The Unified Extensible Firmware Interface
) Secure Boot technology ensures that the system firmware checks whether the system boot loader is signed with a cryptographic key authorized by a database of public keys contained in the firmware. With signature verification in the next-stage boot loader and kernel, it is possible to prevent the execution of kernel space code which has not been signed by a trusted key.
A chain of trust is established from the firmware to the signed drivers and kernel modules as follows. The first-stage boot loader,
shim.efi, is signed by a UEFI private key and authenticated by a public key, signed by a certificate authority (CA), stored in the firmware database. The
shim.efi contains the Red Hat public key, “Red Hat Secure Boot (CA key 1)”, which is used to authenticate both the GRUB 2 boot loader,
grubx64.efi, and the Red Hat kernel. The kernel in turn contains public keys to authenticate drivers and modules.
Secure Boot is the boot path validation component of the Unified Extensible Firmware Interface (UEFI) specification. The specification defines:
a programming interface for cryptographically protected UEFI variables in non-volatile storage,
how the trusted X.509 root certificates are stored in UEFI variables,
validation of UEFI applications like boot loaders and drivers,
procedures to revoke known-bad certificates and application hashes.
UEFI Secure Boot does not prevent the installation or removal of second-stage boot loaders, nor require explicit user confirmation of such changes. Signatures are verified during booting, not when the boot loader is installed or updated. Therefore, UEFI Secure Boot does not stop boot path manipulations, it helps in the detection of unauthorized changes. A new boot loader or kernel will work as long as it is signed by a key trusted by the system.
26.11.1. UEFI Secure Boot Support in Red Hat Enterprise Linux 7
Red Hat Enterprise Linux 7 includes support for the UEFI Secure Boot feature, which means that Red Hat Enterprise Linux 7 can be installed and run on systems where UEFI Secure Boot is enabled. On UEFI-based systems with the Secure Boot technology enabled, all drivers that are loaded must be signed with a trusted key, otherwise the system will not accept them. All drivers provided by Red Hat are signed by one of Red Hat's private keys and authenticated by the corresponding Red Hat public key in the kernel.
If you want to load externally built drivers, drivers that are not provided on the Red Hat Enterprise Linux DVD, you must make sure these drivers are signed as well.
Restrictions Imposed by UEFI Secure Boot
As UEFI Secure Boot support in Red Hat Enterprise Linux 7 is designed to ensure that the system only runs kernel mode code after its signature has been properly authenticated, certain restrictions exist.
GRUB 2 module loading is disabled as there is no infrastructure for signing and verification of GRUB 2 modules, which means allowing them to be loaded would constitute execution of untrusted code inside the security perimeter that Secure Boot defines. Instead, Red Hat provides a signed GRUB 2 binary that has all the modules supported on Red Hat Enterprise Linux 7 already included.