NSA's Ghidra - Can It Do MIPS?

You won't find the solution to the challenge here. That is against root-me.org rules. We are simply looking at a ghidra analysis of the executable.

The previous test on Ghidra impressed me, and so I decided to test it on another simple crackme challenge from root-me.org

In this case the executable is MIPS ELF:

$ file ch27.bin

ch27.bin: ELF 32-bit LSB executable, MIPS, MIPS32 rel2 version 1 (SYSV), dynamically linked, interpreter /lib/ld-uClibc.so.0, stripped

Lets see if Ghidra can handle it:

And it has no problem with this. In fact, the decompiler may make this crackme challenge kind of a boring exercise - IF the code is correct. Let's try!

  size_t sVar1;

  int local_58;

  char local_54 [4];

  char local_50;

  char local_4f;

  char local_4e;

  char local_4d;

  char local_43;

  char local_42;

  undefined4 local_c;

  puts("crack-me for Root-me by s4r");

  puts("Enter password please");

  fgets(local_54,0x40,stdin);                    // read up to 0x40 characters from

                                                 // user entry (including 0)

                                                 // note that local_54 is technically

                                                 // only has space for 4 characters

  sVar1 = strlen(local_54);                      // how many chars actually got entered

  *(undefined *)((int)&local_58 + sVar1 + 3) = 0;// terminate string at

                                                 // local_58+strlen+3 with 0

  sVar1 = strlen(local_54);

  if (sVar1 == 0x13) {                           // the password has to be 0x13 = 19 chars long

    local_58 = 8;                                // initialize local_58 to 8

    while (local_58 < 0x11) {                    // just a for loop from 8 to 16

      if (local_54[local_58] != 'i') {           // local_54[8] to local_54[16] have to be 'i'

        FUN_00400814();

        return 0;

      local_58 = local_58 + 1;                   // just a for loop from 8 to 16

    if (local_42 == 's') {                       // local_54[18]

      if (local_43 == 'p') {                     // local_54[17]

        if (local_4d == 'm') {                   // local_54[7]

          if ((local_54[2] == 'n') && (local_4e == 'n')) { // local_54[2] and local_54[6]

            if (local_54[0] == 'c') {            // local_54[0]

              if (local_54[1] == 'a') {          // local_54[1]

                if (local_54[3] == 't') {        // local_54[3]

                  if (local_50 == 'r') {         // local_54[4]

                    if (local_4f == 'u') {       // local_54[5]

                      FUN_004007c0();

                      return local_c;

I left out all the else statement, because they all go to the same function, which fails the password check.

Lets look at the local variables

char local_54 [4];

  char local_50;

  char local_4f;

  char local_4e;

  char local_4d;

  char local_43;

  char local_42;

The stack grows downward here, and the naming of the variables is a bit unfortunate, because they are named by how far down in the stack they are. Here a bit of the disassembly window:

Stack[-0x4]:4  local_4

Stack[-0x8]:4  local_8

Stack[-0xc]:4  local_c

Stack[-0x10]:4 local_10

Stack[-0x42]:1 local_42

Stack[-0x43]:1 local_43

Stack[-0x4c]:1 local_4c

Stack[-0x4d]:1 local_4d

Stack[-0x4e]:1 local_4e

Stack[-0x4f]:1 local_4f

Stack[-0x50]:1 local_50

Stack[-0x51]:1 local_51

Stack[-0x52]:1 local_52

Stack[-0x53]:1 local_53

Stack[-0x54]:1 local_54

Stack[-0x58]:4 local_58

Stack[-0x60]:4 local_60

So local_53 means 0x53 down from the stack pointer. This is important, because it implies that

fgets(local_54,0x40,stdin);

allows us to overwrite the other local variables. Hence,

  char local_54 [4]; //local_54[0..3]

  char local_50;     //local_54[4]

  char local_4f;     //local_54[5]

  char local_4e;     //local_54[6]

  char local_4d;     //local_54[7]

  char local_4c;     //local_54[8]

  char local_4b;     //local_54[9]

  char local_4a;     //local_54[10]

  char local_49;     //local_54[11]

  char local_48;     //local_54[12]

  char local_47;     //local_54[13]

  char local_46;     //local_54[14]

  char local_45;     //local_54[15]

  char local_44;     //local_54[16]

  char local_43;     //local_54[17]

  char local_42;     //local_54[18]

  undefined4 local_c;//local_54[19] extra space for 0 termination

and now the password is easy to deduce. local_54[8] to local_54[16] have to be 'i's and the other characters are specified in the if statements.

The decompiler made this very very easy in deed. I rarely look at MIPS assembly code and this would have been very tedious to solve for me without the pseudocode.

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