someone can help me recognize the vulnerability

Question

I get this code and I can not find the vulnerability. I think this code may be vulnerable to buffer overflow. How do I prove it?

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

typedef struct _AC_DATA
{   
    int the;
    int fan;
    bool rec;
} AC_DATA, *PAC_DATA;

typedef struct _RADIO_DATA
{
    char speakers_volume[4];
    int station;
} RADIO_DATA, *PRADION_DATA;

typedef struct _GLOBAL_DATA
{
    AC_DATA ac_data;
    RADIO_DATA radio_data;
}GLOBAL_DATA, *PGLOBAL_DATA;

void good()
{
    printf("Good!\n");
}

int update_volume(PGLOBAL_DATA pglobal_data, int index, int new_volume)
{
    char arr[100];

    if(index > 4)
    {
        printf("error invalid speaker.\n");
        return -1;
    }
    printf("updating the speaker %d to volume %d\n", index, new_volume);
    pglobal_data -> radio_data.speakers_volume[index] = new_volume;
}

void hacked()
{
    printf("Hacked!\n");
}


int main () {

    GLOBAL_DATA global_data = {0};
    int index;
    int new_volume;
    printf("address of main: 0x%X.\n", main);
    printf("enter volume index:");
    scanf("%d", &index);
    printf("enter new volume:");
    scanf("%d", &new_volume);
    update_volume(&global_data, index, new_volume);

    return 0;
}

Answer 1

The bug is in update_volume.

pglobal_data -> radio_data.speakers_volume[index] = new_volume;

index is an int, it can take negative values too. Ideally you should not be able to access -ve indices in an array as it gives you read/write over memory area preceding the array. Here we just have a check to limit its max value to 4 but you can use negative integers to INT_MIN and overwrite a byte.

For exploitation, global_data belongs to main's stack frame. Accessing negative indices you can overwrite a byte in return address of update_volume (somewhere in main) on the stack. This gives you the primitive to jump to maybe hacked or good functions.

Resources: Use ASAN to verify your claims while fuzzing with inputs.

Here is the ASAN dump for this case.

ASAN_SYMBOLIZER_PATH=/usr/lib/llvm-6.0/bin/llvm-symbolizer ./test
address of main: 0xCA77FE6F.
enter volume index:-36
enter new volume:65
updating the speaker -36 to volume 65
=================================================================
==27759==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffebfb3ff98 at pc 0x55f1ca77fe6f bp 0x7ffebfb3fed0 sp 0x7ffebfb3fec0
WRITE of size 1 at 0x7ffebfb3ff98 thread T0
    #0 0x55f1ca77fe6e in update_volume /tmp/re.c:39
    #1 0x55f1ca78004b in main /tmp/re.c:57
    #2 0x7f237a433b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #3 0x55f1ca77fcf9 in _start (/tmp/test+0xcf9)

Address 0x7ffebfb3ff98 is located in stack of thread T0 at offset 136 in frame
    #0 0x55f1ca77fe7e in main /tmp/re.c:47

  This frame has 3 object(s):
    [32, 36) 'index'
    [96, 100) 'new_volume'
    [160, 180) 'global_data' <== Memory access at offset 136 underflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /tmp/re.c:39 in update_volume
Shadow bytes around the buggy address:
  0x100057f5ffa0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100057f5ffb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100057f5ffc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100057f5ffd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100057f5ffe0: 00 00 f1 f1 f1 f1 04 f2 f2 f2 f2 f2 f2 f2 04 f2
=>0x100057f5fff0: f2 f2 f2[f2]f2 f2 00 00 04 f2 00 00 00 00 00 00
  0x100057f60000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100057f60010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100057f60020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100057f60030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100057f60040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==27759==ABORTING

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