linux-kernel - Re: [PATCH V5] virt: sev: Prevent IV reuse in SNP guest driver

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Date:   Thu, 17 Nov 2022 07:19:17 -0700
From:   Peter Gonda <pgonda@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Borislav Petkov <bp@...e.de>, Michael Roth <michael.roth@....com>,
        Haowen Bai <baihaowen@...zu.com>,
        Yang Yingliang <yangyingliang@...wei.com>,
        Marc Orr <marcorr@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Dionna Glaze <dionnaglaze@...gle.com>,
        Ashish Kalra <Ashish.Kalra@....com>, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V5] virt: sev: Prevent IV reuse in SNP guest driver

On Wed, Nov 16, 2022 at 12:02 PM Tom Lendacky <thomas.lendacky@....com> wrote:
>
> On 11/16/22 11:55, Peter Gonda wrote:
> > The AMD Secure Processor (ASP) and an SNP guest use a series of
> > AES-GCM keys called VMPCKs to communicate securely with each other.
> > The IV to this scheme is a sequence number that both the ASP and the
> > guest track. Currently this sequence number in a guest request must
> > exactly match the sequence number tracked by the ASP. This means that
> > if the guest sees an error from the host during a request it can only
> > retry that exact request or disable the VMPCK to prevent an IV reuse.
> > AES-GCM cannot tolerate IV reuse see: "Authentication Failures in NIST
> > version of GCM" - Antoine Joux et al.
> >
> > In order to address this make handle_guest_request() delete the VMPCK
> > on any non successful return. To allow userspace querying the cert_data
> > length make handle_guest_request() safe the number of pages required by
>
> s/safe/save/
>
> > the host, then handle_guest_request() retry the request without
>
> ... then have handle_guest_request() ...
>
> > requesting the extended data, then return the number of pages required
> > back to userspace.
> >
> > Fixes: fce96cf044308 ("virt: Add SEV-SNP guest driver")
> > Signed-off-by: Peter Gonda <pgonda@...gle.com>
> > Reported-by: Peter Gonda <pgonda@...gle.com>
>
> Just some nits on the commit message and comments below, otherwise
>
> Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

Thanks Tom. I'll update with all the feedback after Boris chimes in.