Re: [RFC PATCH 08/35] KVM: SVM: Prevent debugging under SEV-ES

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, Sep 14, 2020 at 03:15:22PM -0500, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> Since the guest register state of an SEV-ES guest is encrypted, debugging
> is not supported. Update the code to prevent guest debugging when the
> guest is an SEV-ES guest. This includes adding a callable function that
> is used to determine if the guest supports being debugged.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 ++
>  arch/x86/kvm/svm/svm.c          | 16 ++++++++++++++++
>  arch/x86/kvm/vmx/vmx.c          |  7 +++++++
>  arch/x86/kvm/x86.c              |  3 +++
>  4 files changed, 28 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index c900992701d6..3e2a3d2a8ba8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1234,6 +1234,8 @@ struct kvm_x86_ops {
>  	void (*reg_read_override)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
>  	void (*reg_write_override)(struct kvm_vcpu *vcpu, enum kvm_reg reg,
>  				   unsigned long val);
> +
> +	bool (*allow_debug)(struct kvm *kvm);

Why add both allow_debug() and vmsa_encrypted?  I assume there are scenarios
where allow_debug() != vmsa_encrypted?  E.g. is there a debug mode for SEV-ES
where the VMSA is not encrypted, but KVM (ironically) can't intercept #DBs or
something?

Alternatively, have you explored using a new VM_TYPE for SEV-ES guests?  With
a genericized vmsa_encrypted, that would allow something like the following
for scenarios where the VMSA is not (yet?) encrypted for an SEV-ES guest.  I
don't love bleeding the VM type into x86.c, but for one-off quirks like this
I think it'd be preferable to adding a kvm_x86_ops hook.

int kvm_arch_vcpu_ioctl_set_guest_debug(...)
{
	if (vcpu->arch.guest_state_protected ||
	    kvm->arch.vm_type == KVM_X86_SEV_ES_VM)
		return -EINVAL;
}