* [CVE-2026-89460][MODERATE REGULAR] s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks
@ 2026-09-12 7:11 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 7:11 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89460
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks
Commit: 777d040c2f91d8d929b4a32333dfe4102b27caea
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=777d040c2f91d8d929b4a32333dfe4102b27caea
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89460
Analysis date: Sat, 12 Sep 2026 03:11:39 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89460 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89460
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 REGULAR
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-2026-89460 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89460 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:L/I:L/A:H';**CWE-476;CWE-362;*CWE-416;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A NULL pointer dereference in the s390 CPUMF perf path can occur when a per task perf event is created while some CPUs are offline and a CPU is hotplug added during the same run. The newly online CPU may lack the per CPU cpu_cf_events infrastructure, and cpumf_pmu_add can then dereference a NULL pointer after the scheduler places the task on that CPU, causing a kernel panic. For the CVSS the PR:L is used for the paranoid score because a local user may be able to create per task perf events while CPU hotplug is supplied by the environment or delegated operations, although reliable direct triggering normally needs administrative CPU hotplug control. The issue is not network reachable and is limited to local s390 systems using the CPUMF facility and perf. Impact is at least local denial of service via kernel crash and in worst case may allow limited confidentiality or integrity impact because the commit also mentions stale data access by interrupt handlers before freed per CPU data is cleared.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) YES OOB DANGER UAF KERNEL_PANIC_PLUS_UAF HARDWARE LINUS YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=1
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=YES
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::
No floor was applied. REAL UAF FLOOR 4 does not apply because the complete direct trigger requires host-admin CPU hotplug control, and the stale access is narrow per-CPU teardown or interrupt-handler state rather than a demonstrated attacker-controlled reuse primitive.
2. Signal breakdown
Positive signals:
* +2 Real UAF / lifetime violation: the commit explicitly says cpum_cf_free_root() and cpum_cf_free_cpu() must clear reference pointers before freeing data to prevent interrupt handlers from accessing stale data.
* +1 Reliable kernel crash / strong DoS: the supplied trace shows a kernel panic from NULL pointer dereference in cpumf_pmu_add() after a CPU is hotplug-added during a per-task perf run.
Negative signals:
* -2 Host-admin-equivalent trigger requirement: the demonstrated reliable sequence requires writing to /sys/devices/system/cpu/cpu1/online, which is host-admin CPU hotplug control.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity impact not awarded because no disclosure, object replacement, directed corruption, or privilege-escalation primitive is established.
* Remote reachability not established.
* Extra availability point not awarded because the same kernel panic is already counted.
* Concrete repeated release/refcount signal not awarded. No first release followed by a second lifetime-mutating operation on the exact same dangling reference is shown.
* Structured stale-object leverage not awarded. The affected per-CPU CPUMF state is active kernel state, but attacker-controlled replacement or meaningful post-free field consumption is not established.
* Weak LPE concern not awarded solely from the UAF/stale-data wording.
* Paranoid repeated-refcount leverage not awarded because no second stale refcount/lifetime mutation is shown.
* FLOOR 5 not applied because there is no concrete repeated release/refcount mutation, controlled reclaim, or meaningful reuse primitive.
3. Reachability analysis
The direct reproducer requires CPU hotplug operations: taking a CPU offline, starting a per-task perf event, and bringing the CPU online during the perf run. CPU hotplug via /sys/devices/system/cpu/cpu*/online is normally host-admin-equivalent.
A local non-root user may be able to create some perf events depending on perf_event_paranoid and system policy, but the complete reliable trigger also needs CPU hotplug timing. The supplied evidence does not show that an ordinary local user can independently perform the full trigger without host-admin control.
No namespace-scoped capability path is established. CPU hotplug is host-global and not a normal user or network namespace operation.
No network trigger is present. The issue is local to s390 systems using the CPUMF facility and perf or /dev/hwctr paths.
Special platform context exists: this is s390 CPUMF-specific code. I did not apply an additional hardware rarity penalty because the host-admin CPU-hotplug requirement already captures the main practical restriction for this triage score.
Race/timing penalty was not applied separately. The sequence involves timing with CPU hotplug and scheduler placement, but the provided reproducer indicates the condition can be deliberately arranged by an administrator rather than being an uncontrollable narrow race.
4. Primitive and severity interpretation
The demonstrated failure is a NULL pointer dereference: a per-task perf event is created while a CPU is offline, the CPU is later hotplug-added, and the scheduler can place the task on that CPU before per-CPU CPUMF data exists. cpumf_pmu_add() then receives NULL from this_cpu_cfhw() and the kernel panics.
The patch context also directly implies a stale post-lifetime access risk because it clears per-CPU reference pointers before freeing to prevent interrupt handlers from accessing stale data. This is enough for RealUAF=YES and PrimitiveClass=CLASS_B, but the primitive is constrained.
This is closer to a narrow per-CPU lifetime/teardown bug than an attacker-controlled active-object UAF. No controlled reclaim, attacker-shaped replacement, callback use, arbitrary read/write, or privilege escalation chain is established.
