I bought my computer in 2011. From 2011 to 2015 the computer was accessible to a cousin of mine who is a developer. He doesn't work with IT security, but has more than a decade of experience programming.
I have reasons to believe he might have installed some monitoring software on my computer, around 2011-2015, when he had physical access to the device. My cousin is very manipulative and always envied me. A friend of my parents strongly implied to me they at least one time searched my computer to see what I was doing. But if they installed something, I always thought it would be software only.
Since then, I have reinstalled the OS (Ubuntu) a few times and I also replaced my HDD. So, keyloggers or other malicious software are not a problem. What I am thinking is an infected BIOS or other hardware.
How easy is to an experienced developer to develop (or to acquire) a BIOS or hardware virus to target a single computer? Should I throw away my computer? Just flashing the bios again will be good enough?
I already read a lot of answers about BIOS virus and how it is not worth the trouble because every single manufacturer will have its own design etc. But I'm thinking about someone trying to access a specific computer, not a wide attack.