--- date: 2023-10-04 13:12 description: Introduction, Phase 1 and Phase 2 of Bomb Lab for CSCI 2400 Lab - 2 tags: gdb, reverse-engineering, c++, csci2400, assembly --- # Bomb Lab ## Introduction Lab 2 for CSCI 2400 - Computer Systems. ## Phase 1 ``` jovyan@jupyter-nach6988:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex 'break phase_1' -ex 'break explode_bomb' -ex 'run' ./bomb GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1 Copyright (C) 2022 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-linux-gnu". Type "show configuration" for configuration details. For bug reporting instructions, please see: . Find the GDB manual and other documentation resources online at: . For help, type "help". Type "apropos word" to search for commands related to "word"... Reading symbols from ./bomb... Breakpoint 1 at 0x15c7 Breakpoint 2 at 0x1d4a Starting program: /home/jovyan/lab2-bomblab-navanchauhan/bombbomb/bomb [Thread debugging using libthread_db enabled] Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1". Welcome to my fiendish little bomb. You have 6 phases with which to blow yourself up. Have a nice day! test string Breakpoint 1, 0x00005555555555c7 in phase_1 () (gdb) dias phase_1 Undefined command: "dias". Try "help". (gdb) disas phase_1 Dump of assembler code for function phase_1: => 0x00005555555555c7 <+0>: endbr64 0x00005555555555cb <+4>: sub $0x8,%rsp 0x00005555555555cf <+8>: lea 0x1b7a(%rip),%rsi # 0x555555557150 0x00005555555555d6 <+15>: call 0x555555555b31 0x00005555555555db <+20>: test %eax,%eax 0x00005555555555dd <+22>: jne 0x5555555555e4 0x00005555555555df <+24>: add $0x8,%rsp 0x00005555555555e3 <+28>: ret 0x00005555555555e4 <+29>: call 0x555555555d4a 0x00005555555555e9 <+34>: jmp 0x5555555555df End of assembler dump. (gdb) print 0x555555557150 $1 = 93824992244048 (gdb) x/1s 0x555555557150 0x555555557150: "Controlling complexity is the essence of computer programming." (gdb) ``` ## Phase 2 ``` Phase 1 defused. How about the next one? 1 2 3 4 5 6 Breakpoint 1, 0x00005555555555eb in phase_2 () (gdb) disas Dump of assembler code for function phase_2: => 0x00005555555555eb <+0>: endbr64 0x00005555555555ef <+4>: push %rbp 0x00005555555555f0 <+5>: push %rbx 0x00005555555555f1 <+6>: sub $0x28,%rsp 0x00005555555555f5 <+10>: mov %rsp,%rsi 0x00005555555555f8 <+13>: call 0x555555555d97 0x00005555555555fd <+18>: cmpl $0x0,(%rsp) 0x0000555555555601 <+22>: js 0x55555555560d 0x0000555555555603 <+24>: mov %rsp,%rbp 0x0000555555555606 <+27>: mov $0x1,%ebx 0x000055555555560b <+32>: jmp 0x555555555620 0x000055555555560d <+34>: call 0x555555555d4a 0x0000555555555612 <+39>: jmp 0x555555555603 0x0000555555555614 <+41>: add $0x1,%ebx 0x0000555555555617 <+44>: add $0x4,%rbp 0x000055555555561b <+48>: cmp $0x6,%ebx 0x000055555555561e <+51>: je 0x555555555631 0x0000555555555620 <+53>: mov %ebx,%eax 0x0000555555555622 <+55>: add 0x0(%rbp),%eax 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) 0x0000555555555628 <+61>: je 0x555555555614 0x000055555555562a <+63>: call 0x555555555d4a 0x000055555555562f <+68>: jmp 0x555555555614 0x0000555555555631 <+70>: add $0x28,%rsp 0x0000555555555635 <+74>: pop %rbx 0x0000555555555636 <+75>: pop %rbp 0x0000555555555637 <+76>: ret End of assembler dump. (gdb) ``` ``` 0x00005555555555fd <+18>: cmpl $0x0,(%rsp) 0x0000555555555601 <+22>: js 0x55555555560d ... 0x000055555555560d <+34>: call 0x555555555d4a ``` The program first compares if the first number is not 0. If the number is not 0, then the `cmpl` instruction returns a negative value. The `js` instruction stands for jump if sign -> causing a jump to the specified address if the sign bit is set. This would result in the explode_bomb function being called. ``` 0x0000555555555603 <+24>: mov %rsp,%rbp 0x0000555555555606 <+27>: mov $0x1,%ebx ``` `%rsp` in x86-64 asm, is the stack pointer i.e. it points to the top of the current stack frame. Since the program just read six numbers, the top of the stack (`%rsp`) contains the address of the first number. By executing `mov %rsp,%rbp` we are setting the base pointer (`%rbp`) to point to this address. Now, for the second instruction `mov $0x1,%ebx`, we are initalising the `%ebx` register with the value 1. Based on the assembly code, you can see that this is being used as a counter/index for the loop. ``` 0x000055555555560b <+32>: jmp 0x555555555620 ``` The program now jumps to ``` 0x0000555555555620 <+53>: mov %ebx,%eax 0x0000555555555622 <+55>: add 0x0(%rbp),%eax 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) 0x0000555555555628 <+61>: je 0x555555555614 ``` Here, the value from `%ebx` is copied to the `%eax` register. For this iteration, the value should be 1. Then, the value at the memory location pointed by `%rbp` is added to the value in `%eax`. For now, 0 is added (the first number that we read). `cmp %eax,0x4(%rbp)` - The instruction compares the value in %eax to the value at the memory address `%rbp + 4`. Since Integers in this context are stored using a word of memory of 4 bytes, this indicates it checks against the second number in the sequence. `je 0x555555555614 ` - The program will jump to `phase_2+41` if the previous `cmp` instruction determined the values as equal. ``` 0x0000555555555614 <+41>: add $0x1,%ebx 0x0000555555555617 <+44>: add $0x4,%rbp 0x000055555555561b <+48>: cmp $0x6,%ebx 0x000055555555561e <+51>: je 0x555555555631 0x0000555555555620 <+53>: mov %ebx,%eax 0x0000555555555622 <+55>: add 0x0(%rbp),%eax 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) 0x0000555555555628 <+61>: je 0x555555555614 ``` Here, we can see that the program increments `%ebx` by 1, adds a 4 byte offset to `%rbp` (the number we will be matching now), and checks if `%ebx` is equal to 6. If it is, it breaks the loop and jumps to `` succesfully finishing this stage. Now, given that we know the first two numbers in the sequence are `0 1`, we can calculate the other numbers by following the pattern of adding the counter and the value of the previous number. Thus, * 3rd number = 1 (previous value) + 2 = 3 * 4th number = 3 (prev value) + 3 = 6 * 5th number = 6 (prev value) + 4 = 10 * 6th number = 10 (prev value) + 5 = 15 ``` ... Phase 1 defused. How about the next one? 0 1 3 6 10 15 Breakpoint 1, 0x00005555555555eb in phase_2 () (gdb) continue Continuing. That's number 2. Keep going! ```