linux-kernel - Re: [PATCH V4] virt: sev: Prevent IV reuse in SNP guest driver

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Message-ID: <c79eeb20-98c1-824c-9572-1474aa12a0f8@amd.com>
Date:   Fri, 4 Nov 2022 12:39:07 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     Dionna Glaze <dionnaglaze@...gle.com>,
        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>,
        Ashish Kalra <Ashish.Kalra@....com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH V4] virt: sev: Prevent IV reuse in SNP guest driver

On 11/3/22 10:23, Peter Gonda wrote:
> The 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:
> https://csrc.nist.gov/csrc/media/projects/block-cipher-techniques/documents/bcm/comments/800-38-series-drafts/gcm/joux_comments.pdf
> 
> To handle userspace querying the cert_data length handle_guest_request()
> now: saves the number of pages required by the host, retries the request
> without requesting the extended data, then returns the number of pages
> required.
> 
> Fixes: fce96cf044308 ("virt: Add SEV-SNP guest driver")
> Signed-off-by: Peter Gonda <pgonda@...gle.com>
> Reported-by: Peter Gonda <pgonda@...gle.com>
> Cc: Dionna Glaze <dionnaglaze@...gle.com>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Michael Roth <michael.roth@....com>
> Cc: Haowen Bai <baihaowen@...zu.com>
> Cc: Yang Yingliang <yangyingliang@...wei.com>
> Cc: Marc Orr <marcorr@...gle.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Ashish Kalra <Ashish.Kalra@....com>
> Cc: linux-kernel@...r.kernel.org
> Cc: kvm@...r.kernel.org

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

> ---
> Tested by placing each of the guest requests: attestation quote,
> extended attestation quote, and get key. Then tested the extended
> attestation quote certificate length querying.
> 
> V4
>   * As suggested by Dionna moved the extended request retry logic into
>     the driver.
>   * Due to big change in patch dropped any reviewed-by tags.
> 
> ---