#18 Buffer overflow (READ of size 1) in the get_nxt_word() function

[bsdb0y]

Hi,

While fuzzing ATasm 1.09, I found a buffer overflow in the get_nxt_word() function, asm.c

Attaching a reproducer, issue can be reproduced by running:

atasm get_nxt_word_test01

ATasm 1.09 (A mostly Mac65 compatible 6502 cross-assembler)
Pass 1: =================================================================
==11689==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000000e1771f at pc 0x0000004ceb53 bp 0x7fffa438d130 sp 0x7fffa438d128
READ of size 1 at 0x000000e1771f thread T0
    #0 0x4ceb52 in get_nxt_word /src/atasm109/src/asm.c:478:13
    #1 0x4e8add in assemble /src/atasm109/src/asm.c:2007:11
    #2 0x4eb4c7 in main /src/atasm109/src/asm.c:2421:3
    #3 0x7f4962f290b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #4 0x41c5ed in _start (/src/atasm109/src/atasm+0x41c5ed)

0x000000e1771f is located 1 bytes to the left of global variable 'line' defined in 'asm.c:372:25' (0xe17720) of size 256
0x000000e1771f is located 63 bytes to the right of global variable 'buf' defined in 'asm.c:372:15' (0xe175e0) of size 256
SUMMARY: AddressSanitizer: global-buffer-overflow /src/atasm109/src/asm.c:478:13 in get_nxt_word
Shadow bytes around the buggy address:
  0x0000801bae90: f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9 00 f9 f9 f9
  0x0000801baea0: f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9
  0x0000801baeb0: f9 f9 f9 f9 00 f9 f9 f9 f9 f9 f9 f9 00 00 00 00
  0x0000801baec0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0000801baed0: 00 00 00 00 00 00 00 00 00 00 00 00 f9 f9 f9 f9
=>0x0000801baee0: f9 f9 f9[f9]00 00 00 00 00 00 00 00 00 00 00 00
  0x0000801baef0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0000801baf00: 00 00 00 00 f9 f9 f9 f9 f9 f9 f9 f9 00 f9 f9 f9
  0x0000801baf10: f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9 00 00 00 00
  0x0000801baf20: 00 00 f9 f9 f9 f9 f9 f9 02 f9 f9 f9 f9 f9 f9 f9
  0x0000801baf30: 04 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9
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
  Shadow gap:              cc
==11689==ABORTING