s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks

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Summary

CVECVE-2026-89460
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-11 20:19:26 UTC
Updated2026-09-11 20:19:26 UTC
DescriptionIn 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. [[email protected]: change commit message]

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 9b9cf3c77e7e090b30657b3d2c288deb58dfb4f2 777d040c2f91d8d929b4a32333dfe4102b27caea git Not specified
CNA Linux Linux affected 9b9cf3c77e7e090b30657b3d2c288deb58dfb4f2 dcce7a06ea690d5cf46a705a9b74cf0398ac091f git Not specified
CNA Linux Linux affected 9b9cf3c77e7e090b30657b3d2c288deb58dfb4f2 71d46de9153e067c63a15f3b00e6364e1c0230f8 git Not specified
CNA Linux Linux affected 9b9cf3c77e7e090b30657b3d2c288deb58dfb4f2 337bd95507a16063687cfc286ea90de5cca48c37 git Not specified
CNA Linux Linux affected 6.5 Not specified
CNA Linux Linux unaffected 6.5 semver Not specified
CNA Linux Linux unaffected 6.12.109 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.50 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.4 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3-rc1 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/337bd95507a16063687cfc286ea90de5cca48c37 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/777d040c2f91d8d929b4a32333dfe4102b27caea 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/71d46de9153e067c63a15f3b00e6364e1c0230f8 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/dcce7a06ea690d5cf46a705a9b74cf0398ac091f 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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