I wonder if one could bind-mount some other proc file (say /proc/$EVIL_PID/cmdline) over cpuinfo to fake content. Would meet root and be visible in mount output though.
You could even just modify the kernel to report arbitrary strings in /proc/cpuinfo.
So they use the CPUID instruction instead. The CPUID instruction can be trapped to throw a SIGSEGV instead of returning real values on x86 with arch_prctl(ARCH_SET_CPUID, 0). So an injected SIGSEGV handler could then spoof it.
But even then, you could also trap the CPUID instruction in the kernel, and spoof it from there, which would be even harder to detect from user space.
In the end, the benchmark program always needs to trust the kernel. Is it really worth trying to detect spoofing?
Other systems do better by not depending on that.