[edk2-devel] [PATCH V3 5/9] OvmfPkg/IntelTdx: Measure Td HobList and Configuration FV

[Yao, Jiewen]

Adding CFV and TD_HOB to MRTD is technically possible, but not desired.

In a typical trust boot use case, the verifier should have a way to distinguish the *code* from *configuration*.
If you look at the TCG specification, the TPM has 24 PCRs. 8 of them are allocated for BIOS. Each PCRs record one type of measurements.
Technically, you can merge all PCR into one. But no one will do that in reality.

I would say: merging everything into one MRTD is a terrible idea.

Thank you
Yao Jiewen

> -----Original Message-----
> From: Gerd Hoffmann <kraxel at redhat.com>
> Sent: Thursday, April 21, 2022 5:15 PM
> To: Yao, Jiewen <jiewen.yao at intel.com>
> Cc: James Bottomley <jejb at linux.ibm.com>; devel at edk2.groups.io; Xu, Min M
> <min.m.xu at intel.com>; Ard Biesheuvel <ardb+tianocore at kernel.org>; Justen,
> Jordan L <jordan.l.justen at intel.com>; Brijesh Singh <brijesh.singh at amd.com>;
> Aktas, Erdem <erdemaktas at google.com>; Tom Lendacky
> <thomas.lendacky at amd.com>
> Subject: Re: [edk2-devel] [PATCH V3 5/9] OvmfPkg/IntelTdx: Measure Td
> HobList and Configuration FV
> 
> On Wed, Apr 20, 2022 at 10:29:11PM +0000, Yao, Jiewen wrote:
> > The Root-of-Trust for Measurement (RTM) for TDX is TDX-Module. The TDX-
> Module will enforce the MRTD calculation for the TDVF code.
> > Then TDVF can then act as Chain-of-Trust for Measurement (CTM) to setup
> RTMR and continue the rest.
> >
> > It is described in [TDX-Module] Chapter 11, [TDVF] Chapter 8.
> >
> > [TDX-Module]
> https://www.intel.com/content/dam/develop/external/us/en/documents/tdx-
> module-1.0-public-spec-v0.931.pdf
> > [TDVF]
> https://www.intel.com/content/dam/develop/external/us/en/documents/tdx-
> virtual-firmware-design-guide-rev-1.01.pdf
> 
> Ok.  So it all works via TDH.MEM.PAGE.ADD (initial set of accepted
> pages) and TDH.MR.EXTEND (measure into MRTD) functions.
> 
> Looking at our binary ...
> 
> # virt-fw-dump -i Build/IntelTdx/DEBUG_GCC5/FV/OVMF.fd --ovmf-meta
> image=Build/IntelTdx/DEBUG_GCC5/FV/OVMF.fd
>   resetvector size=0x9b0
>     [ ... sev metadata snipped ... ]
>     guid:TdxMetadataOffset size=0x16 data=50080000
>       mbase=0xffc84000 msize=0x37c000 type=BFV (code) fbase=0x84000
> fsize=0x37c000 flags=0x1
>       mbase=0xffc00000 msize=0x84000 type=CFV (vars) fbase=0x0 fsize=0x84000
>       mbase=0x810000 msize=0x10000 type=MEM
>       mbase=0x80b000 msize=0x2000 type=MEM
>       mbase=0x809000 msize=0x2000 type=TD Hob
>       mbase=0x800000 msize=0x6000 type=MEM
> 
> ... BFV is measured (bit 0 of flags) whereas CFV and TD Hob are only
> added but not measured.
> 
> Adding CFV and TH Hob to the initial launch measurement should be
> possible by just updating flags, correct?
> 
> I think this should be done for the CFV.  The firmware will be loaded
> via "qemu -bios OVMF.fd".  No separate images for CODE and VARS. So
> splitting the measurement looks rather pointless to me.
> 
> TD Hob could be part of the initial launch measurement too, which would
> avoid the need to measure anything in SEC.  On the other hand the that
> would make the launch measurement depend not only on the firmware image
> but also the guest configuration (memory size), which would likely make
> things more complexity elsewhere, so probably not a good idea.
> 
> take care,
>   Gerd



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