[ruby-core:98484] [Ruby master Bug#16907] Probable use-after-free in VM assertion

Issue #16907 has been reported by jeremyevans0 (Jeremy Evans).

----------------------------------------
Bug #16907: Probable use-after-free in VM assertion
https://bugs.ruby-lang.org/issues/16907

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* ruby -v: ruby 2.8.0dev (2020-05-22) [x86_64-openbsd6.7]
* Backport: 2.5: DONTNEED, 2.6: DONTNEED, 2.7: DONTNEED
----------------------------------------
The following Ruby program fails with VM assertions enabled on OpenBSD (code taken from `test_caller_to_enum` in `test/ruby/test_backtrace.rb`):

```ruby
      def foo
        return to_enum(__method__) unless block_given?
        raise
        yield 1
      end

      enum = foo
      enum.next
```

This is due to the following assertion in `rb_current_vm` in `vm_core.h`:

```c
    VM_ASSERT(ruby_current_vm_ptr == NULL ||
	      ruby_current_execution_context_ptr == NULL ||
	      rb_ec_thread_ptr(GET_EC()) == NULL ||
	      rb_ec_vm_ptr(GET_EC()) == ruby_current_vm_ptr);

```

Adding some debugging code, `rb_ec_vm_ptr(GET_EC())` is `0xdfdfdfdfdfdfdfdf`.  This is the memory pattern that OpenBSD free(3) writes to memory in order to detect use-after-free.  So it is quite likely that this is operating on freed memory.

My guess as to what is happening here is that the enumerator fiber stack is freed, but this VM assertion is still accessing the memory.  However, that's just a guess, and not a particularly educated one.  I am not sure how to fix it.



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