CVE-2024-42126: 
Linux Kernel vulnerability analysis and mitigation

CVE-2024-42126 affects the Linux kernel's PowerPC architecture implementation. The vulnerability involves nmi_enter()/nmi_exit() functions that handle per-CPU variables during real mode interrupt processing. The issue was discovered in July 2024 and affects systems where percpu allocation comes from vmalloc area, particularly when CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK is enabled (Kernel Git).

The vulnerability occurs when nmi_enter()/nmi_exit() functions access per-CPU variables during real mode interrupt handling (e.g., early HMI/MCE interrupt handler). The issue manifests specifically when percpu allocation comes from vmalloc area rather than the embedded first chunk. Early HMI/MCE handlers called through DEFINE_INTERRUPT_HANDLER_NMI() wrapper invoke these problematic nmi_enter/nmi_exit calls. The issue can be triggered using the kernel command line parameter 'percpu_alloc=page' to force percpu allocation from vmalloc area (Kernel Git).

When exploited, this vulnerability can lead to a kernel crash during machine_check_early execution. This is particularly evident in the crash trace showing failure in rcu_nmi_enter+0x24/0x110 during machine_check_early execution (Kernel Git).

The vulnerability requires specific conditions to be exploited, including having CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK enabled and percpu allocation configured to use vmalloc area. It primarily affects PowerPC architecture systems under these specific configurations (Kernel Git).

The issue has been fixed by avoiding the use of nmi_enter()/nmi_exit() in real mode when the percpu first chunk is not embedded. The fix has been implemented in various Linux kernel versions, including Ubuntu 24.04 LTS and 22.04 LTS. System administrators should update to the patched kernel versions that include this fix (Ubuntu Security).