Assembly segmentation fault after making a system call, at the end of my code

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You can’t ret from start; it isn’t a function and there’s no return address on the stack. The stack pointer points at argc on process entry.

As n.m. said in the comments, the issue is that you aren’t exiting the program, so execution runs off into garbage code and you get a segfault.

What you need is:

;; Linux 32-bit x86
%define ___SYSCALL_EXIT 1

// ... at the end of _start:
    mov eax, ___SYSCALL_EXIT
    mov ebx, 0
    int 0x80

(The above is 32-bit code. In 64-bit code you want mov eax, 231 (exit_group) / syscall, with the exit status in EDI. For example:

;; Linux x86-64
    xor   edi, edi     ;  or mov edi, eax    if you have a ret val in EAX
    mov   eax, 231     ; __NR_exit_group
    syscall

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