linux-kernel - Re: [PATCH v7 44/45] virt: sevguest: Add support to derive key

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Message-ID: <f6214e86-019e-6b34-194d-c2b36001a19a@amd.com>
Date:   Thu, 18 Nov 2021 11:43:23 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v7 44/45] virt: sevguest: Add support to derive key



On 11/18/21 10:43 AM, Peter Gonda wrote:
...
>> +       u8 buf[89];
> 
> Could we document this magic number?
> 

Yes, I will document from where this number came.

>> +
>> +       if (!arg->req_data || !arg->resp_data)
>> +               return -EINVAL;
>> +
>> +       /* Copy the request payload from userspace */
>> +       if (copy_from_user(&req, (void __user *)arg->req_data, sizeof(req)))
>> +               return -EFAULT;
>> +
>> +       /* Message version must be non-zero */
>> +       if (!req.msg_version)
>> +               return -EINVAL;
>> +
>> +       /*
>> +        * The intermediate response buffer is used while decrypting the
>> +        * response payload. Make sure that it has enough space to cover the
>> +        * authtag.
>> +        */
>> +       resp_len = sizeof(resp.data) + crypto->a_len;
>> +       if (sizeof(buf) < resp_len)
>> +               return -ENOMEM;
>> +
>> +       /* Issue the command to get the attestation report */
>> +       rc = handle_guest_request(snp_dev, SVM_VMGEXIT_GUEST_REQUEST, req.msg_version,
>> +                                 SNP_MSG_KEY_REQ, &req.data, sizeof(req.data), buf, resp_len,
>> +                                 &arg->fw_err);
>> +       if (rc)
>> +               goto e_free;
> 
> Should we check the first 32 bits of |data| here since that is a
> status field? If we see 16h here we could return -EINVAL, or better to
> let userspace deal with that error handling?
> 

I was trying to avoid looking into a response structure to keep the 
flexibility; e.g if SNP firmware changes a response format then we don't 
need to have any changes in the driver. The userspace should be able to 
deal with it and it can check the "status" or a new field etc.

thanks