[CVE-2026-98530][LOW] kprobes: Fix permanent hang when flushing the kprobe optimizer

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

CVE: CVE-2026-98530
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: kprobes: Fix permanent hang when flushing the kprobe optimizer
Commit: f05a01fc7d4f731b4c100d2aef1e85ad77728862
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f05a01fc7d4f731b4c100d2aef1e85ad77728862
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98530
Analysis date: Sat, 24 Oct 2026 07:36:03 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
A host administrator can trigger a permanent kprobe-optimizer flush wait that blocks later optimization-sysctl accesses, without demonstrated memory corruption or security-boundary bypass.

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

The original Linux kernel CVE announcement for CVE-2026-98530 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98530

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-98530	LOW	https://www.kernelcve.org/list/?q=CVE-2026-98530 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=LOW RESULT=DIFF TEMP_DISTANCE_MERGE_SKIP=AEGIS_NO_POSITIVE_SEVERITY_EVIDENCE (final Legacy and Aegis differ; FINAL: LOW, Aegis: 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:N/I:N/A:L';CWE-362;CWE-667;*CWE-833;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '2.5';DESCR 'A synchronization flaw in kprobe optimizer flushing can cause a permanent lost wakeup. A single flusher may already wait forever because kprobe_optimizer only called complete when the completion was already done. Concurrent sysctl and debugfs flushers can also reinitialize the shared completion and remove an earlier waiter from its queue. The blocked process remains in D state and the sysctl writer retains kprobe_sysctl_mutex, causing later reads or writes of kprobes-optimization to hang. This is a local availability issue with no demonstrated UAF, out of bounds access, information leak, or memory corruption primitive. For the CVSS the PR:H value reflects that configuring and enabling kprobes and writing the relevant controls normally requires administrative tracing privileges. The paranoid PR:L alternative covers environments where tracing control is delegated to a restricted service or reduced capability administrator. AC:H reflects the concurrent flusher case and timing window, although the incorrect completion condition may also make a single flush hang reliably. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 2.5 and the production ActionableScore returned 0 with recommendation Ordinary Moderate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. ActionableScore thresholds stage MODERATE to LOW because the ActionableScore threshold rule was applied. final reconciliation stage LOW to LOW because KP specialist did not establish a qualifying panic and the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS and DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 and KPANIC_UNSUPPORTED_REMOVED and V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED. The independent weaker Aegis shadow evaluated CVSS 2.2 and ActionableScore 1 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is LOW.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)	MAYBE	OOB LOCK INIT DEBUGFS LINUS DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3 KPANIC_UNSUPPORTED_REMOVED V64_AEGIS_UNSUPPORTED_PROMOTION_BLOCKED	-	-	checked

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

ActionableScore=0
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: -1.
* Conservative score: 0 after clamping the negative result to the minimum score.
* Raw paranoid score: -1.
* Paranoid score: 0 after clamping; no technically supported stronger primitive or lower-privilege complete trigger changes the arithmetic.
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Availability impact: the incorrect completion handling permanently blocks the flushing writer in D state and leaves later accesses to the optimization sysctl blocked.
* -2 Proven host-administrator-equivalent trigger: the demonstrated path configures/enables kprobes and writes the root-controlled kprobes optimization sysctl. No complete below-host-admin path is established.
* No race penalty: although concurrent flushers can lose a waiter, the inverted `completion_done()` condition can make a single flush hang; hard timing is not required for the basic defect.
* No memory-corruption, UAF, OOB, stale-object, authorization-bypass, disclosure, integrity, or host-crash signal is shown.
* NEW52/B5-R does not apply: there is no heap OOB write, retry-accumulated offset, or later consumption of corrupted structured state.
* No extra availability point: the blocked tasks and inaccessible sysctl are the same availability consequence.

3. Reachability analysis

The supplied reproducer uses an administrator to create and enable a kprobe and then write the optimization sysctl. The complete path is not shown to work through a user namespace or namespace-scoped capability. Delegated tracing permissions are conceivable but do not establish delegated access to every required operation, particularly the sysctl write. Numeric host UID 0 is therefore not proven as an invariant requirement, while effective host-administrator powers are shown for the supplied path. No special hardware is required.

4. Primitive and severity reading

The defect is a lost-wakeup/permanent-wait synchronization failure: the optimizer conditionally completes an unfinished completion, and concurrent waiters can reinitialize the shared completion. The fix replaces it with a monotonically increasing pass counter and wait-variable signaling.

