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Date: Mon, 29 Apr 2024 07:27:22 -0700
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Subject: Re: [PATCH v14 21/22] crypto: ccp: Add the SNP_{PAUSE,RESUME}_ATTESTATION
 commands
From: Sean Christopherson <seanjc@google.com>
To: Michael Roth <michael.roth@amd.com>
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On Fri, Apr 26, 2024, Michael Roth wrote:
> On Fri, Apr 26, 2024 at 05:10:10PM -0700, Sean Christopherson wrote:
> > e.g. put the cert in a directory along with a lock.  Actually, IIUC, there doesn't
> > even need to be a separate lock file.  I know very little about userspace programming,
> > but common sense and a quick search tells me that file locks are a solved problem.
> > 
> > E.g. it took me ~5 minutes of Googling to come up with this, which AFAICT does
> > exactly what you want.
> > 
> > touch ~/vlek.cert
> > (
> >   flock -e 200
> >   echo "Locked the cert, sleeping for 10 seconds"
> >   sleep 10
> >   echo "Igor, it's alive!!!!!!"
> > ) 200< vlek.cert
> > 
> > touch ~/vlek.cert
> > (
> >   flock -s 201
> >   echo "Got me a shared lock, no updates for you!"
> > ) 201< vlek.cert
> > 
> 
> Hmm... I did completely miss this option. But I think there are still some
> issues here. IIUC you're suggesting (for example):
> 
>   "Management":
>   a) writelock vlek.cert
>   b) perform SNP_LOAD_VLEK and update vlek.cert contents
>   c) unlock vlek.cert
> 
>   "QEMU":
>   a) readlock vlek.cert
>   b) copy cert into guest buffer
>   c) unlock vlek.cert
> 
> The issue is that after "QEMU" unlocks and return the cert to KVM we'll
> have:
> 
>   "KVM"
>   a) return from EXT_GUEST_REQ exit to userspace
>   b) issue the attestation report to firmware
>   c) return the attestation report and cert to the guest
> 
> Between a) and b), "Management" can complete another entire update, but
> the cert that it passes back to the guest will be stale relative to the
> key used to sign the attestation report.

I was thinking userspace would hold the lock across SEV_CMD_SNP_GUEST_REQUEST.

   QEMU:
    a) readlock vlek.cert
    b) copy cert into guest buffer
    c) set kvm_run->immediate_exit
    d) invoke KVM_RUN
    e) KVM sends SEV_CMD_SNP_GUEST_REQUEST to PSP
    f) KVM exits to userspace with -EINTR
    g) unlock vlek.cert
    h) invoke KVM_RUN (resume the guest)

> If we need to take more time to explore other options it's not
> absolutely necessary to have the kernel solve this now. But every userspace
> will need to solve it in some way so it seemed like it might be nice to
> have a simple reference implementation to start with.

Shoving something into the kernel is not a "reference implementation", especially
not when it impacts the ABI of multiple subsystems.