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

[Tom Lendacky]

On 9/14/20 4:26 PM, Sean Christopherson wrote:
> On Mon, Sep 14, 2020 at 03:15:22PM -0500, Tom Lendacky wrote:
>> From: Tom Lendacky <thomas.lendacky@xxxxxxx>
>>
>> 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@xxxxxxx>
>> ---
>>  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?

No, once the guest has had LAUNCH_UPDATE_VMSA run against the vCPUs, then
the vCPU states are all encrypted. But that doesn't mean that debugging
can't be done in the future.

> 
> 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;
> }
> 

I haven't explored that, I'll look into it.

Thanks,
Tom