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

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Date:   Wed, 16 Sep 2020 10:11:10 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [RFC PATCH 08/35] KVM: SVM: Prevent debugging under SEV-ES

On 9/15/20 3:13 PM, Tom Lendacky wrote:
> 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@....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@....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?
>>>
>>> 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.

So I don't think there's any guarantee that the KVM_SET_GUEST_DEBUG ioctl
couldn't be called before the VMSA is encrypted, meaning I can't check the
arch.guest_state_protected bit for that call. So if we really want to get
rid of the allow_debug() op, I'd need some other way to indicate that this
is an SEV-ES / protected state guest.

How are you planning on blocking this ioctl for TDX? Would the
arch.guest_state_protected bit be sit earlier than is done for SEV-ES?

Thanks,
Tom

> 
> Thanks,
> Tom
> 
>>