Although without checking of the bootloader you could create a grub module which would inject code or silently enable module loading as the kernel booted so that a rootkit could persist. And even with secureboot and disabling of unsigned module loading you can still inject code into the kernel using any kernel vulnerability accessible from userspace and use that to load a module or enable module loading.
It's good that most of these rootkits are clearly made by amateurs, what would a linux rootkit look like if it had the professional resources of cyber-weapons like Stuxnet or Flame?
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