CVE-2024-42126: powerpc: Avoid nmi_enter/nmi_exit in real mode interrupt.

In the Linux kernel, the following vulnerability has been resolved:

powerpc: Avoid nmienter/nmiexit in real mode interrupt.

nmienter()/nmiexit() touches per cpu variables which can lead to kernel crash when invoked during real mode interrupt handling (e.g. early HMI/MCE interrupt handler) if percpu allocation comes from vmalloc area.

Early HMI/MCE handlers are called through DEFINEINTERRUPTHANDLERNMI() wrapper which invokes nmienter/nmiexit calls. We don't see any issue when percpu allocation is from the embedded first chunk. However with CONFIGNEEDPERCPUPAGEFIRSTCHUNK enabled there are chances where percpu allocation can come from the vmalloc area.

With kernel command line "percpualloc=page" we can force percpu allocation to come from vmalloc area and can see kernel crash in machinecheckearly:

[ 1.215714] NIP [c000000000e49eb4] rcunmienter+0x24/0x110 [ 1.215717] LR [c0000000000461a0] machinecheckearly+0xf0/0x2c0 [ 1.215719] --- interrupt: 200 [ 1.215720] [c000000fffd73180] [0000000000000000] 0x0 (unreliable) [ 1.215722] [c000000fffd731b0] [0000000000000000] 0x0 [ 1.215724] [c000000fffd73210] [c000000000008364] machinecheckearlycommon+0x134/0x1f8

Fix this by avoiding use of nmienter()/nmiexit() in real mode if percpu first chunk is not embedded.