[CVE-2026-98258][MODERATE 7.0] posix-cpu-timers: Prevent freeing a timer which is queued on the expiry list

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

CVE: CVE-2026-98258
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: posix-cpu-timers: Prevent freeing a timer which is queued on the expiry list
Commit: 9971dac1a77845ff467146915fcbb9170f6a8209
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9971dac1a77845ff467146915fcbb9170f6a8209
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98258
Analysis date: Sat, 10 Oct 2026 14:30:10 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98258 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98258

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-98258	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98258 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	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-672;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 free a struct k_itimer while the timer is still queued on the private expiry list. The expiry handler can later resume list traversal and read the freed elist.next member, resulting in a concrete use after free and at least a kernel crash. For the CVSS the PR:L is selected because an unprivileged local user can create and modify POSIX CPU timers for a process without requiring administrative capabilities. AV:L applies because local code execution is required and the issue is not network reachable. AC:H applies because exploitation depends on winning a timing race between expiry handling, timer_settime(), and timer deletion. Impact is at least a local denial of service via kernel crash. In the worst defensible case, reuse of the freed timer object during list traversal may affect confidentiality and integrity or support local privilege escalation, although reliable exploitation is not established by the available code.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	INIT RACE UAF TIMER HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* No floor or cap changed either score. The real UAF floor was checked, but the raw conservative score already reached 5.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF. `timer_delete()` can unhash and free the exact `struct k_itimer` while it remains queued on the expiry handlers private list. The handler subsequently reads the freed `elist.next`.
* +1 Local unprivileged trigger. An ordinary local process can create, modify, and delete its own POSIX CPU timers without host-administrator capabilities.
* +1 Reliable kernel crash or strong DoS. Traversal through a freed list node can produce an invalid pointer dereference, list corruption, Oops, or panic.
* +1 Attacker-influenced active-object lifetime. The same local process can create the timer and repeatedly drive expiration, `timer_settime()`, and deletion. These operations directly control the vulnerable lifetime transition.

Paranoid-only signal:

* +1 Structured stale-object exploitation leverage. The stale access consumes `elist.next`, an active list-traversal pointer, rather than merely reading passive metadata. Repeated timer creation and deletion provide a technically defensible reuse surface, although controlled reclaim is not established.

Non-awarded signals:

* Controlled reclaim, attacker-shaped replacement contents, arbitrary read or write, callback control, and credential modification are not demonstrated.
* No confidentiality or integrity point is awarded solely because the freed list node could theoretically be reused.
* No practical LPE-chain point is awarded. The supplied previous LLM assessment identifies possible privilege escalation only as a worst-case interpretation, not as demonstrated exploit evidence.
* No race penalty is applied. Although the race requires a specific interleaving, the attacker can repeatedly drive the relevant timer operations.
* No extra Availability point is awarded because it would double-count the same kernel crash.
* No concrete repeated release or refcount point is awarded. The timer is freed once and later traversed. The stale operation is not a second `put`, `free`, or refcount mutation.
* Paranoid repeated-refcount leverage is therefore withheld.
* B5-R does not apply because no retry-accumulated heap OOB write is present.
* B3-PENDING does not apply because no shared-backing write is present.
* B4-PENDING does not apply because no authorization decision is bypassed.

3. Reachability analysis

The evaluated attacker is an ordinary local user operating on POSIX CPU timers associated with the attackers own process or threads. Creating, arming, changing, and deleting those timers does not normally require UID 0, a capability, a user namespace, or host-administrator-equivalent authority.

No prior privileged setup, special hardware, device assignment, boot option, or unusual kernel control-plane configuration is identified. The path is local and not network reachable.

The complete trigger involves expiration handling racing with `timer_settime()` and timer deletion. The race has a specific three-way ordering, but the relevant operations can be attempted repeatedly by the same local process. This supports `HostAdminRequired=NO` and does not justify a timing penalty. Namespace-scoped privilege is not involved because the interface does not depend on obtaining a namespaced capability.

4. Primitive and severity interpretation

The exact primitive is a post-free traversal of an active structured kernel object. `timer_delete()` may free `struct k_itimer` while its embedded `elist` node remains owned by the expiry handlers private list. When expiry processing resumes, it reads `elist.next` from freed storage and may continue list processing using stale pointer state.

This is broader than an ordinary cleanup-only stale dereference because the post-lifetime use occurs in active expiry-list traversal. However, it is not a concrete repeated-release case. The first lifetime operation frees the timer, but the later operation reads and traverses the stale list node rather than performing a second lifetime or refcount mutation.

The patch-supported consequence is kernel memory-lifetime corruption with realistic crash or DoS. A stronger consequence involving allocator reuse, influenced `elist.next`, further memory accesses, or local privilege escalation is technically plausible but not demonstrated. There is no established arbitrary write, controlled reclaim, disclosure, or completed LPE chain.

The issue is CLASS_B because it establishes a real active-object UAF with meaningful unresolved pointer-traversal behavior. `ImportantCandidate=YES` and `ManualReview=REQUIRED` are appropriate because the complete post-reuse behavior of the list traversal needs exploitation-focused review. `AutoDowngradeAllowed=NO` because automatically treating it as an ordinary crash-only race would discard an unresolved below-host-admin active-object lifetime primitive.

5. One-sentence report phrase

An unprivileged local user can race POSIX CPU timer expiration, modification, and deletion so that the expiry handler traverses a freed `struct k_itimer` list node, causing a concrete kernel use-after-free with at least crash potential and unresolved post-reuse exploitation leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The bug is a below-host-admin active-object UAF involving traversal through a freed list pointer, while controlled reclaim and privilege-escalation feasibility remain unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: posix-cpu-timers: Prevent freeing a timer which is queued on the expiry list
Commit: 9971dac1a77845ff467146915fcbb9170f6a8209
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9971dac1a77845ff467146915fcbb9170f6a8209

Commit description:

Kijo analyzed another race in the POSIX CPU timer code:

Commit bf63568 converted cpu_timer::firing from a tristate value to a
boolean. This lost the distinction between "not owned by the firing list"
and "still owned, but delivery was canceled". The resulting race is:

    expiry handler              timer_settime()        timer_delete()
    --------------              ---------------        --------------
    collect timer onto
    private firing list
    firing = true
                                observes firing = true
                                firing = false
                                return TIMER_RETRY
                                wait for handler
                                                       observes firing = false
                                                       finish deletion
                                                       unhash and free timer
    resume list traversal
    read freed elist.next
    -> UAF

The firing bit is clearly the wrong indicator since that commit.

Check whether the timer is queued on the expiry list or not instead. If it
is queued clear the firing bit to prevent signal delivery as before and
return TIMER_RETRY so the caller unlocks the timer which allows the expiry
code to make progress and remove it from the list.

Fixes: bf63568 ("posix-cpu-timers: Cleanup the firing logic")
Reported-by: Kijo Park <red993688@gmail.com>
Debugged-by: Kijo Park <red993688@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Kijo Park <red993688@gmail.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Cc: stable@vger.kernel.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=9971dac1a77845ff467146915fcbb9170f6a8209

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

The original Linux kernel CVE announcement for CVE-2026-98258 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98258

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: 5
  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).