From ffdcd44d54560f8d56e344580d9eb7839d5fdf8f Mon Sep 17 00:00:00 2001 From: Navan Chauhan Date: Wed, 4 Oct 2023 15:21:13 -0600 Subject: added phase 5 --- Content/posts/2023-10-04-bomb-lab.md | 204 ++++++++++++++++++++++++++++++++++- 1 file changed, 202 insertions(+), 2 deletions(-) (limited to 'Content/posts/2023-10-04-bomb-lab.md') diff --git a/Content/posts/2023-10-04-bomb-lab.md b/Content/posts/2023-10-04-bomb-lab.md index 6a84d9e..d235186 100644 --- a/Content/posts/2023-10-04-bomb-lab.md +++ b/Content/posts/2023-10-04-bomb-lab.md @@ -1,10 +1,10 @@ --- date: 2023-10-04 13:12 -description: Introduction, Phases 1-3 of Bomb Lab for CSCI 2400 Lab - 2 +description: Introduction, Phases 1-4 of Bomb Lab for CSCI 2400 Lab - 2 tags: gdb, reverse-engineering, c++, csci2400, assembly --- -# Bomb Lab Phases 1-3 +# Bomb Lab Phases 1-4 ## Introduction @@ -453,3 +453,203 @@ Breakpoint 1, 0x0000555555555638 in phase_3 () Continuing. Halfway there! ``` + +## Phase 4 + +``` +jovyan@jupyter-nach6988:~/lab2-bomblab-navanchauhan/bombbomb$ gdb -ex 'break phase_4' -ex 'break explode_bomb' -ex 'run' -args ./bomb sol.txt +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 0x17d3 +Breakpoint 2 at 0x1d4a +Starting program: /home/jovyan/lab2-bomblab-navanchauhan/bombbomb/bomb sol.txt +[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! +Phase 1 defused. How about the next one? +That's number 2. Keep going! +Halfway there! +test string + +Breakpoint 1, 0x00005555555557d3 in phase_4 () +(gdb) disas phase_4 +Dump of assembler code for function phase_4: +=> 0x00005555555557d3 <+0>: endbr64 + 0x00005555555557d7 <+4>: sub $0x18,%rsp + 0x00005555555557db <+8>: lea 0x8(%rsp),%rcx + 0x00005555555557e0 <+13>: lea 0xc(%rsp),%rdx + 0x00005555555557e5 <+18>: lea 0x1bba(%rip),%rsi # 0x5555555573a6 + 0x00005555555557ec <+25>: mov $0x0,%eax + 0x00005555555557f1 <+30>: call 0x5555555552e0 <__isoc99_sscanf@plt> + 0x00005555555557f6 <+35>: cmp $0x2,%eax + 0x00005555555557f9 <+38>: jne 0x555555555802 + 0x00005555555557fb <+40>: cmpl $0xe,0xc(%rsp) + 0x0000555555555800 <+45>: jbe 0x555555555807 + 0x0000555555555802 <+47>: call 0x555555555d4a + 0x0000555555555807 <+52>: mov $0xe,%edx + 0x000055555555580c <+57>: mov $0x0,%esi + 0x0000555555555811 <+62>: mov 0xc(%rsp),%edi + 0x0000555555555815 <+66>: call 0x555555555799 + 0x000055555555581a <+71>: cmp $0x2,%eax + 0x000055555555581d <+74>: jne 0x555555555826 + 0x000055555555581f <+76>: cmpl $0x2,0x8(%rsp) + 0x0000555555555824 <+81>: je 0x55555555582b + 0x0000555555555826 <+83>: call 0x555555555d4a + 0x000055555555582b <+88>: add $0x18,%rsp + 0x000055555555582f <+92>: ret +End of assembler dump. +(gdb) +``` + +Again, `gdb` has marked the string being passed to `scanf` + +``` +(gdb) x/1s 0x5555555573a6 +0x5555555573a6: "%d %d" +``` + +Okay, so this time we are supposed to enter 2 numbers. + +``` + 0x00005555555557f6 <+35>: cmp $0x2,%eax + 0x00005555555557f9 <+38>: jne 0x555555555802 +``` + +Checks if there were 2 values read from calling `scanf`, if not -> jump to `` which calls ``. + +``` + 0x00005555555557fb <+40>: cmpl $0xe,0xc(%rsp) + 0x0000555555555800 <+45>: jbe 0x555555555807 +``` + +Compare `0xe` (14 in Decimal) and value stored at `$rsp` + `0xc` bytes (Decimal 12). If this condition is met (<= 14), jump to ``. If not, then explode bomb. + +``` +... + 0x0000555555555807 <+52>: mov $0xe,%edx + 0x000055555555580c <+57>: mov $0x0,%esi + 0x0000555555555811 <+62>: mov 0xc(%rsp),%edi + 0x0000555555555815 <+66>: call 0x555555555799 + 0x000055555555581a <+71>: cmp $0x2,%eax + 0x000055555555581d <+74>: jne 0x555555555826 + 0x000055555555581f <+76>: cmpl $0x2,0x8(%rsp) + 0x0000555555555824 <+81>: je 0x55555555582b + 0x0000555555555826 <+83>: call 0x555555555d4a +``` + +* ` 0x0000555555555815 <+66>: call 0x555555555799 ` calls another function called `func4` +* The returned value is compared with `0x2`, if they are not equal then the program jumps to call ``. This tells us that `func4` should return 2. + +Let us look into `func4` + +``` +(gdb) disas func4 +Dump of assembler code for function func4: + 0x0000555555555799 <+0>: endbr64 + 0x000055555555579d <+4>: sub $0x8,%rsp + 0x00005555555557a1 <+8>: mov %edx,%ecx + 0x00005555555557a3 <+10>: sub %esi,%ecx + 0x00005555555557a5 <+12>: shr %ecx + 0x00005555555557a7 <+14>: add %esi,%ecx + 0x00005555555557a9 <+16>: cmp %edi,%ecx + 0x00005555555557ab <+18>: ja 0x5555555557b9 + 0x00005555555557ad <+20>: mov $0x0,%eax + 0x00005555555557b2 <+25>: jb 0x5555555557c5 + 0x00005555555557b4 <+27>: add $0x8,%rsp + 0x00005555555557b8 <+31>: ret + 0x00005555555557b9 <+32>: lea -0x1(%rcx),%edx + 0x00005555555557bc <+35>: call 0x555555555799 + 0x00005555555557c1 <+40>: add %eax,%eax + 0x00005555555557c3 <+42>: jmp 0x5555555557b4 + 0x00005555555557c5 <+44>: lea 0x1(%rcx),%esi + 0x00005555555557c8 <+47>: call 0x555555555799 + 0x00005555555557cd <+52>: lea 0x1(%rax,%rax,1),%eax + 0x00005555555557d1 <+56>: jmp 0x5555555557b4 +``` + +This looks like a recursive function :( (I hate recursive functions) + +Let's annotate the instructions. + +``` +endbr64 +sub $0x8,%rsp // subtract 8 bytes from the stack pointer +mov %edx,%ecx // Move the value in register %edx to %ecx +sub %esi,%ecx // Subtract the value in %esi from %ecx +shr %ecx // Right shift the value in %ecx by one bit (dividing the value by 2) +add %esi,%ecx // Add the value in %esi to %ecx +cmp %edi,%ecx // Compare +ja 0x5555555557b9 // If %ecx > %edi -> jump to instruction at offset +32 +mov $0x0,%eax // Move 0 to %eax +jb 0x5555555557c5 // If %ecx < %edi -> jump to instruction at offset +44. +add $0x8,%rsp // add 8 bytes to the stack pointer +ret // return +lea -0x1(%rcx),%edx // LEA of $rxc - 1 into $edx +call 0x555555555799 // Call itself +add %eax,%eax // Double the value in %eax +jmp 0x5555555557b4 // jump to the instruction at offset +27 +lea 0x1(%rcx),%esi +call 0x555555555799 +lea 0x1(%rax,%rax,1),%eax // LEA of %rax * 2 + 1 into $eax +jmp 0x5555555557b4 +``` + +We can either try to compute the values by hand, or write a simple script in Python to get the answer. + +``` +def func4(edi, esi=0, edx=20): + ecx = (edx - esi) // 2 + esi + if ecx > edi: + return 2 * func4(edi, esi, ecx - 1) + elif ecx < edi: + return 2 * func4(edi, ecx + 1, edx) + 1 + else: + return 0 + +for x in range(10): + if func4(x) == 2: + print(f"answer is {x}") + break +``` + +Running this code, we get: `answer is 5` + +Okay, so we know that the number needed to be passed to `func4` is 5. But, what about the second digit? + +If we go back to the code for ``, we can see that: + +``` + 0x000055555555581f <+76>: cmpl $0x2,0x8(%rsp) + 0x0000555555555824 <+81>: je 0x55555555582b +``` + +The value at `$rsp+8` should be equal to 2. So, let us try passing `5 2` as our input. + +``` +... +Phase 1 defused. How about the next one? +That's number 2. Keep going! +Halfway there! +5 2 + +Breakpoint 1, 0x00005555555557d3 in phase_4 () +(gdb) continue +Continuing. +So you got that one. Try this one. +``` + -- cgit v1.2.3