InfoSect's Month of Pointless Bonus Bugs (#32)

InfoSect, Canberra's hackerspace, regularly runs public group sessions to perform code review and vulnerability discovery. Over the next 30 days, I'll highlight the source code of 30 unknown vulnerabilities.

Bonus Bug #32

 In the package abootimg, a program for working with android boot images, it's possible to craft a boot image to trigger a buffer overflow and other issues when abootimg accesses it.

struct boot_img_hdr
{
    unsigned char magic[BOOT_MAGIC_SIZE];

     unsigned kernel_size;  /* size in bytes */
    unsigned kernel_addr;  /* physical load addr */

     unsigned ramdisk_size; /* size in bytes */
    unsigned ramdisk_addr; /* physical load addr */

     unsigned second_size;  /* size in bytes */

    unsigned second_addr;  /* physical load addr */

...

    case extract:
      open_bootimg(bootimg, "r");
      read_header(bootimg);
      write_bootimg_config(bootimg);
      extract_kernel(bootimg);
      extract_ramdisk(bootimg);
      extract_second(bootimg);
      break;

    
...

void read_header(t_abootimg* img)
{
  size_t rb = fread(&img->header, sizeof(boot_img_hdr), 1, img->stream);
  if ((rb!=1) || ferror(img->stream))

...

  if (check_boot_img_header(img))
    abort_printf("%s: not a valid Android Boot Image.\n", img->fname);
}

If we can bypass check_boot_img_header we can do some things..

int check_boot_img_header(t_abootimg* img)
{
  if (strncmp(img->header.magic, BOOT_MAGIC, BOOT_MAGIC_SIZE)) {
    fprintf(stderr, "%s: no Android Magic Value\n", img->fname);
    return 1;
  }

...

  unsigned n = (img->header.kernel_size + page_size - 1) / page_size;
  unsigned m = (img->header.ramdisk_size + page_size - 1) / page_size;
  unsigned o = (img->header.second_size + page_size - 1) / page_size;

  unsigned total_size = (1+n+m+o)*page_size;

  if (total_size > img->size) {
    fprintf(stderr, "%s: sizes mismatches in boot image\n", img->fname);
    return 1;
  }

This is a simple integer overflow to calculate total_size. Lets find some potential places for heap overflows..

void extract_second(t_abootimg* img)
{
  unsigned psize = img->header.page_size;
  unsigned ksize = img->header.kernel_size;
  unsigned rsize = img->header.ramdisk_size;
  unsigned ssize = img->header.second_size;

  if (!ssize) // Second Stage not present
    return;

  unsigned n = (rsize + ksize + psize - 1) / psize;
  unsigned soffset = (1+n)*psize;

  printf ("extracting second stage image in %s\n", img->second_fname);

  void* s = malloc(ksize);
  if (!s)
    abort_perror(NULL);

  if (fseek(img->stream, soffset, SEEK_SET))
    abort_perror(img->fname);

  size_t rb = fread(s, ssize, 1, img->stream);
  if ((rb!=1) || ferror(img->stream))
    abort_perror(img->fname);

This is bad code. It's using malloc(ksize) but using ssize for the read. If we use the boot img header check bypass, we should be able to do bad things.


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