[CVE-2026-98260][MODERATE 7.0] exec: Cleanup POSIX timers right after de_thread()

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

CVE: CVE-2026-98260
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: exec: Cleanup POSIX timers right after de_thread()
Commit: 6f1977cea3e85cd8ab55fb337d3e1725fe61d1ce
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f1977cea3e85cd8ab55fb337d3e1725fe61d1ce
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98260
Analysis date: Sat, 10 Oct 2026 14:48:57 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98260 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98260

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-2026-98260	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98260 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:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A lifecycle race in POSIX CPU timer handling can leave a timer node queued on a non-leader thread after de_thread changes ownership of its TID. If exec later fails or is interrupted after the point of no return, exit_itimers can free the k_itimer while its node remains in the task timer tree. Reaping the task later passes the stale node to timerqueue_del and causes a use-after-free during rbtree cleanup. The confirmed impact is a local kernel crash. The stale rbtree node also provides a plausible memory corruption condition that may permit privilege escalation if the freed allocation is reused with attacker influenced contents. For the CVSS the PR:L value is selected because an unprivileged local process can create threads and POSIX CPU timers and invoke execve without administrative capabilities. AC:H is selected because reliable triggering depends on reaching a later exec failure path or fatal signal window after the TID exchange. The issue is not network reachable. Impact is at least a local denial of service via kernel crash and in the worst case may affect confidentiality and integrity through kernel memory corruption.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	READ LOCK DISK INIT UAF TIMER HARDWARE LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=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: 5
* Paranoid score: 5
* No floor or cap changed either score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Positive signals:

* +2 Real UAF. `exit_itimers()` frees the `k_itimer` while its timerqueue node remains linked in the task RB tree. Later task reaping passes that freed node to `timerqueue_del()`.
* +1 Local unprivileged trigger. Creating per-thread POSIX CPU timers, executing from a non-leader thread, sending a permitted signal, and reaping a same-user child do not require host-administrator privileges.
* +1 Attacker-influenced lifetime sequence. The process controls timer creation and arming, the non-leader `execve()`, and repeated attempts to reach the post-`de_thread()` failure window.
* +1 Strong DoS. Consuming the freed RB-tree node during cleanup can corrupt timerqueue bookkeeping or cause a kernel fault and host denial of service.

Non-awarded signals:

* Controlled reclaim or attacker-shaped replacement contents are not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* Strong LPE leverage is not awarded merely because `timerqueue_del()` processes a freed node.
* Structured stale-object leverage is not separately awarded because meaningful control over post-free contents or replacement is not established.
* No extra Availability point is awarded for the same kernel crash.
* No Confidentiality or Integrity point is awarded from theoretical allocator reuse.
* No race penalty is applied. Timing is required, but the attacker controls the surrounding operations and can create new processes for repeated attempts. The evidence does not establish a materially uncontrollable one-shot race.
* This is not a concrete repeated release or refcount case. The first operation frees `k_itimer`, but the later operation is RB-tree deletion rather than a second `put`, `free`, or refcount mutation.
* The dedicated repeated-release +1 was not awarded.
* The paranoid repeated-refcount +1 was not awarded.
* FLOOR 5 was not applied. The raw score already reaches 5.
* NEW52 B5-R does not apply because this is not a retry-accumulated heap OOB write.
* B3-PENDING and B4-PENDING do not apply because there is no shared-backing write or authorization decision bypass.

3. Reachability analysis

An ordinary local user can create threads, arm a `CLOCK_THREAD_CPUTIME_ID` timer, invoke `execve()` from a non-leader thread, and use a same-UID parent to signal and reap the process. No capability or UID 0 requirement is visible in the complete described trigger.

No namespace-scoped capability is needed, so namespace delegation is not part of the attack model. No prior privileged configuration, special hardware, device assignment, or network access is required. POSIX timers must be enabled in the kernel.

The trigger depends on failure or interruption after `de_thread()` and before the original timer cleanup point. This creates a timing requirement, but the operations are locally repeatable and the supplied sequence provides a concrete trigger rather than a purely hypothetical race.

4. Primitive and severity interpretation

The exact primitive is an active structured-object UAF. A `k_itimer` is freed while its embedded timerqueue node remains linked in a live RB tree. Later cleanup treats the stale node as valid and performs `timerqueue_del()`.

