[Fw_Os_Forum] How to correctly sign EFI Firmware Volume?

[Petr Vandrovec]

Richardson, Brian wrote on 10/2/2018 1:00 PM:
> Petr:
> 
> Signing should be covered by UEFI Capsule Update in EDK II.
> https://github.com/tianocore/tianocore.github.io/wiki/Capsule-Based-Firmware-Update-and-Firmware-Recovery
> 
> https://github.com/tianocore/edk2/tree/master/SignedCapsulePkg
> https://github.com/tianocore/edk2/tree/master/FmpDevicePkg

Thanks.

We are looking way how remote attestation can be done for our platform's 
firmware.  As our platform H-RTM measured our 2MB EFI ROM to TPM's PCR0, 
now only thing we need is that 3rd party tool can extract 2MB image, 
verify hash matches with TPM event log, and that image is signed by VMware.

Problem is that 3rd party tool must be able to know where to find 
signature and how to validate it.

And as Tianocore (and so VMware's UEFI) uses EFI Firmware Volume format 
for the ROM content, saying that our firmware volume has embedded 
signature conforming to PI spec 1.6 would be easiest for everybody: 
there is public standard, VMware implements signing & 3rd party 
implements verification, and all is good.

If that's not the way through, then I can take a look at making our ROM 
content being UEFI capsule with firmware volume payload, rather than 
just firmware volume, and then use UEFI capsule to hold signature.

But at that point I could also just wrap another firmware volume around 
our current one, and sign that embedded volume using FV's ability to 
sign individual files.  Or wrap image in Microsoft's PE format, and use 
their authenticode to sign then image.

We've prototyped both, but we feel uneasy with both as for both of them 
there is unsigned area that could be used to compromise ROM without 
breaking signature.

 > If you run into issues or have further questions, please ping the 
edk2-devel mailing list.
 > https://lists.01.org/mailman/listinfo/edk2-devel

Thanks.  I'll ask there.

Petr

> Thanks ... br
> ---
> Brian Richardson, Senior Technical Marketing Engineer, Intel Software


> -----Original Message-----
> From: fw_os_forum-bounces at mailman.uefi.org <fw_os_forum-bounces at mailman.uefi.org> On Behalf Of Petr Vandrovec
> Sent: Tuesday, October 2, 2018 3:36 PM
> To: fw_os_forum at mailman.uefi.org
> Subject: [Fw_Os_Forum] How to correctly sign EFI Firmware Volume?
> 
> Good morning,
> 
> I'm looking at options how to sign our EFI firmware through some industry-standard embedded signature option, and signing whole firmware volume as described in Platform Initialization spec would definitely fit the bill.
> 
> Unfortunately problem is that I cannot make sense of what should be actually signed.  Chapter 3.2.1.1 of PI_Spec_1_6.pdf says:
> 
> <quote>
> 3.2.1.1 EFI Signed Firmware Volumes
> 
> There may be one or more headers with a FormatType of value EFI_FIRMWARE_CONTENTS_SIGNED_GUID.
> 
> A signed firmware volume is a cryptographic signature across the entire volume. To process the contents and verify the integrity of the volume, the EFI_FIRMWARE_VOLUME_EXT_ENTRY_GUID_TYPE Data[] shall contain an instance of WIN_CERTIFICATE_UEFI_GUID where the CertType = EFI_CERT_TYPE_PKCS7_GUID or EFI_CERT_TYPE_RSA2048_SHA256_GUID.
> </quote>
> 
> Part about WIN_CERTIFICATE_UEFI_GUID is easy.  But what should be signed?
> 
> Text says 'A signed firmware volume is a cryptographic signature across the entire volume.' - beside that 'firmware volume' is not a signature, what is 'the entire volume' ?  Clearly Data[] entry holding signature cannot be part of the signature, as otherwise adding signature would invalidate that very same signature, so it cannot be signature of entire volume from first 16 reserved bytes in the header to the last byte of the image, but something else.
> 
> Can someone provide clarification what should be signed?  It seems to me like that intention is to only sign data portion of the volume, from the end of extended header to the end of volume.  But that means that anyone can modify anything in the header or extended header without breaking signature.
> 
> Are there any examples of signed firmware volumes?  Unfortunately there does not seem to be any code in UDK for this feature.
> 
> Thanks,
> Petr Vandrovec
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