Re: [PATCH v2] qapi, target/i386/sev: Add cpu0-id to query-sev-capabilities

[Dov Murik]

On 24/02/2022 10:59, Daniel P. Berrangé wrote:
> On Thu, Feb 24, 2022 at 06:14:05AM +0000, Dov Murik wrote:
>> Add a new field 'cpu0-id' to the response of query-sev-capabilities QMP
>> command.  The value of the field is the base64-encoded 64-byte unique ID
>> of the CPU0 (socket 0), which can be used to retrieve the signed CEK of
>> the CPU from AMD's Key Distribution Service (KDS).
>>
>> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
>>
>> ---
>>
>> v2:
>> - change encoding to Base64 (thanks Daniel)
>> - rename constant to SEV_CPU_UNIQUE_ID_LEN
>> ---
>>  qapi/misc-target.json |  4 ++++
>>  target/i386/sev.c     | 27 +++++++++++++++++++++++++++
>>  2 files changed, 31 insertions(+)
>>
>> diff --git a/qapi/misc-target.json b/qapi/misc-target.json
>> index 4bc45d2474..c6d9ad69e1 100644
>> --- a/qapi/misc-target.json
>> +++ b/qapi/misc-target.json
>> @@ -177,6 +177,8 @@
>>  #
>>  # @cert-chain:  PDH certificate chain (base64 encoded)
>>  #
>> +# @cpu0-id: 64-byte unique ID of CPU0 (base64 encoded) (since 7.0)
>> +#
>>  # @cbitpos: C-bit location in page table entry
>>  #
>>  # @reduced-phys-bits: Number of physical Address bit reduction when SEV is
>> @@ -187,6 +189,7 @@
>>  { 'struct': 'SevCapability',
>>    'data': { 'pdh': 'str',
>>              'cert-chain': 'str',
>> +            'cpu0-id': 'str',
>>              'cbitpos': 'int',
>>              'reduced-phys-bits': 'int'},
>>    'if': 'TARGET_I386' }
>> @@ -205,6 +208,7 @@
>>  #
>>  # -> { "execute": "query-sev-capabilities" }
>>  # <- { "return": { "pdh": "8CCDD8DDD", "cert-chain": "888CCCDDDEE",
>> +#                  "cpu0-id": "2lvmGwo+...61iEinw==",
>>  #                  "cbitpos": 47, "reduced-phys-bits": 5}}
>>  #
>>  ##
>> diff --git a/target/i386/sev.c b/target/i386/sev.c
>> index 025ff7a6f8..d3d2680e16 100644
>> --- a/target/i386/sev.c
>> +++ b/target/i386/sev.c
>> @@ -82,6 +82,8 @@ struct SevGuestState {
>>  #define DEFAULT_GUEST_POLICY    0x1 /* disable debug */
>>  #define DEFAULT_SEV_DEVICE      "/dev/sev"
>>  
>> +#define SEV_CPU_UNIQUE_ID_LEN   64
> 
> Is this fixed size actually guaranteed by AMD ?  In reading the spec
> for "GET_ID" it feels like they're intentionally not specifying a
> fixed length, pushing towards querying once and then re-trying with
> the reported buffer size:
> 
>    "If the value of the ID_LEN field is too small, an 
>     INVALID_LENGTH error is returned and the minimum 
>     required length is written to the field"
> 
> I didn't find where it says 64 bytes exactly.
> 

OK.

I'll change it to dynamically allocate the ID buffer based on the length
returned by the first query, similar to sev_get_pdh_info().

I'll remove the explicit lengths from the qapi description and the
commit message.

-Dov


>> +
>>  #define SEV_INFO_BLOCK_GUID     "00f771de-1a7e-4fcb-890e-68c77e2fb44e"
>>  typedef struct __attribute__((__packed__)) SevInfoBlock {
>>      /* SEV-ES Reset Vector Address */
>> @@ -531,11 +533,31 @@ e_free:
>>      return 1;
>>  }
>>  
>> +static int
>> +sev_get_id(int fd, guchar *id_buf, size_t id_buf_len, Error **errp)
>> +{
>> +    struct sev_user_data_get_id2 id = {
>> +        .address = (unsigned long)id_buf,
>> +        .length = id_buf_len
>> +    };
>> +    int err, r;
>> +
>> +    r = sev_platform_ioctl(fd, SEV_GET_ID2, &id, &err);
>> +    if (r < 0) {
>> +        error_setg(errp, "SEV: Failed to get ID ret=%d fw_err=%d (%s)",
>> +                   r, err, fw_error_to_str(err));
>> +        return 1;
>> +    }
>> +
>> +    return 0;
>> +}
>> +
>>  static SevCapability *sev_get_capabilities(Error **errp)
>>  {
>>      SevCapability *cap = NULL;
>>      guchar *pdh_data = NULL;
>>      guchar *cert_chain_data = NULL;
>> +    guchar cpu0_id[SEV_CPU_UNIQUE_ID_LEN];
>>      size_t pdh_len = 0, cert_chain_len = 0;
>>      uint32_t ebx;
>>      int fd;
>> @@ -561,9 +583,14 @@ static SevCapability *sev_get_capabilities(Error **errp)
>>          goto out;
>>      }
>>  
>> +    if (sev_get_id(fd, cpu0_id, sizeof(cpu0_id), errp)) {
>> +        goto out;
>> +    }
>> +
>>      cap = g_new0(SevCapability, 1);
>>      cap->pdh = g_base64_encode(pdh_data, pdh_len);
>>      cap->cert_chain = g_base64_encode(cert_chain_data, cert_chain_len);
>> +    cap->cpu0_id = g_base64_encode(cpu0_id, sizeof(cpu0_id));
>>  
>>      host_cpuid(0x8000001F, 0, NULL, &ebx, NULL, NULL);
>>      cap->cbitpos = ebx & 0x3f;
>> -- 
>> 2.25.1
>>
> 
> Regards,
> Daniel