[nailer]

CPUID is available in /proc/cpuinfo (or whatever the /sys or modern equivalent is, my Linux is a bit rusty)

Not sure re: UD2 though.

[nenolod]

The point here is that the information in /proc/cpuinfo is virtualized, and therefore may not reflect reality...

[nailer]

Would you have to be a kernel module to access the CPU directly?

[aryastark]

No. All binary programs access the CPU directly. Only certain instructions require being in a certain "ring" (i.e. access level). The CPUID instruction is non-privileged, meaning you can access it from user space.