[vayun]

At which point they will be stopped by Ubuntu login screen which requires user password (or fingerprint, pin, etc).

Basically there are two options for have to enter 1 password: 1. TPM disk encryption + login password. 2. Password for disk encryption and automatic login (doesn't support multiple user accounts well).

[tryauuum]

no, I mean the attacker boots his own copy of Windows or Ubuntu, which is supposedly trusted by the UEFI keys. Will tpm somehow detect that it shouldn't unlock secrets for this boot?

[noisem4ker]

Yes. The PCR state after booting won't match that of the regular system, so the TPM will refuse to give up the key.