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

["Yao, Jiewen" <jiewen.yao@intel.com>]

Gerd
I cannot agree your statement on ordering.

Smart attacker can forge the good measurement based upon the severity of vulnerability.

One famous example in 2011:
https://invisiblethingslab.com/resources/2011/Attacking_Intel_TXT_via_SINIT_hijacking.pdf
Because the attack happens before PCR18 measurement, the PCR18 is forged successfully.




> -----Original Message-----
> From: devel@edk2.groups.io <devel@edk2.groups.io> On Behalf Of Gerd
> Hoffmann
> Sent: Wednesday, April 20, 2022 4:17 PM
> To: Yao, Jiewen <jiewen.yao@intel.com>
> Cc: devel@edk2.groups.io; Xu, Min M <min.m.xu@intel.com>; Ard Biesheuvel
> <ardb+tianocore@kernel.org>; Justen, Jordan L <jordan.l.justen@intel.com>;
> Brijesh Singh <brijesh.singh@amd.com>; Aktas, Erdem
> <erdemaktas@google.com>; James Bottomley <jejb@linux.ibm.com>; Tom
> Lendacky <thomas.lendacky@amd.com>
> Subject: Re: [edk2-devel] [PATCH V3 5/9] OvmfPkg/IntelTdx: Measure Td
> HobList and Configuration FV
> 
>   Hi,
> 
> > > Yes for validation (aka sanity-checking the fields, etc).
> > > But for measurement I don't see why the ordering matters.
> > > Whenever you do that before or after consuming the TdHob
> > > should not make a difference.
> >
> > [Jiewen] I disagree. The order matters from security perspective.
> > If you use it, there is risk that the buggy code will compromise the system
> before you have chance to measure it.
> 
> Measurement will only record hashes for verification later on.
> It will not prevent running possibly buggy/compromised code.
> 
> So, no matter what the order is, you'll figure the system got
> compromised after the fact, when checking the hashes later, and in turn
> take actions like refusing to hand out secrets to the compromised
> system.
> 
> > There was already known attacks: The measurement was in wrong place,
> > which caused the attack can forge the measurement.
> 
> Do you have a link or CVE number for me?
> 
> thanks,
>   Gerd
> 
> 
> 
> 
>