There is no concrete repeated release/refcount operation. This is not the stronger double-release/refcount Class B category.
ImportantCandidate=NO because the known consequences are kernel panic and narrow stale-data avoidance, with no meaningful unresolved exploitation leverage beyond crash shown.
AutoDowngradeAllowed=YES because this appears to be a constrained, host-admin-triggered, platform-specific crash/stale-pointer issue without demonstrated boundary bypass, LPE, controlled corruption, or repeated lifetime mutation.
Likely operational severity: ordinary Moderate for affected s390 systems, mainly DoS under CPU hotplug plus CPUMF/perf conditions.
5. One-sentence report phrase
A s390 CPUMF perf per-CPU lifetime bug can cause a kernel panic when a CPU is hotplug-added during a per-task perf run, while the stale-data cleanup note warrants review but does not establish controlled memory corruption or privilege escalation.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Manual review is recommended because the patch mentions stale data access by interrupt handlers after free, but the complete direct trigger appears host-admin-controlled and the supplied evidence does not establish a stronger exploitation primitive.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks
Commit: 777d040c2f91d8d929b4a32333dfe4102b27caea
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=777d040c2f91d8d929b4a32333dfe4102b27caea
Commit description:
The command 'perf stat -e cycles -- <command>' crashes the kernel
when CPUs are hotplug added during that run.
Root cause is the allocation of struct cpu_cf_events at first
event initialization. The allocation is dynamic and the first
event that has task context creates such a structure for
each online CPU. This is not sufficient. CPUs may be offline
during event creation and can be set online during the
perf run time. For example commands
# echo 0 > /sys/devices/system/cpu/cpu1/online
# perf stat -e cycles -i -- stress-ng -t10s --matrix X
# sleep 1
# echo 1 > /sys/devices/system/cpu/cpu1/online
create an event for CPUs 0,2-X. Since the events are created with
task-context, the scheduler will eventually schedule the program
on CPU1. This CPU has not created and initialized any per
CPU event infrastructure as that CPU was not online at the time
of the perf invocation. Thus when the scheduler runs stress-ng
on CPU1, the function cpumf_pmu_add() refers to a NULL pointer:
struct cpu_cf_events *cpuhw = this_cpu_cfhw();
This function call is invoked after the task stress-ng has been
made runnable on CPU1. And this_cpu_cfhw() returns NULL.
The result is a panic:
Unable to handle kernel pointer dereference in virtual kernel address space
Failing address: 0000000000000000 TEID: 0000000000000483
....
Krnl PSW : 0404d00180000000 000003ef8291fd0c (cpumf_pmu_add+0x3c/0x80)
....
Call Trace:
[<000003ef8291fd0c>] cpumf_pmu_add+0x3c/0x80
[<000003ef82bb5e3e>] event_sched_in+0xae/0x190
[<000003ef82bb60d6>] merge_sched_in+0x1b6/0x390
[<000003ef82bb65b8>] visit_groups_merge.constprop.0.isra.0+0x308/0x5b0
[<000003ef82bb689a>] pmu_groups_sched_in+0x3a/0x50
[<000003ef82bb6a30>] ctx_sched_in+0x180/0x260
[<000003ef82bb780c>] perf_event_context_sched_in+0x11c/0x2d0
[<000003ef82bb79ee>] __perf_event_task_sched_in+0x2e/0xc0
[<000003ef82994834>] finish_task_switch.isra.0+0x1a4/0x250
....
Last Breaking-Event-Address:
[<000003ef8291f1d8>] this_cpu_cfhw+0x38/0x40
The issue arises only in per-task context when the CPUMF facility is
used and the scheduler picks a random CPU for such a process to run on.
The scheduler enables the CPUMF infrastructure via PMU callback
functions pmu::add() and pmu::del().
Introduce a CPU hotplug prepare/dead callback pair which creates and
removes the per CPU counter data while the CPU is offline. Count the
users which track every CPU (cpu == -1), that is perf_event_open()
events with task context and /dev/hwctr device sessions, in the new
counter cpu_cf_root::tskcnt, protected by pmc_reserve_mutex.
This ensures the infrastructure is available when
new CPU is selected to run the per-task context process.
In cpum_cf_free_root() and cpum_cf_free_cpu() ensure the reference
pointer to data structures is set to NULL before the data is freed
to prevent interrupt handlers to access stale data.
[[email protected]: change commit message]
Fixes: 9b9cf3c ("s390/cpum_cf: rework PER_CPU_DEFINE of struct cpu_cf_events")
Cc: [email protected] # v6.5+
Suggested-by: Heiko Carstens <[email protected]>
Suggested-by: Christian Borntraeger <[email protected]>
Assisted-by: Claude:claude-sonnet-5
Signed-off-by: Thomas Richter <[email protected]>
Acked-by: Heiko Carstens <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
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=777d040c2f91d8d929b4a32333dfe4102b27caea
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89460 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89460
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:H
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS score: 5.8
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: 1
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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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