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

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Date:   Wed, 16 Sep 2020 11:38:38 -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 9/16/20 11:02 AM, Sean Christopherson wrote:
> On Wed, Sep 16, 2020 at 10:11:10AM -0500, Tom Lendacky wrote:
>> On 9/15/20 3:13 PM, Tom Lendacky wrote:
>>> On 9/15/20 11:30 AM, Sean Christopherson wrote:
>>>> 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.
> 
> Would anything break if KVM "speculatively" set guest_state_protected before
> LAUNCH_UPDATE_VMSA?  E.g. does KVM need to emulate before LAUNCH_UPDATE_VMSA?

Yes, the way the code is set up, the guest state (VMSA) is initialized in
the same way it is today (mostly) and that state is encrypted by the
LAUNCH_UPDATE_VMSA call. I check the guest_state_protected bit to decide
on whether to direct the updates to the real VMSA (before it's encrypted)
or the GHCB (that's the get_vmsa() function from patch #5).

Thanks,
Tom

> 
>> 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?
> 
> Yep, guest_state_protected is set from time zero (kvm_x86_ops.vm_init) as
> guest state is encrypted/inaccessible from the get go.  The flag actually
> gets turned off for debuggable TDX guests, but that's also forced to happen
> before the KVM_RUN can be invoked (TDX architecture) and is a one-time
> configuration, i.e. userspace can flip the switch exactly once, and only at
> a very specific point in time.
>