CVE-2026-64250

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

LoongArch: Report dying CPU to RCU in stop_this_cpu()

This is a port of MIPS commit 9f3f3bdc6d9dac1 ("MIPS: smp: report dying

CPU to RCU in stop_this_cpu()"). smp_send_stop() parks all secondary

CPUs in stop_this_cpu(). And the function marks the CPU offline for the

scheduler via set_cpu_online(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 smp_send_stop()

this is harmless, which is why it went unnoticed. However, since commit

91840be8f710370 ("irq_work: Fix use-after-free in irq_work_single() on

PREEMPT_RT"), irq_work_sync() calls synchronize_rcu() on architectures

without an irq_work self-IPI, i.e. where arch_irq_work_has_interrupt()

returns false. Any irq_work_sync() issued in the reboot/shutdown/halt

path after smp_send_stop() then blocks on a grace period that can never

complete, hanging the reboot:

WARNING: CPU: 0 PID: 15 at kernel/irq_work.c:144 irq_work_queue_on

...

rcu: INFO: rcu_sched 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 needs some hacks to reproduce, and it was not noticed on

LoongArch because arch_irq_work_has_interrupt() usually returns true.

Call rcutree_report_cpu_dead() 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. LoongArch shuts down all CPUs

here without going through the CPU-hotplug mechanism, so this report is

not otherwise issued.