CVE-2026-89460: s390/cpum_cf: Handle CPU hotplug via prepare/dead callbacks

In the Linux kernel, the following vulnerability has been resolved:

s390/cpumcf: 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 cpucfevents 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 cpumfpmuadd() refers to a NULL pointer:

struct cpucfevents cpuhw = thiscpucfhw();

This function call is invoked after the task stress-ng has been made runnable on CPU1. And thiscpucfhw() 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 (cpumfpmuadd+0x3c/0x80) .... Call Trace: [<000003ef8291fd0c>] cpumfpmuadd+0x3c/0x80 [<000003ef82bb5e3e>] eventschedin+0xae/0x190 [<000003ef82bb60d6>] mergeschedin+0x1b6/0x390 [<000003ef82bb65b8>] visitgroupsmerge.constprop.0.isra.0+0x308/0x5b0 [<000003ef82bb689a>] pmugroupsschedin+0x3a/0x50 [<000003ef82bb6a30>] ctxschedin+0x180/0x260 [<000003ef82bb780c>] perfeventcontextschedin+0x11c/0x2d0 [<000003ef82bb79ee>] perfeventtaskschedin+0x2e/0xc0 [<000003ef82994834>] finishtaskswitch.isra.0+0x1a4/0x250 .... Last Breaking-Event-Address: [<000003ef8291f1d8>] thiscpucfhw+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 perfeventopen() events with task context and /dev/hwctr device sessions, in the new counter cpucfroot::tskcnt, protected by pmcreservemutex. This ensures the infrastructure is available when new CPU is selected to run the per-task context process.

In cpumcffreeroot() and cpumcffreecpu() 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]