The technical documentation about Secure Boot says that SB is not a mechanisms to steal ownership of your device. It is a spurious claim because the design specification is only a reference and not a requirement. Gentoo has further documentation that can be found describing KeyTool, a package that enables booting directly into UEFI to change the keys manually if your implemented UEFI bootloader lacks the functional implementation required to sign your own keys. I’ve never tried it personally. I merely know of its existence.