linux-kernel - Re: [PATCH v10 48/50] KVM: SEV: Provide support for SNP_GUEST

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Message-ID: <ZU6zGgvfhga0Oiob@google.com>
Date:   Fri, 10 Nov 2023 14:47:54 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Michael Roth <michael.roth@....com>
Cc:     Alexey Kardashevskiy <aik@....com>,
        Dionna Amalie Glaze <dionnaglaze@...gle.com>,
        kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, vkuznets@...hat.com,
        jmattson@...gle.com, luto@...nel.org, dave.hansen@...ux.intel.com,
        slp@...hat.com, pgonda@...gle.com, peterz@...radead.org,
        srinivas.pandruvada@...ux.intel.com, rientjes@...gle.com,
        dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de,
        vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com,
        tony.luck@...el.com, sathyanarayanan.kuppuswamy@...ux.intel.com,
        alpergun@...gle.com, jarkko@...nel.org, ashish.kalra@....com,
        nikunj.dadhania@....com, pankaj.gupta@....com,
        liam.merwick@...cle.com, zhi.a.wang@...el.com,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v10 48/50] KVM: SEV: Provide support for SNP_GUEST_REQUEST
 NAE event

On Fri, Nov 10, 2023, Michael Roth wrote:
> On Wed, Oct 18, 2023 at 06:48:59AM -0700, Sean Christopherson wrote:
> > On Wed, Oct 18, 2023, Alexey Kardashevskiy wrote:
> > Anyways, back to punting to userspace.  Here's a rough sketch.  The only new uAPI
> > is the definition of KVM_HC_SNP_GET_CERTS and its arguments.
> 
> This sketch seems like a good, flexible way to handle per-VM certs, but
> it does complicate things from a userspace perspective. As a basic
> requirement, all userspaces will need to provide a way to specify the
> initial blob (either a very verbose base64-encoded userspace cmdline param,
> or a filepatch that needs additional management to store and handle
> permissions/etc.), and also a means to update it (e.g. a HMP/QMP command
> for QEMU, some libvirt wrappers, etc.).
>
> That's all well and good if you want to make use of per-VM certs, but we
> don't necessarily expect that most deployments will necessarily want to deal
> with per-VM certs, and would be happy with a system-wide one where they could
> simply issue the /dev/sev ioctl to inject one automatically for all guests.
> 
> So we're sort of complicating the more common case to support a more niche
> one (as far as userspace is concerned anyway; as far as kernel goes, your
> approach is certainly simplest :)).
> 
> Instead, maybe a compromise is warranted so the requirements on userspace
> side are less complicated for a more basic deployment:
> 
>   1) If /dev/sev is used to set a global certificate, then that will be
>      used unconditionally by KVM, protected by simple dumb mutex during
>      usage/update.
>   2) If /dev/sev is not used to set the global certificate is the value
>      is NULL, we assume userspace wants full responsibility for managing
>      certificates and exit to userspace to request the certs in the manner
>      you suggested.
> 
> Sean, Dionna, would this cover your concerns and address the certificate
> update use-case?

Honestly, no.  I see zero reason for the kernel to be involved.  IIUC, there's no
privileged operations that require kernel intervention, which means that shoving
a global cert into /dev/sev is using the CCP driver as middleman.  Just use a
userspace daemon.  I have a very hard time believing that passing around large-ish
blobs of data in userspace isn't already a solved problem.