[CVE-2026-64560][MODERATE 7.0] posix-cpu-timers: Prevent UAF caused by non-leader exec() race

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

CVE: CVE-2026-64560
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: posix-cpu-timers: Prevent UAF caused by non-leader exec() race
Commit: ad1cafa1bdaa71da85d71cac053838bbe97852b6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ad1cafa1bdaa71da85d71cac053838bbe97852b6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64560
Analysis date: Wed, 29 Jul 2026 13:51:11 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: **Actionable Moderate at minimum. Strong Important candidate only under paranoid UAF triage.
Manual review required: YES

Summary:
A local race in POSIX CPU timer handling during non-leader exec can leave an armed timer queued after its `k_itimer` is freed, creating a kernel UAF that can cause DoS and warrants manual review for possible LPE impact.

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

The original Linux kernel CVE announcement for CVE-2026-64560 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64560

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-2026-64560	MODERATE	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 race in POSIX CPU timer handling can leave an armed TGID targeted timer queued while sys_timer_delete or posix_cpu_timer_set loses the task sighand during non-leader exec. The k_itimer object can then be freed while its timerqueue node remains reachable from run_posix_cpu_timers or other timerqueue add and delete paths, creating a use-after-free condition in kernel context. For the CVSS the PR:L is used because a local unprivileged process can create POSIX CPU timers, use threads, exec from a non-leader, and race timer delete or set operations. The issue is not network reachable. Impact is at least local denial of service via kernel crash. For the paranoid score, choose the highest still-defensible interpretation supported by the bug class and patch context, with preference for manual-review sensitivity over autoclosed false negatives. Because this is a real UAF in timerqueue state, worst case impact may include privilege escalation, so CIA:H,H,H is defensible for manual triage.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	LOCK INIT RACE UAF TIMER HARDWARE LINUS KPANIC  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	guess

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: **4**
* Paranoid score: **5**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**:: **Actionable Moderate at minimum. Strong Important candidate only under paranoid UAF triage.**

## 2. Signal breakdown

Conservative signals:

* **Local unprivileged trigger: +1**
  POSIX CPU timers, threads, `exec()`, `timer_delete()`, `timer_settime()`, and nanosleep-related paths are reachable by local user processes without root.

* **Memory corruption, weak race-UAF primitive: +1**
  The commit explicitly describes a UAF where a freed `k_itimer` can remain reachable through its `timerqueue` node. However, no attacker-controlled reclaim, type confusion, callback dispatch, arbitrary write, or controlled object replacement is shown.

* **Real lifetime corruption: +1**
  The bug is a true lifetime issue: the timer object can be freed while still logically queued or reachable from POSIX CPU timer machinery.

* **Reliable kernel crash / strong DoS: +1**
  Access to a freed timerqueue node from `run_posix_cpu_timers()` or timerqueue add/delete operations can realistically lead to kernel crash.

* **Broad/default/common subsystem: +1**
  POSIX CPU timers and process/thread exit/exec handling are core kernel facilities, not a rare driver or special hardware path.

Negative signals:

* **Hard race / special timing required: -1**
  Triggering requires a race involving non-leader `exec()`, leader switching, task teardown, and concurrent timer delete/set/rearm paths.

Paranoid interpretation:

* The same facts justify raising the practical triage score to **5**, because this is not a mere NULL dereference or warning. It is a real UAF in core process/timer infrastructure, and local UAFs historically deserve manual review even when no exploit primitive is demonstrated.

## 3. Reachability analysis

A local unprivileged user can plausibly reach the affected logic by creating POSIX CPU timers, using multithreaded processes, triggering non-leader `exec()`, and racing timer delete or set operations. No network reachability is indicated. Namespaces do not appear to be required for reachability, and they do not materially reduce the local nature of the bug. The affected functionality is part of normal kernel process and POSIX timer handling, so exposure is broad, but reliable exploitation depends on hitting a narrow race window.

