Re: [PATCH v3 2/4] x86/tdx: Add tdx_hcall_get_quote() API support

Isaku Yamahata
2022-04-22T17:27:16Z

On Wed, Apr 20, 2022 at 12:16:04AM -0700,
Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@linux.intel.com> wrote:

>
>
> On 4/19/22 8:39 PM, Aubrey Li wrote:
> > On 2022/4/16 上午6:01, Kuppuswamy Sathyanarayanan wrote:
> > > Attestation is the process used by two un-trusted entities to prove to
> > > each other that it can be trusted. In TDX guest, attestation is mainly
> > > used to verify the trustworthiness of a TD to the 3rd party key
> > > servers.
> > >
> > > First step in the attestation process is to generate the TDREPORT data.
> > > This support is added using tdx_mcall_tdreport() API. The second stage
> > > in the attestation process is for the guest to request the VMM generate
> > > and sign a quote based on the TDREPORT acquired earlier. More details
> > > about the steps involved in attestation process can be found in TDX
> > > Guest-Host Communication Interface (GHCI) for Intel TDX 1.5, section
> > > titled "TD attestation"
> > >
> > > Add tdx_hcall_get_quote() helper function to implement the GetQuote
> > > hypercall.
> > >
> > > More details about the GetQuote TDVMCALL are in the Guest-Host
> > > Communication Interface (GHCI) Specification, sec 3.3, titled
> > > "VP.VMCALL<GetQuote>".
> > >
> > > This will be used by the TD attestation driver in follow-on patches.
> > >
> > > Reviewed-by: Tony Luck <tony.luck@intel.com>
> > > Reviewed-by: Andi Kleen <ak@linux.intel.com>
> > > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> > > ---
> > > arch/x86/coco/tdx/tdx.c | 38 ++++++++++++++++++++++++++++++++++++++
> > > arch/x86/include/asm/tdx.h | 2 ++
> > > 2 files changed, 40 insertions(+)
> > >
> > > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> > > index 3e409b618d3f..c259d81a5d7f 100644
> > > --- a/arch/x86/coco/tdx/tdx.c
> > > +++ b/arch/x86/coco/tdx/tdx.c
> > > @@ -21,6 +21,7 @@
> > > /* TDX hypercall Leaf IDs */
> > > #define TDVMCALL_MAP_GPA 0x10001
> > > +#define TDVMCALL_GET_QUOTE 0x10002
> > > /* MMIO direction */
> > > #define EPT_READ 0
> > > @@ -144,6 +145,43 @@ long tdx_mcall_tdreport(void *data, void *reportdata)
> > > }
> > > EXPORT_SYMBOL_GPL(tdx_mcall_tdreport);
> > > +/*
> > > + * tdx_hcall_get_quote() - Generate TDQUOTE using TDREPORT_STRUCT.
> > > + *
> > > + * @data : Address of 8KB GPA memory which contains
> > > + * TDREPORT_STRUCT.
> > > + * @len : Length of the GPA in bytes.
> > > + *
> > > + * return 0 on success or failure error number.
> > > + */
> > > +long tdx_hcall_get_quote(void *data, u64 len)
> > > +{
> > > + u64 ret;
> > > +
> > > + /*
> > > + * Use confidential guest TDX check to ensure this API is only
> > > + * used by TDX guest platforms.
> > > + */
> > > + if (!data || !cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
> > > + return -EINVAL;
> > > +
> > > + /*
> > > + * Pass the physical address of tdreport data to the VMM
> > > + * and trigger the tdquote generation. Quote data will be
> > > + * stored back in the same physical address space. More info
> > > + * about ABI can be found in TDX Guest-Host-Communication
> > > + * Interface (GHCI), sec titled "TDG.VP.VMCALL<GetQuote>".
> > > + */
> > > + ret = _tdx_hypercall(TDVMCALL_GET_QUOTE, cc_mkdec(virt_to_phys(data)),
> > > + len, 0, 0);
> > > +
> >
> > I commented here in v2 but no response, so let me try again.
> >
> > IIUC, virt_to_phys(data) (GPA) will be stored in the register when
> > TDCALL brings the context back to the VMX root mode, and hypervisor(QEMU)
> > will find the mapped host virtual address(HVA) with the GPA in the register,
> > and the subsequent ops will be HVA<->HVA in hypervisor, EPT will not be
> > involved so no need to cc_mkdec() this GPA.
> >
> > Please help to correct me if I was wrong.
>
> It was done to meet the expectation from VMM. For shared GPA address,
> VMM expects shared bit set. All cc_mkdec() does is to set this bit.

This is to conform to the guest-host communicate interface(GHCI) spec.
The input value is defined as "shared GPA as input". Shared GPA is GPA with
shared bit set.

table TDG.VP.VMCALL<GetQuote> - Input Operands
R12 Shared GPA as input – the memory contains a TDREPORT_STRUCT.
The same buffer is used as output – the memory contains a TD Quote.

Userspace VMM(qemu) can be implemented to accept GPA with shared bit set or not.
It's not a big issue.
--
Isaku Yamahata <isaku.yamahata@gmail.com>