Linux Kernel RCU Grace Period Deadlock via Unreported Dying CPU on MIPS
CVE-2026-64248 Published on July 24, 2026
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 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.
Products Associated with CVE-2026-64248
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Affected Versions
Linux:- Version 2dc79362302922cb18f35e262712b5e58de65442 and below f8a1ef884013dc99f712d3eb75624c7cd3fd94f6 is affected.
- Version eef4f71b46a9929ac33e968538c9dd5d96a02460 and below e1919d026706544cb6e7251ec06e908edd6f34ee is affected.
- Version 684a78183c54c23e70d1cba320f7fc184604210b and below 6eda71977ee11c222f8ad4cae4d18d50448e56f4 is affected.
- Version 18c0456ea2615b1a743a6db739c74411c3b42bc6 and below f9b57a0015c241274651f4b36627f56b1b5a8651 is affected.
- Version 91840be8f710370607f949a627e070896faeddb8 and below 9fef09df42df55ab819b285ea892e0fc1b95a9c4 is affected.
- Version 91840be8f710370607f949a627e070896faeddb8 and below 9f3f3bdc6d9dac1a5a8262ee7ad0f2ff1527a7e7 is affected.
- Version 81b582784518196eff1050212a046bc29d3a05dd is affected.
- Version 6.1.175 and below 6.1.178 is affected.
- Version 6.6.142 and below 6.6.145 is affected.
- Version 6.12.92 and below 6.12.95 is affected.
- Version 6.18.34 and below 6.18.38 is affected.
- Version 7.0.11 and below 7.1 is affected.
- Version 7.1 is affected.
- Before 7.1 is unaffected.
- Version 6.1.178, <= 6.1.* is unaffected.
- Version 6.6.145, <= 6.6.* is unaffected.
- Version 6.12.95, <= 6.12.* is unaffected.
- Version 6.18.38, <= 6.18.* is unaffected.
- Version 7.1.3, <= 7.1.* is unaffected.
- Version 7.2-rc1, <= * is unaffected.