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

[Tom Lendacky]

On 9/15/20 11:30 AM, Sean Christopherson wrote:
> On Tue, Sep 15, 2020 at 08:37:12AM -0500, Tom Lendacky wrote:
>> 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.
> 
> I don't quite follow the "doesn't mean debugging can't be done in the future".
> Does that imply that debugging could be supported for SEV-ES guests, even if
> they have an encrypted VMSA?

Almost anything can be done with software. It would require a lot of
hypervisor and guest code and changes to the GHCB spec, etc. So given
that, probably just the check for arch.guest_state_protected is enough for
now. I'll just need to be sure none of the debugging paths can be taken
before the VMSA is encrypted.

Thanks,
Tom

>