admin privileges are not required to create ordinary processes and POSIX CPU timer activity. The issue is not network reachable.

Namespace-scoped privilege is not relevant here because the trigger is not based on a namespace capability such as `CAP_NET_ADMIN`. No prior privileged host setup is required beyond a kernel configuration where this vulnerable POSIX CPU timer path exists. The commit notes that `CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y` reduces or avoids the vulnerable ordering because `exit_task_work()` runs before `exit_notify()`, but the guard is still valid.

The race is timing-sensitive because it requires an exiting non-autoreaping task after `exit_notify()`, IRQ-side CPU timer handling, and concurrent `posix_cpu_timer_del()`. However, repeated local attempts are plausible through repeated process and timer lifecycle operations.

## 4. Primitive and severity interpretation

The exact primitive is a local race causing a task/POSIX CPU timer lifetime mismatch. `handle_posix_cpu_timers()` may continue while `release_task(tsk)` becomes possible, and a concurrent `posix_cpu_timer_del()` may fail to observe `timer->it.cpu.firing`.

This is an active structured lifetime bug, not fixed stale storage. It is not a simple NULL dereference or warning-only issue. The demonstrated consequence is at least local kernel crash/DoS. The speculative consequence is privilege escalation through kernel UAF exploitation, but the supplied patch does not demonstrate controlled reclaim, arbitrary write, or a complete LPE chain.

PrimitiveClass is CLASS_B because a real kernel lifetime/UAF primitive is established, but CLASS_A exploitation primitives are not. ImportantCandidate is YES because below-host-admin active kernel UAF with unresolved exploitation leverage should not be automatically downgraded. AutoDowngradeAllowed is NO because automatic closure would lose a meaningful unresolved lifetime-corruption primitive.

Likely operational severity is actionable Moderate with Strong Important review sensitivity.

## 5. One-sentence report phrase

A race in POSIX CPU timer handling can allow timer processing to continue on an exiting task while concurrent timer deletion misses the firing state, creating a local kernel UAF candidate that can crash the system and requires manual review for possible privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: real local kernel lifetime/UAF behavior is directly implied, and the supplied context indicates possible privilege escalation even though a complete exploitation primitive is not demonstrated.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: posix-cpu-timers: fix race between handle_posix_cpu_timers() and posix_cpu_timer_del()
Commit: 78a4b8e3795b31dae58762bc091bb0f4f74a2200
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=78a4b8e3795b31dae58762bc091bb0f4f74a2200

Commit description:

If an exiting non-autoreaping task has already passed exit_notify() and
calls handle_posix_cpu_timers() from IRQ, it can be reaped by its parent
or debugger right after unlock_task_sighand().

If a concurrent posix_cpu_timer_del() runs at that moment, it won't be
able to detect timer->it.cpu.firing != 0: cpu_timer_task_rcu() and/or
lock_task_sighand() will fail.

Add the tsk->exit_state check into run_posix_cpu_timers() to fix this.

This fix is not needed if CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y, because
exit_task_work() is called before exit_notify(). But the check still
makes sense, task_work_add(&tsk->posix_cputimers_work.work) will fail
anyway in this case.

Cc: stable@vger.kernel.org
Reported-by: Benoît Sevens <bsevens@google.com>
Fixes: 0bdd2ed ("sched: run_posix_cpu_timers: Don't check ->exit_state, use lock_task_sighand()")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/time/posix-cpu-timers.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=78a4b8e3795b31dae58762bc091bb0f4f74a2200

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

The original Linux kernel CVE announcement for CVE-2025-38352 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2025-38352

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: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: 4
  Paranoid ActionableScore: 6

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).