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Bug #16907

Probable use-after-free in VM assertion

Added by jeremyevans0 (Jeremy Evans) 7 months ago. Updated 4 months ago.

Status:
Closed
Priority:
Normal
Target version:
-
ruby -v:
ruby 2.8.0dev (2020-05-22) [x86_64-openbsd6.7]
[ruby-core:98484]

Description

The following Ruby program fails with VM assertions enabled on OpenBSD (code taken from test_caller_to_enum in test/ruby/test_backtrace.rb):

      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:

    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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