Out-of-bounds Read in mrb_get_args in mruby/mruby

Affected commit

3cf291f72224715942beaf8553e42ba8891ab3c6

Proof of Concept

0..% = [0,0,0,0,0,0,0,0,0,0,0,0,0,**{}] = 0

Below is the output from mruby ASAN build:

AddressSanitizer:DEADLYSIGNAL
=================================================================
==36059==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000011 (pc 0x000000418ac7 bp 0x7ffe61994cc0 sp 0x7ffe619949b0 T0)
==36059==The signal is caused by a READ memory access.
==36059==Hint: address points to the zero page.
    #0 0x418ac7 in mrb_get_args /root/mruby/src/class.c:1009
    #1 0x456ac6 in mrb_str_aset_m /root/mruby/src/string.c:1334
    #2 0x4840ba in mrb_vm_exec /root/mruby/src/vm.c:1638
    #3 0x475dcc in mrb_vm_run /root/mruby/src/vm.c:1132
    #4 0x4bf928 in mrb_top_run /root/mruby/src/vm.c:3045
    #5 0x4eba38 in mrb_load_exec mrbgems/mruby-compiler/core/parse.y:6891
    #6 0x4ebcd5 in mrb_load_detect_file_cxt mrbgems/mruby-compiler/core/parse.y:6934
    #7 0x40672c in main /root/mruby/mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:357
    #8 0x7f8cad9030b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #9 0x403a7d in _start (/root/mruby/bin/mruby+0x403a7d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /root/mruby/src/class.c:1009 in mrb_get_args
==36059==ABORTING

Test Platform:

Ubuntu 18.04

Acknowledgements

This bug was found by Ken Wong(@wwkenwong) from Black Bauhinia(@blackb6a) and Alex Cheung