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Date: Tue, 31 Aug 2021 07:13:05 +0200
From: "Gerd Hoffmann" <kraxel@redhat.com>
To: devel@edk2.groups.io, min.m.xu@intel.com
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>,
	"Justen, Jordan L" <jordan.l.justen@intel.com>,
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	Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [edk2-devel] [PATCH V5 1/2] OvmfPkg: Introduce Tdx BFV/CFV PCDs and PcdOvmfImageSizeInKb
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  Hi,

> > From a security point of view I don't think it is a good idea to hard code any
> > assumptions about the layout of the vars volume.
> Do you mean I cannot assume the layout of VarStore? 
> At least in Ovmf the VarStore.fdf.inc defines the layout of VarStore like below.

What prevents an attacker from creating a varstore with a different
layout?  Place the variables at the end of the file, which isn't
measured (because you assume it is the spare part), then being able
to change variables without the guest noticing?

take care,
  Gerd