CVE-2021-47454

In the Linux kernel, the following vulnerability has been resolved:
powerpc/smp: do not decrement idle task preempt count in CPU offline With
PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we get:
BUG: scheduling while atomic: swapper/1/0/0x00000000 no locks held by
swapper/1/0. CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100
Call Trace: dump_stack_lvl+0xac/0x108 __schedule_bug+0xac/0xe0
__schedule+0xcf8/0x10d0 schedule_idle+0x3c/0x70 do_idle+0x2d8/0x4a0
cpu_startup_entry+0x38/0x40 start_secondary+0x2ec/0x3a0
start_secondary_prolog+0x10/0x14 This is because powerpc’s
arch_cpu_idle_dead() decrements the idle task’s preempt count, for reasons
explained in commit a7c2bb8279d2 (“powerpc: Re-enable preemption before
cpu_die()”), specifically “start_secondary() expects a preempt_count() of
0.” However, since commit 2c669ef6979c (“powerpc/preempt: Don’t touch the
idle task’s preempt_count during hotplug”) and commit f1a0a376ca0c
(“sched/core: Initialize the idle task with preemption disabled”), that
justification no longer holds. The idle task isn’t supposed to re-enable
preemption, so remove the vestigial preempt_enable() from the CPU offline
path. Tested with pseries and powernv in qemu, and pseries on PowerVM.