Re: [PATCH v3 2/4] KVM: x86: Partially allow KVM_SET_CPUID{,2} after KVM_RUN

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 18, 2022, Vitaly Kuznetsov wrote:
> +/* Check whether the supplied CPUID data is equal to what is already set for the vCPU. */
> +static int kvm_cpuid_check_equal(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
> +				 int nent)
> +{
> +	struct kvm_cpuid_entry2 *orig;
> +	int i;
> +
> +	if (nent != vcpu->arch.cpuid_nent)
> +		return -EINVAL;
> +
> +	for (i = 0; i < nent; i++) {
> +		orig = &vcpu->arch.cpuid_entries[i];
> +		if (e2[i].function != orig->function ||
> +		    e2[i].index != orig->index ||
> +		    e2[i].eax != orig->eax || e2[i].ebx != orig->ebx ||
> +		    e2[i].ecx != orig->ecx || e2[i].edx != orig->edx)
> +			return -EINVAL;

This needs to check .flags for the above check on .index to be meaningful, and at
that point, can't we be even more agressive and just do?

	if (memcmp(e2, vcpu->arch.cpuid_entries, nent * sizeof(e2)))
		return -EINVAL;

	return 0;

> +	}
> +
> +	return 0;
> +}
> +
>  static void kvm_update_kvm_cpuid_base(struct kvm_vcpu *vcpu)
>  {
>  	u32 function;
> @@ -313,6 +335,20 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
>  
>  	__kvm_update_cpuid_runtime(vcpu, e2, nent);
> +	/*
> +	 * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
> +	 * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
> +	 * tracked in kvm_mmu_page_role.  As a result, KVM may miss guest page
> +	 * faults due to reusing SPs/SPTEs. In practice no sane VMM mucks with
> +	 * the core vCPU model on the fly. It would've been better to forbid any
> +	 * KVM_SET_CPUID{,2} calls after KVM_RUN altogether but unfortunately
> +	 * some VMMs (e.g. QEMU) reuse vCPU fds for CPU hotplug/unplug and do
> +	 * KVM_SET_CPUID{,2} again. To support this legacy behavior, check
> +	 * whether the supplied CPUID data is equal to what's already set.

This is misleading/wrong.  KVM_RUN isn't the only problematic ioctl(), it's just
the one that we decided to use to detect that userspace is being stupid.  And
forbidding KVM_SET_CPUID after KVM_RUN (or even all problematic ioctls()) wouldn't
solve problem as providing different CPUID configurations for vCPUs in a VM will
also cause the MMU to fall on its face.

> +	if (vcpu->arch.last_vmentry_cpu != -1)
> +		return kvm_cpuid_check_equal(vcpu, e2, nent);

And technically, checking last_vmentry_cpu doesn't forbid changing CPUID after
KVM_RUN, it forbids changing CPUID after successfully entering the guest (or
emulating instructions on VMX).

I realize I'm being very pedantic, as a well-intended userspace is obviously not
going to change CPUID after -EINTR or whatever.  But I do want to highlight that
this approach is by no means bulletproof, and that what is/isn't allowed with
respect to guest CPUID isn't necessarily associated with what is/isn't "safe".
In other words, this check doesn't guarantee that userspace can't misuse KVM_SET_CPUID,
and on the flip side it disallows using KVM_SET_CPUID in ways that are perfectly ok
(if userspace is careful and deliberate).