[Fw_Os_Forum] How to correctly sign EFI Firmware Volume?

[Richardson, Brian]

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 

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 ... br
---
Brian Richardson, Senior Technical Marketing Engineer, Intel Software
brian.richardson at intel.com -- @intel_brian (Twitter & WeChat)
https://software.intel.com/en-us/meet-the-developers/evangelists/team/brian-richardson 

-----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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