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

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Message-ID: <9988f485-ce78-4df4-b294-32cc7743b6b2@amd.com>
Date:   Wed, 16 Sep 2020 15:27:13 -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 11:49 AM, Sean Christopherson wrote:
> On Wed, Sep 16, 2020 at 11:38:38AM -0500, Tom Lendacky wrote:
>>
>>
>> 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).
> 
> Ah, gotcha.  Would it work to set guest_state_protected[*] from time zero,
> and move vmsa_encrypted to struct vcpu_svm?  I.e. keep vmsa_encrypted, but
> use it only for guiding get_vmsa() and related behavior.

It is mainly __set_sregs() that needs to know when to allow the register
writes and when not to. During guest initialization, __set_sregs is how
some of the VMSA is initialized by Qemu.

Thanks,
Tom

>