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diff --git a/Content/posts/2023-10-04-bomb-lab.md b/Content/posts/2023-10-04-bomb-lab.md new file mode 100644 index 0000000..4919f61 --- /dev/null +++ b/Content/posts/2023-10-04-bomb-lab.md @@ -0,0 +1,181 @@ +--- +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 <http://gnu.org/licenses/gpl.html> +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: +<https://www.gnu.org/software/gdb/bugs/>. +Find the GDB manual and other documentation resources online at: + <http://www.gnu.org/software/gdb/documentation/>. + +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 <strings_not_equal> + 0x00005555555555db <+20>: test %eax,%eax + 0x00005555555555dd <+22>: jne 0x5555555555e4 <phase_1+29> + 0x00005555555555df <+24>: add $0x8,%rsp + 0x00005555555555e3 <+28>: ret + 0x00005555555555e4 <+29>: call 0x555555555d4a <explode_bomb> + 0x00005555555555e9 <+34>: jmp 0x5555555555df <phase_1+24> +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 <read_six_numbers> + 0x00005555555555fd <+18>: cmpl $0x0,(%rsp) + 0x0000555555555601 <+22>: js 0x55555555560d <phase_2+34> + 0x0000555555555603 <+24>: mov %rsp,%rbp + 0x0000555555555606 <+27>: mov $0x1,%ebx + 0x000055555555560b <+32>: jmp 0x555555555620 <phase_2+53> + 0x000055555555560d <+34>: call 0x555555555d4a <explode_bomb> + 0x0000555555555612 <+39>: jmp 0x555555555603 <phase_2+24> + 0x0000555555555614 <+41>: add $0x1,%ebx + 0x0000555555555617 <+44>: add $0x4,%rbp + 0x000055555555561b <+48>: cmp $0x6,%ebx + 0x000055555555561e <+51>: je 0x555555555631 <phase_2+70> + 0x0000555555555620 <+53>: mov %ebx,%eax + 0x0000555555555622 <+55>: add 0x0(%rbp),%eax + 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) + 0x0000555555555628 <+61>: je 0x555555555614 <phase_2+41> + 0x000055555555562a <+63>: call 0x555555555d4a <explode_bomb> + 0x000055555555562f <+68>: jmp 0x555555555614 <phase_2+41> + 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 <phase_2+34> +... + 0x000055555555560d <+34>: call 0x555555555d4a <explode_bomb> +``` + +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 <phase_2+53> +``` + +The program now jumps to <phase_2+53> + +``` + 0x0000555555555620 <+53>: mov %ebx,%eax + 0x0000555555555622 <+55>: add 0x0(%rbp),%eax + 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) + 0x0000555555555628 <+61>: je 0x555555555614 <phase_2+41> +``` + +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 <phase_2+41>` - 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 <phase_2+70> + 0x0000555555555620 <+53>: mov %ebx,%eax + 0x0000555555555622 <+55>: add 0x0(%rbp),%eax + 0x0000555555555625 <+58>: cmp %eax,0x4(%rbp) + 0x0000555555555628 <+61>: je 0x555555555614 <phase_2+41> +``` + +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 `<phase_2+70>` 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! +``` |