Oj: Use-After-Free in Oj::Doc Iterators via Reentrant Close

### Summary `Oj::Doc` iterators (`each_value`, `each_child`, `each_leaf`) are vulnerable to a heap use-after-free. When a Ruby block yielded during iteration calls `doc.close` or `d.close`, the document's heap memory is freed while the C iterator is still running. When control returns from the block, the iterator reads from the freed region, producing a use-after-free accessible from pure Ruby. ### Version - **Software**: oj gem - **Affected**: all versions with `ext/oj/fast.c` - **Latest tested**: 3.17.1 (confirmed present) ### Details The iterators in `ext/oj/fast.c` follow the pattern: ```c // fast.c:1505 (doc_each_child) static VALUE doc_each_child(VALUE self, ...) { ... while (cur != NULL) { rb_yield(...); // ← Ruby block executes here cur = cur->next; // ← cur is now freed if block called close() } } ``` `rb_yield` can invoke arbitrary Ruby code, including calling `close()` on the `Doc` or any child node, which calls `ruby_sized_xfree` on the backing buffer. On return, the C code reads `cur->next` from the freed region. All three iterators are affected. ASAN report (each_child variant): ``` ==253632==ERROR: AddressSanitizer: heap-use-after-free on address 0x5210000bd080 READ of size 8 at 0x5210000bd080 thread T0 #0 doc_each_child /ext/oj/fast.c:1505 0x5210000bd080 is located 896 bytes inside of 4064-byte region [0x5210000bcd00, 0x5210000bdce0) freed by thread T0 here: #0 free #1 ruby_sized_xfree (libruby-3.3.so.3.3) ``` All three iterators trigger the same freed region (`fd` shadow bytes): ``` 0x5210000bd080:[fd]fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd ``` ### Reproduce ```ruby require 'oj' # each_child Oj::Doc.open('[1,2]') { |doc| doc.each_child { |d| d.close } } # each_value Oj::Doc.open('[1,2]') { |doc| doc.each_value { |v| doc.close } } # each_leaf Oj::Doc.open('[1,[2]]') { |doc| doc.each_leaf { |d| d.close } } ```

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