CVE-2026-64248: MIPS: smp: 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:

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

smp_send_stop() parks all secondary CPUs in stop_this_cpu(). 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. Since commit
91840be8f710 ("irq_work: Fix use-after-free in irq_work_single() on PREEMPT_RT")
however, irq_work_sync() calls synchronize_rcu() on architectures without
an irq_work self-IPI, i.e. where arch_irq_work_has_interrupt() returns
false. That is the asm-generic default used by MIPS. Any irq_work_sync()
issued in the reboot/shutdown 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 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 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. 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 cpu_die_early().
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.

The Linux kernel CVE team has assigned CVE-2026-64248 to this issue.


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

	Issue introduced in 6.1.175 with commit 2dc79362302922cb18f35e262712b5e58de65442 and fixed in 6.1.178 with commit f8a1ef884013dc99f712d3eb75624c7cd3fd94f6
	Issue introduced in 6.6.142 with commit eef4f71b46a9929ac33e968538c9dd5d96a02460 and fixed in 6.6.145 with commit e1919d026706544cb6e7251ec06e908edd6f34ee
	Issue introduced in 6.12.92 with commit 684a78183c54c23e70d1cba320f7fc184604210b and fixed in 6.12.95 with commit 6eda71977ee11c222f8ad4cae4d18d50448e56f4
	Issue introduced in 6.18.34 with commit 18c0456ea2615b1a743a6db739c74411c3b42bc6 and fixed in 6.18.38 with commit f9b57a0015c241274651f4b36627f56b1b5a8651
	Issue introduced in 7.1 with commit 91840be8f710370607f949a627e070896faeddb8 and fixed in 7.1.3 with commit 9fef09df42df55ab819b285ea892e0fc1b95a9c4
	Issue introduced in 7.1 with commit 91840be8f710370607f949a627e070896faeddb8 and fixed in 7.2-rc1 with commit 9f3f3bdc6d9dac1a5a8262ee7ad0f2ff1527a7e7
	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-2026-64248
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/mips/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/f8a1ef884013dc99f712d3eb75624c7cd3fd94f6
	https://git.kernel.org/stable/c/e1919d026706544cb6e7251ec06e908edd6f34ee
	https://git.kernel.org/stable/c/6eda71977ee11c222f8ad4cae4d18d50448e56f4
	https://git.kernel.org/stable/c/f9b57a0015c241274651f4b36627f56b1b5a8651
	https://git.kernel.org/stable/c/9fef09df42df55ab819b285ea892e0fc1b95a9c4
	https://git.kernel.org/stable/c/9f3f3bdc6d9dac1a5a8262ee7ad0f2ff1527a7e7