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

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