If legacy boot is off, UEFI and TPM/secure boot offers code signing for everything running on your computer. UEFI checks first stage boot loaders and/or kernel

If legacy boot is off, UEFI and TPM/secure boot offers code signing for everything running on your computer. UEFI checks first stage boot loaders and/or kernels for a signature backed by a key/cert in the TPM. AFAIK know, out of the box, that means Microsoft, RedHat/IBM, Canonical or a handful of others have signed your bootloader/kernel. Whatever you run next is supposed to check that any further code is signed - at least code allowed into kernel space. If there exist any signed snippets with exploitable errors, the whole card house collapses (but you can limit exposure by only allowing code you yourself signed, such as a single build of the Linux kernel). Linux can/will require signed drivers with secure boot enabled - so that can lead to some issues (that can be fixed, eg by adding a signing key and signing the drivers). In theory, you'll never run untrusted code in the kernel - no blue pill hypervisor root kit etc.