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

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Message-ID: <52f3eb69-cd5a-5234-e222-7ca483f0f424@amd.com>
Date:   Thu, 17 Sep 2020 11:27:25 -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 5:50 PM, Sean Christopherson wrote:
> On Wed, Sep 16, 2020 at 03:27:13PM -0500, Tom Lendacky wrote:
>> 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.
> 
> Hmm.  I assume that also means KVM_SET_REGS and KVM_GET_XCRS are also legal
> before the VMSA is encrypted?  If so, then the current behavior of setting
> vmsa_encrypted "late" make sense.  KVM_SET_FPU/XSAVE can be handled by not
> allocating guest_fpu, i.e. they can be disallowed from time zero without
> adding an SEV-ES specific check.
> 
> Which brings us back to KVM_SET_GUEST_DEBUG.  What would happen if that were
> allowed prior to VMSA encryption?  If LAUNCH_UPDATE_VMSA acts as a sort of
> reset, one thought would be to allow KVM_SET_GUEST_DEBUG and then sanitize
> KVM's state during LAUNCH_UPDATE_VMSA.  Or perhaps even better, disallow
> LAUNCH_UPDATE_VMSA if vcpu->guest_debug!=0.  That would allow using debug
> capabilities up until LAUNCH_UPDATE_VMSA without adding much burden to KVM.

I think the vcpu->guest_debug check before the LAUNCH_UPDATE_VMSA would be 
good. I'll remove the allow_debug() op and replace it with the 
guest_state_protected check in its place.

Thanks,
Tom

>