NetBSD/src VTAMXyp — sys/arch/amd64/amd64 machdep.c, sys/compat/linux/arch/amd64 linux_machdep.c
Fix a tiny race in setregs and linux_setregs. Between the moment we set
pcb_flags to zero, and the moment cpu_segregs64_zero resets pcb_gs, we may
be preempted.
If this happens, and if the calling LWP was a 32bit thread, when switching
back to that LWP, the context switcher sees that PCB_COMPAT32 is not set in
pcb_flags and tries to perform a 64bit context switch; but pcb_gs contains
a 32bit GDT descriptor, and not a 64bit GS.base value. The wrmsr therefore
faults because the value is non-canonical, and this fault is fatal.
Rearrange the code so that the update of pcb_flags and pcb_gs/pcb_fs is non
interruptible. This fixes the problem, tested with a reproducer (which
therefore doesn't work anymore).
Likely fixes PR/53993.
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