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coreboot-kgpe-d16/Documentation/vendorcode/eltan/security.md

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Eltan Security

This code enables measured boot and verified boot support. Verified boot is available in coreboot, but based on ChromeOS. This vendorcode security solution is intended to be used for system without ChromeOS support.

This solution allows implementing verified boot support for systems that do not contain a TPM.

Hashing Library

The API functions of 3rdparty/vboot/firmware are used.

Measured boot

Measured boot support requires a TPM2 device.

The items specified in mb_log_list[] and *_verify_list[] will be measured.

The mb_log_list[] should only contain items that are not contained in one of the verify_lists below (except for the bootblock_verify_list[]).

The list can contain the following items: config, revision, cmos_layout.bin. oemmanifest.bin should be added to the list when Verified boot is enabled.

Verified boot

Verified boot support will use the OEM manifest to verify the items.

The verification process is controlled using the following verify lists:

  • bootblock_verify_list[] (will not be measured, verified in bootblock)
  • romstage_verify_list[] (verified in early romstage)
  • postcar_verify_list[] (verified in just before postcar loading)
  • ramstage_verify_list[] (verified in just before ramstage loading)
  • payload_verify_list[] (verified in just before payload loading)
  • oprom_verify_list[] (verified before option rom execution)

A verify_list entry contains a related_items member. This can point to an additional verify_list which will be verified before the specified item is verified. As an example the grub entry in payload_verify_list[] can point to the grub_additional_list[] that contains the items used by the grub payload and the seabios entry in payload_verify_list[] can point to the seabios_additional_list[] that contains the items used by the seabios payload. By doing this the entries that are verified (and measured) depend on the payload selected at runtime.

Creating private and public keys

Create private key in RSA2048 format: openssl genrsa -F4 -out <private_key_file> 2048

Create public key using private key: futility --vb1 create <private_key_file> <public_key_file_without_extension>

The public key will be included into coreboot and used for verified boot only.

Enabling support

To enable measured boot support:

  • Enabled VENDORCODE_ELTAN_MBOOT
  • Create mb_log_list table with list of items to measure

To enable verified boot support:

  • Enable VENDORCODE_ELTAN_VBOOT
  • Create the verify lists *_verify_list[]
  • VENDORCODE_ELTAN_VBOOT_KEY_FILE must point to location of the public key file created with futility

Creating signed binary

During build of coreboot binary an empty oemmanifest.bin is added to the binary.

This binary must be replaced by a correct (signed) binary when VENDORCODE_ELTAN_VBOOT is enabled

The oemmanifest.bin file contains the SHA-256 (or SHA-512) hashes of all the different parts contained in verify_lists.

When VENDORCODE_ELTAN_VBOOT_SIGNED_MANIFEST is enabled the manifest should be signed and the signature should appended to the manifest.

Please make sure the public key is in the RO part of the coreboot image. The oemmanifest.bin file should be in the RW part of the coreboot image.

Hashing

The oemmanifest.bin file contains the hashes of different binaries parts of the binary e.g.: bootblock, romstage, postcar, ramstage, fsp etc.

The total number of items must match VENDORCODE_ELTAN_OEM_MANIFEST_ITEMS.

For every part the SHA (SHA-256) must be calculated. First extract the binary from the coreboot image using: cbfstool <coreboot_file_name> extract -n <cbfs_name> -f <item_binary_file_name> followed by: openssl dgst -sha256 -binary -out <hash_file_name> <item_binary_file_name>

Replace -sha256 with -sha512 when VENDORCODE_ELTAN_VBOOT_USE_SHA512 is enabled.

All the hashes must be combined to a hash binary. The hashes need to be placed in the same order as defined by the HASH_IDX_XXX values.

Signing

The oemmanifest needs to be signed when VENDORCODE_ELTAN_VBOOT_SIGNED_MANIFEST is enabled.

This can be done with the following command: openssl dgst -sign <private_key_file_name> -sha256 -out <signature_binary> <hash_binary>

The signed manifest can be created by adding the signature to the manifest: cat <hash_binary> <signature_binary> >hash_table.bin

Create binary

The oemmanifest.bin file must be replaced in the coreboot binary by the generated hash_table.bin.

To replace the binary: Remove using: cbfstool <coreboot_file_name> remove -n oemmanifest.bin Then add the new image using: cbfstool coreboot.bin add -f <hash_table_file_name> -n oemmanifest.bin -t raw \ -b <CONFIG_VENDORCODE_ELTAN_OEM_MANIFEST_LOC>

Debugging

You can enable verbose console output in menuconfig.