Description
In the Linux kernel, the following vulnerability has been resolved:
s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks
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.
[gor@linux.ibm.com: change commit message]
s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks
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.
[gor@linux.ibm.com: change commit message]
Analysis
Analysis and contextual insights are available on OpenCVE Cloud.
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | |||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
|
|||||||||||||||||||||
| Linux | Linux | affected |
|
No data.
No data.
No data available yet.
Remediation
No vendor fix or workaround currently provided.
Additional remediation guidance may be available on OpenCVE Cloud.
Tracking
Sign in to view the affected projects.
Advisories
No advisories yet.
References
History
Sat, 12 Sep 2026 00:15:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Weaknesses | CWE-476 | |
| References |
| |
| Metrics |
threat_severity
|
cvssV3_1
|
Fri, 11 Sep 2026 23:45:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| Description | In the Linux kernel, the following vulnerability has been resolved: s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks 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. [gor@linux.ibm.com: change commit message] | |
| Title | s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-09-11T19:43:22.664Z
Reserved: 2026-09-11T19:38:34.708Z
Link: CVE-2026-89460
Vulnrichment
No data.
NVD
Status : Received
Published: 2026-09-11T20:19:26.893
Modified: 2026-09-11T20:19:26.893
Link: CVE-2026-89460
Redhat
OpenCVE Enrichment
No data.
Weaknesses
-
CWE-476
NULL Pointer Dereference