Oj: Stack Buffer Overflow in Oj::Doc#each_child via Deeply Nested Input

### Summary `Oj::Doc#each_child`, when invoked recursively over a deeply nested JSON document, overflows a fixed-size stack buffer and aborts the process. This is a denial of service reachable from untrusted JSON. ### Details Two-step chain in `ext/oj/fast.c`: 1. **`doc_each_child` (~line 1501)** increments `doc->where` past the `where_path[MAX_STACK = 100]` array with no bounds check, and never restores it (`doc->where--` is missing). Calling `each_child` recursively from inside the yield block therefore drives `doc->where` beyond the array. 2. **On the next entry (~line 1478)** the function copies the path into a stack-local buffer: ```c Leaf save_path[MAX_STACK]; // 800-byte stack buffer size_t wlen = doc->where - doc->where_path; if (0 < wlen) { memcpy(save_path, doc->where_path, sizeof(Leaf) * (wlen + 1)); } ``` When the previous recursive call left `doc->where` past `where_path[100]`, `wlen` exceeds `MAX_STACK` and the `memcpy` overflows `save_path` on the C stack. The `Oj::Doc` parser imposes no JSON nesting-depth limit (it relies on a C-stack pressure check), so deeply nested attacker input reaches this path. ### Proof of Concept ```ruby require 'oj' depth = 200 payload = '[' * depth + '1' + ']' * depth Oj::Doc.open(payload) do |doc| r = lambda { doc.each_child { |_| r.call } } r.call end ``` Recursion depth <= 99 iterates normally; depth >= 101 aborts. lldb backtrace on the affected build (`ruby 3.3.8 / arm64-darwin24`): ``` SIGABRT #2 __abort #3 __stack_chk_fail #4 doc_each_child (oj.bundle, fast.c) ``` ### Impact Reliable denial of service: any endpoint that calls `Oj::Doc.open(untrusted) { |d| d.each_child ... }` recursively can be crashed with a small deeply-nested payload. On builds with a stack protector (the default, `-fstack-protector-strong`) the canary aborts the process before the saved return address is used. The Step-1 heap OOB writes into `struct _doc` fields do occur, but are masked in practice because the Step-2 stack overflow crashes first; turning them into anything beyond a crash has not been demonstrated. ### Patches Fixed in **3.17.3**: `doc_each_child` now bounds-checks before incrementing `doc->where` (raising `Oj::DepthError`) and restores `doc->where` after the loop, matching the existing `each_leaf` pattern. Verified on the fixed build: depth >= 101 raises a clean `Oj::DepthError` instead of aborting. ### Credit Reported by Zac Wang (@7a6163).

Quoted source text, attributed separately from HOL analysis.