heap out of bounds read in get_fat()

[hannob]

An invalid memory read (heap oob) can happen with a malformed filesystem in the function get_fat().

This was found with american fuzzy lop and address sanitizer.

ASAN stack trace:

==17221==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x61400000fdc8 at pc 0x0000004f5289 bp 0x7fff4589e890 sp 0x7fff4589e888
READ of size 4 at 0x61400000fdc8 thread T0
    #0 0x4f5288 in get_fat /mnt/ram/dosfstools/src/fat.c:68:19
    #1 0x4f5288 in read_fat /mnt/ram/dosfstools/src/fat.c:165
    #2 0x4e018c in main /mnt/ram/dosfstools/src/fsck.fat.c:187:12
    #3 0x7f140fa897af in __libc_start_main (/lib64/libc.so.6+0x207af)
    #4 0x4181e8 in _start (/mnt/ram/dosfstools/fsck.fat+0x4181e8)

0x61400000fdc8 is located 0 bytes to the right of 392-byte region [0x61400000fc40,0x61400000fdc8)
allocated by thread T0 here:
    #0 0x4b3888 in __interceptor_malloc (/mnt/ram/dosfstools/fsck.fat+0x4b3888)
    #1 0x4f332c in alloc /mnt/ram/dosfstools/src/common.c:66:17

SUMMARY: AddressSanitizer: heap-buffer-overflow /mnt/ram/dosfstools/src/fat.c:68:19 in get_fat
Shadow bytes around the buggy address:
  0x0c287fff9f60: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c287fff9f70: fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa fa
  0x0c287fff9f80: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
  0x0c287fff9f90: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c287fff9fa0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c287fff9fb0: 00 00 00 00 00 00 00 00 00[fa]fa fa fa fa fa fa
  0x0c287fff9fc0: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
  0x0c287fff9fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c287fff9fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c287fff9ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fa fa
  0x0c287fffa000: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
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
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  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

dosfstools-get_fat-oob-heap-read.zip