To bypass a secure boot for System on Chip (SoC), some attacks target the phase of copying the bootloader from where it is stored to the main memory. Instead, a malicious code will be copied to the main memory. In that case, the bootloader which is the fisrt element of the chain of trust is not even booted. In such attacks, the whole secure boot mechanism is bypassed (see the paper  where the bootloader is stored in an external flash memory and the attack targets the action of coping the original first stage bootloder BL1 into the main memory).
Is successfully booting the bootloder (safely copied to the main memory) guarantees a successful secure boot process?
1: "Controlling PC on ARM using Fault Injection"