CVE-2021-47454

What it actually does

The flaw leaves the idle task's preempt count decremented after CPU offline on powerpc, violating the expectation that idle tasks start with preemption disabled and triggering scheduling-while-atomic faults on subsequent CPU online.

Why it matters to the business

Systems using powerpc processors may experience kernel crashes or hangs during routine CPU hotplug operations such as maintenance, virtualization scaling, or power management, leading to unplanned downtime on affected hardware.

How an attack unfolds

An attacker with the ability to trigger repeated CPU offline/online cycles (via privileged interfaces or hypervisor control) can force the kernel into an atomic scheduling state, resulting in a crash that denies service; the low EPSS and lack of KEV listing indicate this is primarily a stability issue rather than a remote code execution vector.

How to fix it

Update the Linux kernel to a version containing commit 62fb9874f5da54fdb243003b386128037319b219 or later; rebuild and reboot with the corrected powerpc/smp code that removes the vestigial preempt_enable from arch_cpu_idle_dead.

How to confirm the fix worked

Confirm the running kernel contains commit 62fb9874f5da54fdb243003b386128037319b219, then exercise CPU offline/online under PREEMPT_COUNT=y and verify absence of scheduling-while-atomic BUG messages.

Typical attack path

1. Obtain privileges sufficient to control CPU hotplug on a powerpc system
2. Repeatedly offline and online a CPU to exercise the idle-task preempt count path
3. Trigger the scheduling-while-atomic condition during cpu_startup_entry
4. Cause kernel panic or system hang

How to detect it

Search powerpc kernel logs for BUG: scheduling while atomic messages referencing swapper and start_secondary; correlate with CPU hotplug events in dmesg or /var/log/messages around the time of any observed panics.

What your security team should do

• Prioritize patching powerpc-based systems that perform frequent CPU hotplug
• Monitor kernel logs for scheduling-while-atomic messages on powerpc during maintenance windows
• Apply the fixed commit in any custom or downstream kernels before enabling CPU hotplug in production