heap-buffer-overflow in dither_func_fs at tosixel.c:861

[SuhwanSong]

version : img2sixel 1.8.2

There is a heap-buffer-overflow in dither_func_fs at tosixel.c:861
please run following cmd to reproduce it.

img2sixel --high-color $PoC

poc
ASAN LOG

==39700==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7fcc6219e808 at pc 0x7fcc6ad13008 bp 0x7ffe2a935b70 sp 0x7ffe2a935b68
READ of size 1 at 0x7fcc6219e808 thread T0
    #0 0x7fcc6ad13007 in dither_func_fs /home/tmp/libsixel/src/tosixel.c:861:9
    #1 0x7fcc6ad13007 in sixel_apply_15bpp_dither /home/tmp/libsixel/src/tosixel.c:1223
    #2 0x7fcc6ad13007 in sixel_encode_highcolor /home/tmp/libsixel/src/tosixel.c:1334
    #3 0x7fcc6ad13007 in sixel_encode /home/tmp/libsixel/src/tosixel.c:1485
    #4 0x7fcc6b0478a4 in sixel_encoder_output_without_macro /home/tmp/libsixel/src/encoder.c:820:14
    #5 0x7fcc6b0478a4 in sixel_encoder_encode_frame /home/tmp/libsixel/src/encoder.c:1050
    #6 0x7fcc6adfdd91 in sixel_helper_load_image_file /home/tmp/libsixel/src/loader.c:913:14
    #7 0x7fcc6b03fd4f in sixel_encoder_encode /home/tmp/libsixel/src/encoder.c:1737:14
    #8 0x51787f in main /home/tmp/libsixel/converters/img2sixel.c:457:22
    #9 0x7fcc693a2b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #10 0x41a379 in _start (/home/tmp/img2sixel+0x41a379)

0x7fcc6219e808 is located 0 bytes to the right of 573448-byte region [0x7fcc62112800,0x7fcc6219e808)
allocated by thread T0 here:
    #0 0x4da230 in __interceptor_malloc (/home/tmp/img2sixel+0x4da230)
    #1 0x7fcc6acfcc7f in sixel_encode_highcolor /home/tmp/libsixel/src/tosixel.c:1309:40
    #2 0x7fcc6acfcc7f in sixel_encode /home/tmp/libsixel/src/tosixel.c:1485
    #3 0x7fcc6b0478a4 in sixel_encoder_output_without_macro /home/tmp/libsixel/src/encoder.c:820:14
    #4 0x7fcc6b0478a4 in sixel_encoder_encode_frame /home/tmp/libsixel/src/encoder.c:1050
    #5 0x7fcc6adfdd91 in sixel_helper_load_image_file /home/tmp/libsixel/src/loader.c:913:14
    #6 0x7fcc6b03fd4f in sixel_encoder_encode /home/tmp/libsixel/src/encoder.c:1737:14
    #7 0x51787f in main /home/tmp/libsixel/converters/img2sixel.c:457:22
    #8 0x7fcc693a2b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310

SUMMARY: AddressSanitizer: heap-buffer-overflow /home/tmp/libsixel/src/tosixel.c:861:9 in dither_func_fs
Shadow bytes around the buggy address:
  0x0ffa0c42bcb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffa0c42bcc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffa0c42bcd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffa0c42bce0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0ffa0c42bcf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0ffa0c42bd00: 00[fa]fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ffa0c42bd10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ffa0c42bd20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ffa0c42bd30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ffa0c42bd40: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0ffa0c42bd50: 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
  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
==39700==ABORTING

[saitoha]

9d0a7ff for #116 also fixed this problem.