Processor features are not checked

[penberg]

The x86-64 bootstrap code, for example, assumes the presence of various MSRs but fails to check if the processor actually supports them or not (in particular, the "no execute" feature).

[tapaswenipathak]

Hi @penberg: Can add few outlines here as well?

[penberg]

The OS configures some CPU features, but does not check if the CPU supports them.

For example, the OS enables the no execute (MX) bit by writing to the IA32_EFER MSR. However, before we can do that, we should check if that is even supported with the CPUID instruction (see Section 5.13.1 (“Detecting and Enabling the Execute-Disable Capability”) of the Intel Architecture’s Software Developer Manual (SDM) for details).

You could, for example, have something like:

enum cpu_feature {
   CPU_FEATURE_NX,
};

bool cpu_has_feature(enum cpu_feature feature);

and simply call panic() if that NX feature is not supported before writing to IA32_EFER.

I think there are some other places too, which should check for CPU feature, so auditing the code after we have the infrastructure is needed.