Level 0: Candle (10 pts)

The function getbuf is called within bufbomb by a function test having the following C code: 

      void test()
      {
          int val;
          volatile int local = 0xdeadbeef;
          val = getbuf();
          /* Check for corrupted stack */
          if (local != 0xdeadbeef) {
              printf("Sabotaged!: the stack has been corrupted\n");
          }
          else if (val == cookie) {
              printf("Boom!: getbuf returned 0x%x\n", val);
              validate(3);
          }
          else {
              printf("Dud: getbuf returned 0x%x\n", val);
          }
      }
When getbuf executes its return statement (line 5 of getbuf), the program ordinarily resumes execution within function test (at line 5 of this function).  Within the file bufbomb, there is a function smoke having the following C code: 
    void smoke()
    {
        printf("Smoke!: You called smoke()\n");
        validate(0);
        exit(0);
    }
Your task is to get bufbomb to execute the code for smoke when getbuf executes its return statement, rather than returning to test.  You can do this by supplying an exploit string that overwrites the stored return pointer in the stack frame for getbuf with the address of the first instruction in smoke.  Note that your exploit string may also corrupt other parts of the stack state, but this will not cause a problem, since smoke causes the program to exit directly. 

Some Advice:

  • All the information you need to devise your exploit string for this level can be determined by examining a diassembled version of bufbomb. 
  • Be careful about byte ordering. 
  • You might want to use gdb to step the program through the last few instructions of getbuf to make sure it is doing the right thing. 
  • The placement of buf within the stack frame for getbuf depends on which version of gcc was used to compile bufbomb.  You will need to pad the beginning of your exploit string with the proper number of bytes to overwrite the return pointer.  The values of these bytes can be arbitrary. 
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