added phase 5

1 files changed, 202 insertions, 2 deletions

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 <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 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 <phase_4+47>
+   0x00005555555557fb <+40>:    cmpl   $0xe,0xc(%rsp)
+   0x0000555555555800 <+45>:    jbe    0x555555555807 <phase_4+52>
+   0x0000555555555802 <+47>:    call   0x555555555d4a <explode_bomb>
+   0x0000555555555807 <+52>:    mov    $0xe,%edx
+   0x000055555555580c <+57>:    mov    $0x0,%esi
+   0x0000555555555811 <+62>:    mov    0xc(%rsp),%edi
+   0x0000555555555815 <+66>:    call   0x555555555799 <func4>
+   0x000055555555581a <+71>:    cmp    $0x2,%eax
+   0x000055555555581d <+74>:    jne    0x555555555826 <phase_4+83>
+   0x000055555555581f <+76>:    cmpl   $0x2,0x8(%rsp)
+   0x0000555555555824 <+81>:    je     0x55555555582b <phase_4+88>
+   0x0000555555555826 <+83>:    call   0x555555555d4a <explode_bomb>
+   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 <phase_4+47>
+```
+
+Checks if there were 2 values read from calling `scanf`, if not -> jump to `<phase_4+47>` which calls `<explode_bomb>`.
+
+```
+   0x00005555555557fb <+40>:    cmpl   $0xe,0xc(%rsp)
+   0x0000555555555800 <+45>:    jbe    0x555555555807 <phase_4+52>
+```
+
+Compare `0xe` (14 in Decimal) and value stored at `$rsp` + `0xc` bytes (Decimal 12). If this condition is met (<= 14), jump to `<phase_4+52>`. 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 <func4>
+   0x000055555555581a <+71>:    cmp    $0x2,%eax
+   0x000055555555581d <+74>:    jne    0x555555555826 <phase_4+83>
+   0x000055555555581f <+76>:    cmpl   $0x2,0x8(%rsp)
+   0x0000555555555824 <+81>:    je     0x55555555582b <phase_4+88>
+   0x0000555555555826 <+83>:    call   0x555555555d4a <explode_bomb>
+```
+
+* `   0x0000555555555815 <+66>:    call   0x555555555799 <func4>` calls another function called `func4`
+* The returned value is compared with `0x2`, if they are not equal then the program jumps to call `<explode_bomb>`. 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 <func4+32>
+   0x00005555555557ad <+20>:    mov    $0x0,%eax
+   0x00005555555557b2 <+25>:    jb     0x5555555557c5 <func4+44>
+   0x00005555555557b4 <+27>:    add    $0x8,%rsp
+   0x00005555555557b8 <+31>:    ret    
+   0x00005555555557b9 <+32>:    lea    -0x1(%rcx),%edx
+   0x00005555555557bc <+35>:    call   0x555555555799 <func4>
+   0x00005555555557c1 <+40>:    add    %eax,%eax
+   0x00005555555557c3 <+42>:    jmp    0x5555555557b4 <func4+27>
+   0x00005555555557c5 <+44>:    lea    0x1(%rcx),%esi
+   0x00005555555557c8 <+47>:    call   0x555555555799 <func4>
+   0x00005555555557cd <+52>:    lea    0x1(%rax,%rax,1),%eax
+   0x00005555555557d1 <+56>:    jmp    0x5555555557b4 <func4+27>
+```
+
+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 <func4+32> // If %ecx > %edi -> jump to instruction at offset +32
+mov $0x0,%eax  // Move 0 to %eax
+jb 0x5555555557c5 <func4+44> // 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 <func4> // Call itself
+add %eax,%eax  // Double the value in %eax
+jmp 0x5555555557b4 <func4+27> // jump to the instruction at offset +27
+lea 0x1(%rcx),%esi
+call 0x555555555799 <func4>
+lea 0x1(%rax,%rax,1),%eax // LEA of %rax * 2 + 1 into $eax 
+jmp 0x5555555557b4 <func4+27>
+```
+
+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 `<phase_4>`, we can see that:
+
+```
+   0x000055555555581f <+76>:    cmpl   $0x2,0x8(%rsp)
+   0x0000555555555824 <+81>:    je     0x55555555582b <phase_4+88>
+```
+
+The value at `$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.
+```
+