Re: [PATCH 1/2] KVM: TDX: Handle TDG.VP.VMCALL<GetQuote>

[Binbin Wu <binbin.wu@linux.intel.com>]

On 4/2/2025 8:53 PM, Binbin Wu wrote:
[...]
>>> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
>>> index b952bc673271..535200446c21 100644
>>> --- a/arch/x86/kvm/vmx/tdx.c
>>> +++ b/arch/x86/kvm/vmx/tdx.c
>>> @@ -1463,6 +1463,39 @@ static int tdx_get_td_vm_call_info(struct kvm_vcpu *vcpu)
>>>       return 1;
>>>   }
>>>   +static int tdx_complete_get_quote(struct kvm_vcpu *vcpu)
>>> +{
>>> +    tdvmcall_set_return_code(vcpu, vcpu->run->tdx_get_quote.ret);
>>> +    return 1;
>>> +}
>>> +
>>> +static int tdx_get_quote(struct kvm_vcpu *vcpu)
>>> +{
>>> +    struct vcpu_tdx *tdx = to_tdx(vcpu);
>>> +
>>> +    u64 gpa = tdx->vp_enter_args.r12;
>>> +    u64 size = tdx->vp_enter_args.r13;
>>> +
>>> +    /* The buffer must be shared memory. */
>>> +    if (vt_is_tdx_private_gpa(vcpu->kvm, gpa) || size == 0) {
>>> +        tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_INVALID_OPERAND);
>>> +        return 1;
>>> +    }
>> It is a little bit confusing about the shared buffer check here.  There are two
>> perspectives here:
>>
>> 1) the buffer has already been converted to shared, i.e., the attributes are
>> stored in the Xarray.
>> 2) the GPA passed in the GetQuote must have the shared bit set.
>>
>> The key is we need 1) here.  From the spec, we need the 2) as well because it
>> *seems* that the spec requires GetQuote to provide the GPA with shared bit set,
>> as it says "Shared GPA as input".
>>
>> The above check only does 2).  I think we need to check 1) as well, because once
>> you forward this GetQuote to userspace, userspace is able to access it freely.
>
> Right.
>
> Another discussion is whether KVM should skip the sanity checks for GetQuote
> and let the userspace take the job.
> Considering checking the buffer is shared memory or not, KVM seems to be a
> better place.
A second thought. If the userspace could do the shared memory check, the
whole sanity checks can be done in userspace to keep KVM as small as possible.

>
>>
>> As a result, the comment
>>
>>    /* The buffer must be shared memory. */
>>
>> should also be updated to something like:
>>
>>    /*
>>     * The buffer must be shared. GetQuote requires the GPA to have
>>     * shared bit set.
>>     */
>
>