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:

// 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

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 } }