Otherwise— if unsigned code runs before updates— the unsigned code will have been modified by the attacker, it will execute a kernel exploit, and the exploit will undermine the update process— not just DOS it but make it look successful while keeping the machine compromised.
Or to put it more simply— What _goal_ (not mechanism) of an attacker will SecureBoot in Fedora thwart. It's advertised on windows as preventing unremovable rootkits, but I've explained why it can't do that at least on Fedora/Linux without signing a substantial hunk of userspace or moving a lot of networking code into init/systemd.
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