Re: [RFC PATCH 27/27] KVM: TDX: Replace hardcoded CPUID filtering with the allowed mask

[Binbin Wu <binbin.wu@linux.intel.com>]

Below are responses to issues reported by sashiko. [1]

[1] https://sashiko.dev/#/patchset/20260417073610.3246316-1-binbin.wu%40linux.intel.com?part=27
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index a996e7f761ed4..2b980335b6675 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
[ ... ]
>  static void tdx_clear_unsupported_cpuid(struct kvm_cpuid_entry2 *entry)
>  {
> -	if (has_tsx(entry))
> -		clear_tsx(entry);
> +	u32 *reg = &entry->eax;
>  
> -	if (has_waitpkg(entry))
> -		clear_waitpkg(entry);
> +	for (int i = CPUID_EAX; i <= CPUID_EDX; i++)
> +		reg[i] &= kvm_cpuid_get_allowed_mask(entry->function, entry->index,
> +						     i, CPUID_OL_TDX);
>  }
Issue #1:
"Does this out-of-bounds memory access trigger undefined behavior?
Since entry->eax is a scalar member of struct kvm_cpuid_entry2, not
an array, accessing reg[i] for indices 1 through 3 goes past the bounds
of the eax object. Compilers with bounds-checking instrumentation like
UBSAN might detect this as a bounds violation and trigger a runtime trap.
Could we use an established helper like cpuid_get_reg_unsafe() or simply
unroll the loop to explicitly access the distinct struct members?"

Yes, will unroll the loop.

>  static bool tdx_unsupported_cpuid(const struct kvm_cpuid_entry2 *entry)
>  {
> -	return has_tsx(entry) || has_waitpkg(entry);
> +	const u32 *reg = &entry->eax;
> +
> +	for (int i = CPUID_EAX; i <= CPUID_EDX; i++) {
> +		if (reg[i] & ~kvm_cpuid_get_allowed_mask(entry->function, entry->index,
> +							 i, CPUID_OL_TDX))
> +			return true;
> +	}
> +
> +	return false;
>  }
Issue #2:
"Does this pointer arithmetic on entry->eax also trigger the same bounds
violation as above?"

Same as above.