Heap buffer overflow in mrb_env_unshare

[clayton-shopify]

The following input demonstrates a crash:

def a
  yield
ensure
  GC.start
  lambda { return }.call
end
a { return }.call

ASAN report:

==90441==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x61e00000f070 at pc 0x000100d64f4e bp 0x7fff5efe9500 sp 0x7fff5efe94f8
READ of size 4 at 0x61e00000f070 thread T0
    #0 0x100d64f4d in mrb_env_unshare (mruby:x86_64+0x100157f4d)
    #1 0x100d694d2 in cipop (mruby:x86_64+0x10015c4d2)
    #2 0x100d86b80 in mrb_vm_exec (mruby:x86_64+0x100179b80)
    #3 0x100d72ad9 in mrb_vm_run (mruby:x86_64+0x100165ad9)
    #4 0x100da5549 in mrb_top_run (mruby:x86_64+0x100198549)
    #5 0x100e76805 in mrb_load_exec (mruby:x86_64+0x100269805)
    #6 0x100e77615 in mrb_load_file_cxt (mruby:x86_64+0x10026a615)
    #7 0x100c0f2e6 in main mruby.c:227
    #8 0x7fffbbbba234 in start (libdyld.dylib:x86_64+0x5234)

0x61e00000f070 is located 16 bytes to the left of 2560-byte region [0x61e00000f080,0x61e00000fa80)
allocated by thread T0 here:
    #0 0x10101e520 in wrap_realloc (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x56520)
    #1 0x100d078a5 in mrb_default_allocf (mruby:x86_64+0x1000fa8a5)
    #2 0x100c88be8 in mrb_realloc_simple gc.c:202
    #3 0x100c892ce in mrb_realloc gc.c:216
    #4 0x100c89d53 in mrb_malloc gc.c:237
    #5 0x100c89ded in mrb_calloc gc.c:255
    #6 0x100d69b55 in stack_init (mruby:x86_64+0x10015cb55)
    #7 0x100d66bd7 in mrb_funcall_with_block (mruby:x86_64+0x100159bd7)
    #8 0x100d6655a in mrb_funcall_with_block (mruby:x86_64+0x10015955a)
    #9 0x100d65d47 in mrb_funcall_argv (mruby:x86_64+0x100158d47)
    #10 0x100c4d045 in mrb_obj_new (mruby:x86_64+0x100040045)
    #11 0x100c71b9d in mrb_exc_new_str (mruby:x86_64+0x100064b9d)
    #12 0x100c7b967 in mrb_init_exception (mruby:x86_64+0x10006e967)
    #13 0x100cac990 in mrb_init_core (mruby:x86_64+0x10009f990)
    #14 0x100d0783e in mrb_open_core (mruby:x86_64+0x1000fa83e)
    #15 0x100d07a0c in mrb_open_allocf (mruby:x86_64+0x1000faa0c)
    #16 0x100d079d7 in mrb_open (mruby:x86_64+0x1000fa9d7)
    #17 0x100c0e1f8 in main mruby.c:171
    #18 0x7fffbbbba234 in start (libdyld.dylib:x86_64+0x5234)

SUMMARY: AddressSanitizer: heap-buffer-overflow (mruby:x86_64+0x100157f4d) in mrb_env_unshare
Shadow bytes around the buggy address:
  0x1c3c00001db0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x1c3c00001dc0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x1c3c00001dd0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x1c3c00001de0: fd fd fd fd fd fd fd fd fd fd fa fa fa fa fa fa
  0x1c3c00001df0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x1c3c00001e00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa[fa]fa
  0x1c3c00001e10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x1c3c00001e20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x1c3c00001e30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x1c3c00001e40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x1c3c00001e50: 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
==90441==ABORTING
Abort trap: 6

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