potrace: heap-based buffer overflow in bm_readbody_bmp (bitmap_io.c) (incomplete fix for CVE-2016-8698)

["Agostino Sarubbo" <ago () gentoo org>]

Description:
potrace is a utility that transforms bitmaps into vector graphics.

A fuzz on 1.14 showed that an overflow previously reported as CVE-2016-8698 was not really fixed. Since there isn’t a 
public git repository, I uploaded the patch on my ‘poc’ repository on github. The patch was sent from the 
upstream maintainer, Mr. Peter Selinger.

The complete ASan output:

# potrace $FILE
==7325==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60200000efd0 at pc 0x00000051dc51 bp 0x7ffc766b1a30 
sp 0x7ffc766b1a28
READ of size 8 at 0x60200000efd0 thread T0
    #0 0x51dc50 in bm_readbody_bmp /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/bitmap_io.c:754:4
    #1 0x51dc50 in bm_read /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/bitmap_io.c:138
    #2 0x510a45 in process_file /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/main.c:1058:9
    #3 0x50dd56 in main /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/main.c:1214:7
    #4 0x7f6c7333e78f in __libc_start_main 
/tmp/portage/sys-libs/glibc-2.23-r3/work/glibc-2.23/csu/../csu/libc-start.c:289
    #5 0x419b68 in getenv (/usr/bin/potrace+0x419b68)

0x60200000efd1 is located 0 bytes to the right of 1-byte region [0x60200000efd0,0x60200000efd1)
allocated by thread T0 here:
    #0 0x4d2b25 in calloc 
/tmp/portage/sys-devel/llvm-3.9.1-r1/work/llvm-3.9.1.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:72
    #1 0x519776 in bm_new /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/bitmap.h:121:30
    #2 0x519776 in bm_readbody_bmp /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/bitmap_io.c:574
    #3 0x519776 in bm_read /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/bitmap_io.c:138
    #4 0x510a45 in process_file /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/main.c:1058:9
    #5 0x50dd56 in main /tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/main.c:1214:7
    #6 0x7f6c7333e78f in __libc_start_main 
/tmp/portage/sys-libs/glibc-2.23-r3/work/glibc-2.23/csu/../csu/libc-start.c:289

SUMMARY: AddressSanitizer: heap-buffer-overflow 
/tmp/portage/media-gfx/potrace-1.14/work/potrace-1.14/src/bitmap_io.c:754:4 in bm_readbody_bmp
Shadow bytes around the buggy address:
  0x0c047fff9da0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9db0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9dc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9dd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9de0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x0c047fff9df0: fa fa fa fa fa fa fa fa fa fa[01]fa fa fa 04 fa
  0x0c047fff9e00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9e10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9e20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9e30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c047fff9e40: 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
==7325==ABORTING

Affected version:
1.14

Fixed version:
1.15

Commit fix:
https://github.com/asarubbo/poc/blob/master/00219-potrace-heapoverflow-bm_readbody_bmp-PATCH

Credit:
This bug was discovered by Agostino Sarubbo of Gentoo.

CVE:
N/A

Reproducer:
https://github.com/asarubbo/poc/blob/master/00210-potrace-heapoverflow-bm_readbody_bmp

Timeline:
2017-02-26: bug discovered and reported to upstream
2017-02-28: upstream released a patch
2017-03-03: blog post about the issue

Note:
This bug was found with American Fuzzy Lop.

Permalink:
https://blogs.gentoo.org/ago/2017/03/03/potrace-heap-based-buffer-overflow-in-bm_readbody_bmp-bitmap_io-c-incomplete-fix-for-cve-2016-8698

--
Agostino Sarubbo
Gentoo Linux Developer