I wasn't familiar with brk. You should ask this question on CW Discord server, topics #asm channel. There's some clever people who should notice your question and answer.

Ah I forgot which kata this was. I haven't solved it in NASM, so I can't see your code, or what testing functions look like :/ When I tried NASM, I was too familiar with signal 6 and situations like yours :p

You could try posting your code here for someone to analyze.

You probably don't want brackets around rdi, so you save value in the register directly.

the input is a matrix: int **, i.e. an array of 8-byte pointers to 4-byte int. [rdi] gives you the (address of the) first row of the matrix, not the first element of the first row

Please use the question label to ask for help, an issue is a provable bug within the kata. Your code has a bug, your second while statement keeps looping past the end of the string, which is undefined behavior.