This is more significant than a fixed stale-buffer read because RB-tree deletion traverses and modifies active tree bookkeeping using a post-lifetime node. It is nevertheless CLASS_B rather than CLASS_A because controlled reclaim, attacker-controlled replacement data, a directed write, callback control, information disclosure, and demonstrated privilege escalation are not established.

This is not category B concrete repeated lifetime corruption as defined for a second `put` or refcount mutation. It is a stale structured cleanup operation after a free. The demonstrated consequence is kernel crash or tree corruption. A stronger local privilege-escalation consequence remains plausible but unproven.

ImportantCandidate is YES because the bug is below-host-admin reachable and the stale use performs nontrivial mutation of a live kernel RB tree rather than only an immediate invalid read. Manual analysis is required to determine allocator reuse opportunities and whether stale RB metadata can be shaped into a useful write primitive.

AutoDowngradeAllowed is NO because automatically reducing the issue to ordinary crash-only Moderate would discard unresolved exploitation leverage from the freed node being consumed by an active tree-deletion operation. The likely operational severity is actionable Moderate, with Important classification dependent on exploitation analysis.

5. One-sentence report phrase

A local unprivileged user can trigger a POSIX CPU timer use-after-free in which task reaping removes an already freed timer node from an active RB tree, causing kernel crash or tree corruption and requiring manual review for possible privilege-escalation leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The UAF is concretely established, reachable below host-admin privilege, and consumed by an RB-tree mutation, but controlled reclaim and a useful non-crash write primitive remain unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: exec: Cleanup POSIX timers right after de_thread()
Commit: 6f1977cea3e85cd8ab55fb337d3e1725fe61d1ce
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6f1977cea3e85cd8ab55fb337d3e1725fe61d1ce

Commit description:

A per-thread CPU timer holds a reference to the PID of the thread it is
attached to and, while it is armed, its node is queued in that thread's
posix_cputimers. The task is looked up by that PID.

When a non-leader thread exec()s, de_thread() changes which task owns
that PID. pid_task(timer->it.cpu.pid, PIDTYPE_PID) then returns NULL,
but the node is still queued on tsk, which is alive. timer_lock_sighand()
takes a failed lookup to mean that the node is already dequeued, so it
has nothing to undo.

begin_new_exec() calls posix_cpu_timers_exit(me) right after
exec_task_namespaces() and that removes the leftover node, so the state
normally stays invisible. But bprm->point_of_no_return is set before
de_thread(), so if unshare_files(), set_mm_exe_file(), exec_mmap() or
exec_task_namespaces() fails, the task dies before it gets there.
exit_itimers() then frees the k_itimer while its node is still queued,
and reaping tsk later erases that freed node from the rbtree.

In short:

      the non-leader thread B           the parent

  timer_create(CLOCK_THREAD_CPUTIME_ID)
  timer_settime()
    arm_timer()            // the node is queued on B
  execve()
    de_thread(B)
      exchange_tids(B, leader)  // B's PID now belongs to the leader
      release_task(leader)
        __exit_signal(leader)
          posix_cpu_timers_exit(leader)  // cleans leader's queue, not B's
          __unhash_process(leader)  // that PID has no task anymore
    exec_mmap()
      mmap_read_lock_killable(old_mm)
                                kill(B, SIGKILL)
      // -EINTR
  get_signal()
    do_exit()
      exit_itimers()
        posix_timer_delete()
          posix_cpu_timer_del()
        posix_timer_unhash_and_free()  // freed while still queued
                                wait4()
                                  release_task(B)
                                    posix_cpu_timers_exit(B)
                                      cleanup_timerqueue()
                                        timerqueue_del()  // use-after-free

Move the POSIX timer cleanup right after de_thread() before any of the
later failure conditions brings the task into do_exit().

[ tglx: Move the cleanup right after de_thread() ]

Fixes: 55e8c8e ("posix-cpu-timers: Store a reference to a pid not a task")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Kijo Park <red993688@gmail.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/ao7Q8miiuLAPVnWv@v4bel
Link: https://patch.msgid.link/20260911090541.627712075@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/exec.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=6f1977cea3e85cd8ab55fb337d3e1725fe61d1ce

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

The original Linux kernel CVE announcement for CVE-2026-98260 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98260

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: unknown
  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).