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CVE Record

CVE-2021-47454: powerpc/smp: do not decrement idle task preempt count in CPU offline

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.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This is a Linux kernel bug in the powerpc CPU offline path. On affected systems, taking a CPU offline and bringing it back online can leave scheduling state inconsistent and trigger a kernel scheduling error. The available sources do not show remote exploitation, active exploitation, or data compromise.

Executive priority

Treat this as a targeted reliability issue for powerpc Linux infrastructure. Prioritize patching where CPU hotplug is used or where kernel instability could affect critical services.

Technical view

In powerpc SMP code, arch_cpu_idle_dead() wrongly decremented the idle task preempt count after scheduler changes made that behavior obsolete. With PREEMPT_COUNT enabled, CPU offline/online can produce “scheduling while atomic” in the idle task. The fix removes the vestigial preempt_enable() from the CPU offline path.

Likely exposure

Exposure appears limited to Linux powerpc deployments running affected kernel versions or downstream kernels carrying the vulnerable code, especially environments using CPU hotplug, virtualization, or maintenance workflows that offline and online CPUs.

Exploitation context

CISA KEV is false, and the supplied sources do not report public exploitation. The described failure requires CPU offline and online behavior on affected powerpc kernels, suggesting operational disruption risk rather than confirmed attacker-driven compromise.

Researcher notes

The evidence identifies root cause, affected kernel ranges, and upstream stable fix references, but provides no CVSS, CWE, exploit report, or vendor-specific package mapping. Validate exposure against downstream kernel backports rather than version strings alone.

Mitigation direction

  • Update affected powerpc Linux kernels to vendor builds containing the referenced stable fixes.
  • Check distribution advisories for backported fixes matching your kernel package.
  • Prioritize systems that use CPU hotplug, virtualization, or dynamic CPU maintenance workflows.
  • Follow vendor guidance for any supported interim operational mitigations.

Validation and detection

  • Inventory powerpc Linux hosts and record kernel versions and vendor package revisions.
  • Check whether affected systems enable PREEMPT_COUNT and CPU hotplug functionality.
  • Verify installed kernels include the referenced upstream stable commits or vendor-equivalent backports.
  • Review logs for “scheduling while atomic” during CPU offline or online events.
Prepared
Confidence
medium
Sources
5

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
2ADP providers
4Source links

SSVC decision data

CISA-ADPCISA Coordinator
Timestamp
Version
2.0.3
Exploitation: noneAutomatable: noTechnical Impact: partial

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

    The CVE record metadata indicates this as the latest update time.

ADP provider summaries

CISA-ADPCISA ADP Vulnrichment
other:ssvc
CVECVE Program Container

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinuxbdf4d33e8342b90386156204e1da0cdfdb4bf146, 2c669ef6979c370f98d4b876e54f19613c81e075, 2c669ef6979c370f98d4b876e54f19613c81e075, 2b6148ef2bd6d8ddc76e7873114f7769b6aa25f0, 20a015e948b825afb47855de2efce7cae7c2608funaffected
LinuxLinux5.14, 0, 5.10.76, 5.14.15, 5.15affected
Weakness

CWE details

No CWE listed

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