UEFI (Unified Extensible Firmware Interface)

Current firmware interface for PCs

• Replacing Basic Input and Output System (BIOS)

Its goal was to define a standard way for the operating system to communicate with the platform firmware during the boot process

• Instead of the former primary mechanism, software interrupts

UEFI allows for modular firmware design

• Enables hardware and system designers a greater flexibility in designing firmware for modern computing environments

Secure boot

Firmware validation process.

• Defines how platform firmware manages security certificates, validation of firmware, and a definition of the interface (protocol) between firmware and the operating system.

Goal: validate an OS bootloader

Secure boot keys

Platform Key (PK)

• (Usually only one) asymmetric key pair

• Should be under the control of the platform owner

Key Exchange Key (KEK)

• Set of Asymmetric key pairs

• Controlled by the OEM and OS vendors

KEKs can only be changed by the PK owner.

UEFI secure boot signature databases

Forbidden signatures database (blacklist, dbx).

• Hashes of blacklisted firmware code.

Signature database (whitelist, db).

• Key or certificates of keys that validate signatures over authorized firmware code.

• Hashes of allowed firmware.

KEK owners can update both databases.

UEFI secure boot firmware validation

Firmware code is not allowed to execute if its hash is in DBX.

Firmware code is allowed to execute if:

• It is signed by a key in DB; or

• Has its hash in the DB; or

• Is signed by a KEK key.

UEFI storage of secure boot keys and DBs

They are stored in UEFI secure variables.

• Each variable is bound to a public key.

  • Set upon its creation.

• Variables can only be changed if an authentication descriptor is provided.

  • With a signature made with the corresponding private key.

• Authentication descriptors cannot be reused.

  • To avoid replay attacks.

UEFI setup and user modes

Setup mode

• PK, KEK, db and dbx can be changed without checks.

• The secure boot is off.

User mode

• PK, KEK, db and dbx cannot be changed without verification.

  • PK, KEK -> requires PK signature

  • db, dbz -> requires a KEK signature