CVE-2026-64250: LoongArch: Report dying CPU to RCU in stop_this_cpu()

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-64250 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.1.175 with commit 2dc79362302922cb18f35e262712b5e58de65442 and fixed in 6.1.178 with commit 262dadc619e69ebeb97affd334cd1078a9704e98
	Issue introduced in 6.6.142 with commit eef4f71b46a9929ac33e968538c9dd5d96a02460 and fixed in 6.6.145 with commit 1fa22de588a65880d6fe54c38c87fffe7d519f60
	Issue introduced in 6.12.92 with commit 684a78183c54c23e70d1cba320f7fc184604210b and fixed in 6.12.95 with commit a0269e928728f970c782319fee53d92d4ea4e512
	Issue introduced in 6.18.34 with commit 18c0456ea2615b1a743a6db739c74411c3b42bc6 and fixed in 6.18.38 with commit 90e254f18b8c224460082329dd5c42fd30995c2f
	Issue introduced in 7.1 with commit 91840be8f710370607f949a627e070896faeddb8 and fixed in 7.1.3 with commit 0833b2b84c2fc1387f8165f0cbf6a02d67f647a5
	Issue introduced in 7.1 with commit 91840be8f710370607f949a627e070896faeddb8 and fixed in 7.2-rc1 with commit f2539c56c74691e7a88af6372ba2b48c06ed2fe4
	Issue introduced in 7.0.11 with commit 81b582784518196eff1050212a046bc29d3a05dd

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64250
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/loongarch/kernel/smp.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/262dadc619e69ebeb97affd334cd1078a9704e98
	https://git.kernel.org/stable/c/1fa22de588a65880d6fe54c38c87fffe7d519f60
	https://git.kernel.org/stable/c/a0269e928728f970c782319fee53d92d4ea4e512
	https://git.kernel.org/stable/c/90e254f18b8c224460082329dd5c42fd30995c2f
	https://git.kernel.org/stable/c/0833b2b84c2fc1387f8165f0cbf6a02d67f647a5
	https://git.kernel.org/stable/c/f2539c56c74691e7a88af6372ba2b48c06ed2fe4