Re: [RFC PATCH 16/27] KVM: x86: Init allowed masks for basic CPUID range in paranoid mode

[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=16

[...]

> @@ -876,6 +905,14 @@ void kvm_initialize_cpu_caps(void)
>  	BUILD_BUG_ON(sizeof(kvm_cpu_caps)/NR_CPUID_OL - (NKVMCAPINTS * sizeof(**kvm_cpu_caps)) >
>  		     sizeof(boot_cpu_data.x86_capability));
>  
> +	kvm_cpu_cap_ignore(0, 0, 0,
> +			   BIT(CPUID_EAX) | BIT(CPUID_EBX) | BIT(CPUID_ECX) | BIT(CPUID_EDX),
> +			   F_CPUID_DEFAULT | F_CPUID_TDX);
> +

"Could this cause an array overflow if the vendor module is reloaded?
kvm_initialize_cpu_caps() is called during vendor module initialization (e.g.,
from vmx_set_cpu_caps() when kvm-intel.ko is loaded). It zeroes out
kvm_cpu_caps, but does not reset ignored_set.nr to 0.
If the vendor module is repeatedly unloaded and reloaded, ignored_set.nr
will continually grow until it exceeds KVM_MAX_CPUID_ENTRIES. At that point,
the WARN_ON_ONCE in kvm_cpu_cap_ignore() will trigger, and all subsequent
ignored CPUID entries will be dropped.
This could cause valid userspace CPUID inputs to be rejected and break VM
creation.
Should ignored_set.nr = 0; be added to the beginning of
kvm_initialize_cpu_caps()?"

Yes, ignored_set.nr should be reset at the beginning of
kvm_initialize_cpu_caps().