CVE-2020-21050: AddressSanitizer: stack-buffer-overflow at fromgif.c:310 · Issue #75 · saitoha/libsixel

Our fuzzer detected several crashes when converting gif file against 2df6437 (compiled with Address Sanitizer). The command to trigger that is img2sixel $POC -o /tmp/test.six where $POC can be:
https://github.com/ntu-sec/pocs/blob/master/libsixel-2df6437/crashes/so_fromgif.c%3A310_1.gif
https://github.com/ntu-sec/pocs/blob/master/libsixel-2df6437/crashes/so_fromgif.c%3A310_2.gif

gdb output is like:

GNU gdb (Ubuntu 8.1-0ubuntu3) 8.1.0.20180409-git
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Reading symbols from /home/hongxu/FOT/libsixel/install/bin/img2sixel...done.
Starting program: /home/hongxu/FOT/libsixel/install/bin/img2sixel hbo_fromgif.c:310_1.gif -o /tmp/test.six
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
=================================================================
==17533==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fffffffa7d8 at pc 0x7ffff7b5f160 bp 0x7fffffff5950 sp 0x7fffffff5948
WRITE of size 2 at 0x7fffffffa7d8 thread T0
    #0 0x7ffff7b5f15f in gif_process_raster /home/hongxu/FOT/libsixel/src/fromgif.c:310:31
    #1 0x7ffff7b5c303 in gif_load_next /home/hongxu/FOT/libsixel/src/fromgif.c:462:22
    #2 0x7ffff7b5a25e in load_gif /home/hongxu/FOT/libsixel/src/fromgif.c:599:22
    #3 0x7ffff7b11198 in load_with_builtin /home/hongxu/FOT/libsixel/src/loader.c:858:18
    #4 0x7ffff7b10116 in sixel_helper_load_image_file /home/hongxu/FOT/libsixel/src/loader.c:1352:18
    #5 0x7ffff7b69d98 in sixel_encoder_encode /home/hongxu/FOT/libsixel/src/encoder.c:1737:14
    #6 0x515787 in main /home/hongxu/FOT/libsixel/converters/img2sixel.c:457:22
    #7 0x7ffff61a6b96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #8 0x41a239 in _start (/home/hongxu/FOT/libsixel/install/bin/img2sixel+0x41a239)

Address 0x7fffffffa7d8 is located in stack of thread T0 at offset 18296 in frame
    #0 0x7ffff7b5999f in load_gif /home/hongxu/FOT/libsixel/src/fromgif.c:555

  This frame has 4 object(s):
    [32, 208) 's' (line 556)
    [272, 18296) 'g' (line 557) <== Memory access at offset 18296 overflows this variable
    [18560, 18568) 'frame' (line 559)
    [18592, 18600) 'fnp' (line 560)
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /home/hongxu/FOT/libsixel/src/fromgif.c:310:31 in gif_process_raster
Shadow bytes around the buggy address:
  0x10007fff74a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff74b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff74c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff74d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff74e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10007fff74f0: 00 00 00 00 00 00 00 00 00 00 00[f2]f2 f2 f2 f2
  0x10007fff7500: f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2
  0x10007fff7510: f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 f2 00 f2 f2 f2
  0x10007fff7520: 00 f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7530: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10007fff7540: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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
==17533==ABORTING

Program received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=0x6) at ../sysdeps/unix/sysv/linux/raise.c:51
51  ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
#0  __GI_raise (sig=sig@entry=0x6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007ffff61c5801 in __GI_abort () at abort.c:79
#2  0x000000000050376b in __sanitizer::Abort() ()
#3  0x0000000000500a98 in __sanitizer::Die() ()
#4  0x00000000004e2d1d in __asan::ReportGenericError(unsigned long, unsigned long, unsigned long, unsigned long, bool, unsigned long, unsigned int, bool) ()
#5  0x00000000004e374b in __asan_report_store2 ()
#6  0x00007ffff7b5f160 in gif_process_raster (s=0x7fffffff6080, g=0x7fffffff6170) at fromgif.c:310
#7  0x00007ffff7b5c304 in gif_load_next (s=0x7fffffff6080, g=0x7fffffff6170, bgcolor=0x0) at fromgif.c:462
#8  0x00007ffff7b5a25f in load_gif (buffer=0x62d000000400 "GIF89a\f", size=0xca, bgcolor=0x0, reqcolors=0x100, fuse_palette=0x1, fstatic=0x0, loop_control=0x0, fn_load=0x7ffff7b6a090 <load_image_callback>, context=0x610000000040, allocator=0x604000000190) at fromgif.c:599
#9  0x00007ffff7b11199 in load_with_builtin (pchunk=0x603000000e20, fstatic=0x0, fuse_palette=0x1, reqcolors=0x100, bgcolor=0x0, loop_control=0x0, fn_load=0x7ffff7b6a090 <load_image_callback>, context=0x610000000040) at loader.c:858
#10 0x00007ffff7b10117 in sixel_helper_load_image_file (filename=0x7fffffffc9ed "hbo_fromgif.c:310_1.gif", fstatic=0x0, fuse_palette=0x1, reqcolors=0x100, bgcolor=0x0, loop_control=0x0, fn_load=0x7ffff7b6a090 <load_image_callback>, finsecure=0x0, cancel_flag=0x13b61c0 <signaled>, context=0x610000000040, allocator=0x604000000190) at loader.c:1352
#11 0x00007ffff7b69d99 in sixel_encoder_encode (encoder=0x610000000040, filename=0x7fffffffc9ed "hbo_fromgif.c:310_1.gif") at encoder.c:1737
#12 0x0000000000515788 in main (argc=0x4, argv=0x7fffffffc488) at img2sixel.c:457