> How is TPM-backed full-disk encryption more secure than using a
> passphrase when (if I understand correctly) the device just
> starts up without needing any user input at boot?
> While TPM can prevent evil-maid attacks, how does it prevent
> someone from just turning on and using your laptop without
> any passphrase?

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If your computer boots directly into your desktop without any password, then you're right. However, you should not get to a point where you have access to your data on the computer, without having to put in a password. This does mean that you're dependent on the security of the OS not to let anyone get around the login screen.

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The TPM doesn't release the encryption key unconditionally.

It does so only when certain conditions are met as measured by the platform firmware, and this generally enables the owner to unlock the disk only for a known, signed, operating system.

The purpose is to make file permissions enforceable in practice. Without it, anyone can boot any OS and read, modify, or execute whatever they like without your knowledge. It is not necessary to remove the internal drive to do this, since by design a PC should be capable of booting from external media, like the flash drive you used to install linux.

Once the disk is decrypted and your OS is running, it can enforce whatever authentication it likes upon the user, including a standard user login.

...

Wwith a tpm unlock you still don't have immediate access to your files. The OS loaded and is up and running, but you still need to input your user password to login, and here you rely on the OS security itself. If you have some form of user auto-login, you skip the DM login somehow, then yes, that's terrible practice and pointless.

That said, it's important to note that a tpm does not release the keys unconditionally. You actually set what conditions guarantee a key release, you can relax this or tighten this as you see fit. Things that can get measured and taken into account to decide are bootloader integrity, kernel integrity, the computer's firmware integrity, and many others.

Once you decide on a group of checks (pcr registers to validate), the tpm will always make sure they're the same. Disk got removed to a new computer? Firmware check fails. You have an unencrypted boot partition, and someone placed a malicious kernel to unlock your OS? Kernel check fails. There's even a check for kernel parameters, so no one can just boot up in single[-user] mode and call it a day.

Of course, all these checks come at a price. It's the same price for the tpm itself. Convenience vs effort. A tpm is all about convenience. "it's my computer, my system, it's me the owner, this whole setup is legit, let me in in few seconds and save me from typing a complex password". But as you add more tpm checks, you're slowly chipping away at this convenience. So you want a kernel check? Now after every kernel update you'll need to boot with luks password, then reset the pcr registers. Convenient? Hassle? Your choice. I personally don't check if it's the same kernel, but instead, I sign the kernel with a personal key (automatic pacman hook, I use arch btw), and trust that my secure boot setup will check that signature and refuse to boot if it's bad. So in that regard, it's also a multipronged approach where different security mechanisms complement each other.

Oh, and I see comments here about how awful it is to use a tpm because you're locked out if you remove the disk and such. There is ALWAYS a master password. You can't have a tpm unlock in a luks slot without another password luks slot.

In conclusion: a tpm is all about convenience. If you think about how most people create passwords, store passwords, and try to recall them, then a tpm is much more secure than having a weak or hackable password. You wanting to still input a complex and secure password on every reboot is a 100% valid choice and I will not argue against it. But if you want ease without security compromise, it takes some effort to create it, test it, and understand the angles of attack you personally might face, and you can have that convenience :)