CVE-2021-47454

/// Technical description

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:

Initializ

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References