CVE-2026-64248: MIPS: smp: report dying CPU to RCU in stop_this_cpu()

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

MIPS: smp: report dying CPU to RCU in stopthiscpu()

smpsendstop() parks all secondary CPUs in stopthiscpu(). The function marks the CPU offline for the scheduler via setcpuonline(false) but never informs RCU, so RCU keeps expecting a quiescent state from CPUs that are now spinning forever with interrupts disabled.

As long as nothing waits for an RCU grace period after smpsendstop() this is harmless, which is why it went unnoticed. Since commit 91840be8f710 ("irqwork: Fix use-after-free in irqworksingle() on PREEMPTRT") however, irqworksync() calls synchronizercu() on architectures without an irqwork self-IPI, i.e. where archirqworkhasinterrupt() returns false. That is the asm-generic default used by MIPS. Any irqworksync() issued in the reboot/shutdown path after smpsendstop() then blocks on a grace period that can never complete, hanging the reboot:

WARNING: CPU: 0 PID: 15 at kernel/irqwork.c:144 irqworkqueueon ... rcu: INFO: rcusched detected stalls on CPUs/tasks: rcu: Offline CPU 1 blocking current GP. rcu: Offline CPU 2 blocking current GP. rcu: Offline CPU 3 blocking current GP.

This issue was noticed on several Realtek MIPS switch SoCs (MIPS interAptiv) and came up during kernel bump downstream in OpenWrt from 6.18.33 to 6.18.34, after the backport of the patch to the 6.18 stable branch. The patch also has been backported all the way back to 6.1.

Call rcutreereportcpudead() once interrupts are disabled, mirroring the generic CPU-hotplug offline path, so RCU stops waiting on the parked CPUs and grace periods can still complete. MIPS shuts down all CPUs here without going through the CPU-hotplug mechanism, so this report is not otherwise issued. Reporting a dying CPU to RCU outside the regular hotplug offline path is not unprecedented: arm64 does the same in cpudieearly(). There it is an exception for a CPU that was coming online and is aborting bringup, rather than the default shutdown action as on MIPS.