Re: [PATCH v5 30/65] i386/tdx: Support user configurable mrconfigid/mrowner/mrownerconfig

[Markus Armbruster]

Xiaoyao Li <xiaoyao.li@intel.com> writes:

> On 2/29/2024 4:37 PM, Markus Armbruster wrote:
>> Xiaoyao Li <xiaoyao.li@intel.com> writes:
>> 
>>> From: Isaku Yamahata <isaku.yamahata@intel.com>
>>>
>>> Three sha384 hash values, mrconfigid, mrowner and mrownerconfig, of a TD
>>> can be provided for TDX attestation. Detailed meaning of them can be
>>> found: 
>>> https://lore.kernel.org/qemu-devel/31d6dbc1-f453-4cef-ab08-4813f4e0ff92@intel.com/
>>>
>>> Allow user to specify those values via property mrconfigid, mrowner and
>>> mrownerconfig. They are all in base64 format.
>>>
>>> example
>>> -object tdx-guest, \
>>>    
>>> mrconfigid=ASNFZ4mrze8BI0VniavN7wEjRWeJq83vASNFZ4mrze8BI0VniavN7wEjRWeJq83v,\
>>>    
>>> mrowner=ASNFZ4mrze8BI0VniavN7wEjRWeJq83vASNFZ4mrze8BI0VniavN7wEjRWeJq83v,\
>>>    
>>> mrownerconfig=ASNFZ4mrze8BI0VniavN7wEjRWeJq83vASNFZ4mrze8BI0VniavN7wEjRWeJq83v
>>>
>>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>>> Co-developed-by: Xiaoyao Li <xiaoyao.li@intel.com>
>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>

[...]

>>> diff --git a/qapi/qom.json b/qapi/qom.json
>>> index 89ed89b9b46e..cac875349a3a 100644
>>> --- a/qapi/qom.json
>>> +++ b/qapi/qom.json
>>> @@ -905,10 +905,25 @@
>>>  #     pages.  Some guest OS (e.g., Linux TD guest) may require this to
>>>  #     be set, otherwise they refuse to boot.
>>>  #
>>> +# @mrconfigid: ID for non-owner-defined configuration of the guest TD,
>>> +#     e.g., run-time or OS configuration (base64 encoded SHA384 digest).
>>> +#     (A default value 0 of SHA384 is used when absent).
>>
>> Suggest to drop the parenthesis in the last sentence.
>>
>> @mrconfigid is a string, so the default value can't be 0.  Actually,
>> it's not just any string, but a base64 encoded SHA384 digest, which
>> means it must be exactly 96 hex digits.  So it can't be "0", either.  It
>> could be
>> "000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000".
>
> I thought value 0 of SHA384 just means it.
>
> That's my fault and my poor english.

"Fault" is too harsh :)  It's not as precise as I want our interface
documentation to be.  We work together to get there.

>> More on this below.
>> 
>>> +#
>>> +# @mrowner: ID for the guest TD’s owner (base64 encoded SHA384 digest).
>>> +#     (A default value 0 of SHA384 is used when absent).
>>> +#
>>> +# @mrownerconfig: ID for owner-defined configuration of the guest TD,
>>> +#     e.g., specific to the workload rather than the run-time or OS
>>> +#     (base64 encoded SHA384 digest). (A default value 0 of SHA384 is
>>> +#     used when absent).
>>> +#
>>>  # Since: 9.0
>>>  ##
>>>  { 'struct': 'TdxGuestProperties',
>>> -  'data': { '*sept-ve-disable': 'bool' } }
>>> +  'data': { '*sept-ve-disable': 'bool',
>>> +            '*mrconfigid': 'str',
>>> +            '*mrowner': 'str',
>>> +            '*mrownerconfig': 'str' } }
>>>  ##
>>>  # @ThreadContextProperties:
>>> diff --git a/target/i386/kvm/tdx.c b/target/i386/kvm/tdx.c
>>> index d0ad4f57b5d0..4ce2f1d082ce 100644
>>> --- a/target/i386/kvm/tdx.c
>>> +++ b/target/i386/kvm/tdx.c
>>> @@ -13,6 +13,7 @@
>>>  #include "qemu/osdep.h"
>>>  #include "qemu/error-report.h"
>>> +#include "qemu/base64.h"
>>>  #include "qapi/error.h"
>>>  #include "qom/object_interfaces.h"
>>>  #include "standard-headers/asm-x86/kvm_para.h"
>>> @@ -516,6 +517,7 @@ int tdx_pre_create_vcpu(CPUState *cpu, Error **errp)
>>>      X86CPU *x86cpu = X86_CPU(cpu);
>>>      CPUX86State *env = &x86cpu->env;
>>>      g_autofree struct kvm_tdx_init_vm *init_vm = NULL;
>>> +    size_t data_len;
>>>      int r = 0;
>>>      object_property_set_bool(OBJECT(cpu), "pmu", false, &error_abort);
>>> @@ -528,6 +530,38 @@ int tdx_pre_create_vcpu(CPUState *cpu, Error **errp)
>>>      init_vm = g_malloc0(sizeof(struct kvm_tdx_init_vm) +
>>>                          sizeof(struct kvm_cpuid_entry2) * 
>>> KVM_MAX_CPUID_ENTRIES);
>>> +#define SHA384_DIGEST_SIZE  48
>>> +
>>> +    if (tdx_guest->mrconfigid) {
>>> +        g_autofree uint8_t *data = qbase64_decode(tdx_guest->mrconfigid,
>>> +                              strlen(tdx_guest->mrconfigid), &data_len, 
>>> errp);
>>> +        if (!data || data_len != SHA384_DIGEST_SIZE) {
>>> +            error_setg(errp, "TDX: failed to decode mrconfigid");
>>> +            return -1;
>>> +        }
>>> +        memcpy(init_vm->mrconfigid, data, data_len);
>>> +    }
>>
>> When @mrconfigid is absent, the property remains null, and this
>> conditional is not executed.  init_vm->mrconfigid[], an array of 6
>> __u64, remains all zero.  How does the kernel treat that?
>
> A all-zero SHA384 value is still a valid value, isn't it?
>
> KVM treats it with no difference.

Can you point me to the spot in the kernel where mrconfigid is used?

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