CVE-2021-47429

What it actually does

The machine check exception handler on powerpc64s incorrectly invoked an asynchronous handler directly from NMI context during unrecoverable MCE events when MSR[RI] was clear, corrupting IRQ state and triggering a BUG_ON in interrupt handling code.

Why it matters to the business

PowerPC-based systems may experience unexpected crashes or instability during hardware error conditions, leading to unplanned downtime and potential data loss in affected environments; the issue is long-published with negligible observed exploitation risk.

How an attack unfolds

An attacker with the ability to trigger unrecoverable machine check exceptions on powerpc64s hardware could force the kernel into the flawed NMI-to-async handler path, resulting in corrupted interrupt state that may be leveraged for denial of service or further memory corruption.

How to fix it

Apply the upstream fix by updating to a Linux kernel containing commit that separates the async and NMI machine check handlers; rebuild and reboot affected powerpc64s systems.

How to confirm the fix worked

Confirm kernel source contains the _async and NMI-specific handler variants, verify absence of direct async call from NMI in machine check code, and test MCE injection on patched systems without triggering the BUG_ON.

Typical attack path

1. Trigger hardware condition leading to unrecoverable MCE on powerpc64s
2. Kernel enters true NMI handler with MSR[RI] clear
3. Direct call to machine_check_exception from NMI context occurs
4. IRQ state corruption and BUG_ON assertion trigger system instability

How to detect it

Review kernel logs for BUG_ON messages referencing arch/powerpc/include/asm/interrupt.h or unexpected NMI context transitions during machine check events; examine crash dumps for corrupted IRQ state on powerpc64s hardware.

What your security team should do

• Prioritize kernel updates on all powerpc64s systems within standard maintenance windows given low EPSS and no KEV listing
• Monitor for future powerpc-specific MCE handling changes in LTS kernel releases
• Include powerpc architecture in regular kernel vulnerability scanning