红帽全球峰会21.2. What is UEFI Secure Boot
With the Unified Extensible Firmware Interface (UEFI) Secure Boot technology, you can prevent the execution of the kernel-space code that is not signed by a trusted key. The system boot loader is signed with a cryptographic key. The database of public keys in the firmware authorizes the process of signing the key. You can subsequently verify a signature in the next-stage boot loader and the kernel.
UEFI Secure Boot establishes a chain of trust from the firmware to the signed drivers and kernel modules as follows:
-
An UEFI private key signs, and a public key authenticates the
shimfirst-stage boot loader. A certificate authority (CA) in turn signs the public key. The CA is stored in the firmware database. -
The
shimfile contains the Red Hat public key Red Hat Secure Boot (CA key 1) to authenticate the GRUB boot loader and the kernel. - The kernel in turn contains public keys to authenticate drivers and modules.
Secure Boot is the boot path validation component of the UEFI specification. The specification defines:
- Programming interface for cryptographically protected UEFI variables in non-volatile storage.
- Storing the trusted X.509 root certificates in UEFI variables.
- Validation of UEFI applications such as boot loaders and drivers.
- Procedures to revoke known-bad certificates and application hashes.
UEFI Secure Boot helps in the detection of unauthorized changes but does not:
- Prevent installation or removal of second-stage boot loaders.
- Require explicit user confirmation of such changes.
- Stop boot path manipulations. Signatures are verified during booting but not when the boot loader is installed or updated.
If the boot loader or the kernel are not signed by a system trusted key, Secure Boot prevents them from starting.
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