## 4. Severity interpretation

This is above an ordinary Moderate because the patch and commit message explicitly identify a UAF involving `k_itimer` and timerqueue state. Realistically, the most demonstrated impact is local DoS through kernel crash. Theoretical LPE concern exists because this is kernel UAF in a broadly reachable subsystem, but the patch does not show attacker-controlled reclaim, controlled overwrite, callback control, or a clear arbitrary write primitive. Therefore the conservative result is borderline/manual-review, while the paranoid result is Actionable Moderate and should not be auto-closed.

## 5. One-sentence report phrase

A local race in POSIX CPU timer handling during non-leader exec can leave an armed timer queued after its `k_itimer` is freed, creating a kernel UAF that can cause DoS and warrants manual review for possible LPE impact.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the issue is a real kernel UAF in core POSIX CPU timer lifetime handling, locally reachable by unprivileged users, with at least reliable crash potential and plausible but unproven privilege-escalation relevance.

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

Patch: posix-cpu-timers: Prevent UAF caused by non-leader exec() race
Commit: ad1cafa1bdaa71da85d71cac053838bbe97852b6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ad1cafa1bdaa71da85d71cac053838bbe97852b6

Commit description:

Wongi and Jungwoo decoded and reported a non-leader exec() related race
which can result in an UAF:

 sys_timer_delete()			exec()
   posix_cpu_timer_del()
   // Observes old leader
   p = pid_task(pid, pid_type);		de_thread()
   					  switch_leader();
					  release_task(old_leader)
					    __exit_signal(old_leader)
					      sighand = lock(old_leader, sighand);
					      posix_cpu_timers*_exit();
   sighand = lock_task_sighand(p)	      unhash_task(old_leader);
     sh = lock(p, sighand)	    	      old_leader->sighand = NULL;
					      unlock(sighand);
     (p->sighand == NULL)
	unlock(sh)
	return NULL;

   // Returns without action
   if(!sighand)
      return 0;
   free_posix_timer();

This is "harmless" unless the deleted timer was armed and enqueued in
p->signal because on exec() a TGID targeted timer is inherited.

As sys_timer_delete() freed the underlying posix timer object
run_posix_cpu_timers() or any timerqueue related add/delete operations on
other timers will access the freed object's timerqueue node, which results
in an UAF.

There is a similar problem vs. posix_cpu_timer_set(). For regular posix
timers it just transiently returns -ESRCH to user space, but for the use
case in do_cpu_nanosleep() it's the same UAF just that the k_itimer is
allocated on the stack.

Also posix_cpu_timer_rearm() fails to rearm the timer, which means it stops
to expire.

While debating solutions Frederic pointed out another problem:

   posix_cpu_timer_del(tmr)
					__exit_signal(p)
					  posix_cpu_timers*_exit(p);
					  unhash_task(p);
					  p->sighand = NULL;
     sh = lock_task_sighand(p)
        sighand = p->sighand;
	if (!sighand)
	    return NULL;
	lock(sighand);

     if (!sh)
	WARN_ON_ONCE(timer_queued(tmr));

On weakly ordered architectures it is not guaranteed that
posix_cpu_timer_del() will observe the stores in posix_cpu_timers*_exit()
when p->sighand is observed as NULL, which means the WARN() can be a false
positive.

Solve these issues by:

  1) Changing the store in __exit_signal() to smp_store_release().

  2) Adding a smp_acquire__after_ctrl_dep() into the !sighand path
     of lock_task_sighand().

  3) Creating a helper function for looking up the task and locking sighand
     which does not return when sighand == NULL. Instead it retries the
     task lookup and only if that fails it gives up.

  4) Using that helper in the three affected functions.

observes all preceeding stores, i.e. the stores in posix_cpu_timers*_exit()
and the ones in unhash_task().

gracefully. When the task lookup returns the old leader, but sighand ==
NULL then it retries. In the non-leader exec() case the subsequent task
lookup will observe the new leader due to #1/#2. In normal exit() scenarios

[Commit description truncated; see the upstream URL below]

Changed files:
  kernel/exit.c
  kernel/signal.c
  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=ad1cafa1bdaa71da85d71cac053838bbe97852b6

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

The original Linux kernel CVE announcement for CVE-2026-64560 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64560

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: 4
  Paranoid ActionableScore: 5

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).