this post was submitted on 27 Aug 2024
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Okay, so my original post I think I was thinking of TPM rather than Secure Boot. TPM would protect against a hostile OS, I think. I think most of my greviences mentioned above are to do with TPM rather than Secure Boot. But they still apply to either.
There are ways to put keyloggers in devices stealthily to where they're not perceptible to a normal person. For example, they could replace one of the USB leads in your bag with one that transmits keystrokes over bluetooth. If you're at home, a maid could just plug a keylogger behind your desktop because most people don't check behind their system when they boot it.
It all feels like a wierd threat model to me. You can either assume that physical access is or isn't covered by it. If you are worried about physical access (beyond your device being stolen), there is a lot more you need to do to secure it. "It's better than nothing" isn't really that convincing to me with regards security models; either you're protected or you aren't.
I'm actually not sure what TPM can guard against, but I think you're right, I think if a malicious OS borked with the bootloader, TPM would catch it and complain before you decrypt the other OS.
Yeah, physical access usually means all bets are off, but you still lock your doors even though a hammer through a window easily circumvents it. Because you don't know what the attacker is willing to do/capable of. If you only ever check for physical devices, you'll miss the attack in software, similarly if you only rely on Secure Boot you'll miss any hardware based attacks. It's there as a tool to plug one attack vector.
Also, my guess is the most common thing this protects against are stupid employees plugging a USB they found in the parking lot into their PC. If they do it while the OS is running, IT can have a policy that blocks it from taking action. But if they leave it there during a reboot, IT is otherwise helpless.