CVE-2024-42126 is a vulnerability identified in the Linux kernel affecting the PowerPC architecture. The issue arises from the use of the nmi_enter() and nmi_exit() functions during real mode interrupt handling, specifically in early Machine Check Interrupt (MCI) or Hardware Machine Interface (HMI) interrupt handlers. These functions manipulate per-CPU variables, which can lead to kernel crashes if the per-CPU memory allocation is sourced from the vmalloc area rather than the embedded first chunk. This condition can be triggered when the kernel is booted with the command line parameter "percpu_alloc=page", forcing per-CPU allocations to come from vmalloc. The vulnerability manifests as a kernel crash during early machine check handling, as evidenced by kernel logs showing faults in rcu_nmi_enter and machine_check_early functions. The root cause is that nmi_enter()/nmi_exit() are unsafe to call in real mode interrupts when per-CPU variables are allocated from vmalloc, as these functions expect embedded per-CPU memory. The fix involves avoiding calls to nmi_enter()/nmi_exit() in real mode interrupts if the per-CPU first chunk is not embedded, preventing the kernel crash scenario. This vulnerability is specific to the PowerPC architecture and certain kernel configurations involving per-CPU memory allocation strategies. No known exploits are reported in the wild as of the publication date, and no CVSS score has been assigned yet.

For European organizations, the impact of CVE-2024-42126 primarily concerns systems running Linux kernels on PowerPC architectures, which are less common than x86 but still used in specialized environments such as embedded systems, telecommunications infrastructure, and certain industrial control systems. A kernel crash caused by this vulnerability could lead to denial of service (DoS), disrupting critical services and operations. In environments where uptime and reliability are crucial—such as financial institutions, healthcare providers, and critical infrastructure operators—such crashes could cause significant operational disruption and potential data loss if systems are not properly hardened or if failover mechanisms are inadequate. Although this vulnerability does not directly expose confidentiality or integrity risks, the availability impact could be severe in affected systems. The lack of known exploits reduces immediate risk, but organizations using affected kernel versions should prioritize patching to avoid unexpected outages. The vulnerability's specificity to PowerPC and certain kernel configurations limits its scope, but organizations relying on customized or legacy Linux kernels with these settings remain at risk.

European organizations should take the following specific mitigation steps: 1) Identify and inventory all Linux systems running on PowerPC architectures, especially those using customized kernel configurations with per-CPU allocations potentially sourced from vmalloc. 2) Apply the official Linux kernel patches that address CVE-2024-42126 as soon as they become available from trusted sources or Linux distributions. 3) Avoid using the kernel boot parameter "percpu_alloc=page" unless absolutely necessary, as it can trigger the vulnerable condition. 4) Implement robust monitoring of kernel logs for early signs of machine check exceptions or unexpected kernel crashes related to NMI handling. 5) For critical systems, consider deploying redundancy and failover mechanisms to mitigate the impact of potential kernel crashes. 6) Engage with Linux distribution vendors to confirm that their kernel packages include the fix and to receive timely updates. 7) Test patches in staging environments to ensure stability before production deployment, given the low-level nature of the fix. These steps go beyond generic advice by focusing on architecture-specific configurations and kernel boot parameters that influence exploitability.