There is no active or fixed stale storage, repeated lifetime mutation, corrupted state consumption, or authorization DENY-to-ALLOW transition. The supported effect is limited to stuck tasks and loss of access to the kprobe optimization control. Thus `PrimitiveClass=NONE`, there is no Important candidate, and automatic Moderate/Low-like handling is safe.

5. One-sentence report phrase

A host administrator can trigger a permanent kprobe-optimizer flush wait that blocks later optimization-sysctl accesses, without demonstrated memory corruption or security-boundary bypass.

6. Manual review advice

NO MANUAL CHECK NEEDED

The patch and call path establish a privileged synchronization DoS with no unresolved corruption or authorization primitive.

7. Call-site confidence

Call-site confidence: high

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

Patch: kprobes: Fix permanent hang when flushing the kprobe optimizer
Commit: f05a01fc7d4f731b4c100d2aef1e85ad77728862
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f05a01fc7d4f731b4c100d2aef1e85ad77728862

Commit description:

Writing 0 to /proc/sys/debug/kprobes-optimization while a kprobe is
jump-optimized never returns. The writer sleeps in D state forever with
kprobe_sysctl_mutex held, so any later read or write of that sysctl
hangs as well. For example, with vfs_read+9 as an optimizable address
in this build:

  # cd /sys/kernel/tracing
  # echo 'p:myprobe vfs_read+9' >> kprobe_events
  # echo 1 > events/kprobes/myprobe/enable
  # # wait until /sys/kernel/debug/kprobes/list shows [OPTIMIZED]
  # echo 0 > /proc/sys/debug/kprobes-optimization

  INFO: task sh:246 blocked for more than 10 seconds.
  Call Trace:
   <TASK>
   __schedule+0x1176/0x4f70
   schedule+0xdc/0x2c0
   schedule_timeout+0x17b/0x260
   wait_for_completion+0x173/0x3c0
   wait_for_kprobe_optimizer_locked+0xbc/0x130
   proc_kprobes_optimization_handler+0x156/0x1b0
   proc_sys_call_handler+0x324/0x490
   vfs_write+0x52d/0xfe0
   ksys_write+0xff/0x200
   do_syscall_64+0x106/0x630
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  ...
  INFO: task cat:265 is blocked on a mutex likely owned by task sh:246.

wait_for_kprobe_optimizer_locked() reinitializes optimizer_completion,
asks the optimizer thread to flush and sleeps in wait_for_completion().
The thread drains the (un)optimizing lists, but calls complete() only
if completion_done() is true, i.e. if the completion is already done,
which never happens while someone waits. disarm_all_kprobes() and
kprobe_trace_self_tests_init() wait the same way.

Calling complete() unconditionally would not be enough: the waiter
drops kprobe_mutex while it sleeps, and nothing else serializes the
sysctl handler against the debugfs "enabled" file. A second flusher
that still finds the lists non-empty, e.g. because a disabled probe is
queued for unoptimizing, reinitializes the completion under the first:

  sysctl write                      debugfs "enabled" write
  unoptimize_all_kprobes()
    wait_for_kprobe_optimizer_locked()
      init_completion(c)
      mutex_unlock(&kprobe_mutex)
      wait_for_completion(c)
                                    disarm_all_kprobes()
                                      wait_for_kprobe_optimizer_locked()
                                        init_completion(c)
                                          // c->wait is reset, the first
                                          // waiter is off the queue
                                        mutex_unlock(&kprobe_mutex)
                                        wait_for_completion(c)
  kprobe_optimizer()
    complete(c)
      // wakes the debugfs writer only

where c is &optimizer_completion. Lining up the two writes during an
optimizer pass loses the sysctl writer this way.

Replace the completion with a counter of optimizer passes, bumped at the
end of each pass and signalled with wake_up_var_locked(), both under
kprobe_mutex. A flusher samples the count and waits with
wait_var_event_mutex(), which drops kprobe_mutex only while sleeping, so
a new count means a whole pass ran in the meantime. Nothing is
reinitialized, so several flushers can sleep in the wait at once.

Link: https://lore.kernel.org/all/20260924092142.199198-1-parri.andrea@gmail.com/

Fixes: 73c12f2 ("kprobes: Use dedicated kthread for kprobe optimizer")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  kernel/kprobes.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=f05a01fc7d4f731b4c100d2aef1e85ad77728862

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

The original Linux kernel CVE announcement for CVE-2026-98530 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98530

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:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS score: 2.5

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: unknown
  Paranoid ActionableScore: 0

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).