Oj: Use-After-Free in Oj::Parser array_class/hash_class GC MarkingCVE-2026-54901

### Summary `Oj::Parser` in usual mode does not mark `array_class` and `hash_class` references during garbage collection. If GC runs after the class is assigned but before a parse, the class object is reclaimed, leaving the parser holding a dangling VALUE. The subsequent `parse` call dereferences the freed object, producing a segfault. ### Version - **Software**: oj gem - **Affected**: all versions with `ext/oj/usual.c` / `ext/oj/parser.c` - **Latest tested**: 3.17.1 (confirmed present) ### Details The `parser_mark` function in `ext/oj/parser.c` is registered as the GC mark callback for the parser's `TypedData`. If `array_class` (stored as `d->array_class` in the `Usual` struct) is not passed to `rb_gc_mark`, the GC does not know it is referenced and may collect it. When `close_array_class` (`usual.c:405`) later calls `rb_funcallv` on the collected class VALUE, it accesses freed memory, crashing at `RIP: 0x7f... / 0x0000000000000000`. Crash output: ``` array_class finalized about to parse [BUG] Segmentation fault at 0x0000000000000000 close_array_class+0x194 /ext/oj/usual.c:405 parse+0x17b3 /ext/oj/parser.c:715 parser_parse+0x10b /ext/oj/parser.c:1408 RIP: 0x7fd1b46d68b7 RBP: 0x0000000000000000 ``` ### Reproduce ```ruby require 'oj' p = Oj::Parser.new(:usual, array_class: (ac = Class.new { def <<(_x); end })) ObjectSpace.define_finalizer(ac, proc { warn 'array_class finalized' }) ac = nil GC.start(full_mark: true, immediate_sweep: true) # collect the class p.parse('[1]') # segfault ```

Quoted source text, attributed separately from HOL analysis.