2nd/10_Wiki/Topic_Programming/Frontend/Pointer Poisoning.md

---
id: wiki-2026-0508-pointer-poisoning
title: Pointer Poisoning
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Pointer Poisoning, Poison Values, Memory Poisoning]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [security, memory-safety, c, cpp, kernel, defensive-programming]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: C
  framework: none
---

# Pointer Poisoning

## 매 한 줄
> **"매 freed pointer 의 detectable invalid value 채움"**. Pointer poisoning 매 use-after-free / double-free / uninitialized-read 의 detection 기법 — free 후 pointer 를 매 0x0, 0xDEADBEEF, kernel-specific magic 으로 set 하여 매 dereference 즉시 crash. Linux kernel `LIST_POISON1/2`, glibc `MALLOC_PERTURB_`, AddressSanitizer 매 광범위 활용. 2026 기준 Rust/Swift 매 ownership 으로 매 회피, C/C++ 매 여전히 핵심 defensive technique.

## 매 핵심

### 매 왜 poison
- **Use-after-free 즉시 detect**: 매 0xDEAD... dereference → page fault.
- **Debugger trail**: stack trace 의 pointer value 만 보고 매 lifecycle stage 추정.
- **Heap exploit 완화**: attacker 매 freed object reuse 의 일부 차단.

### 매 일반적인 poison value
- `0x0` — 매 NULL (가장 단순, page 0 unmapped).
- `0xDEADBEEF` / `0xDEADC0DE` — 매 32-bit human-readable.
- `0xDEADBEEFDEADBEEF` — 매 64-bit.
- Linux kernel: `LIST_POISON1 = 0xdead000000000100`, `LIST_POISON2 = 0xdead000000000122` (매 user-space dereference 시 distinct fault address).
- glibc tcache: poison 매 next-pointer 의 obfuscation (safe-linking, 2.32+).

### 매 응용
1. Linux kernel list_del — `LIST_POISON1/2` 매 next/prev.
2. glibc `MALLOC_PERTURB_` — alloc/free 시 매 byte fill.
3. AddressSanitizer — shadow memory + redzone poison.
4. Embedded firmware — magic pattern 매 stack canary.
5. Safe-linking — heap freelist pointer XOR mask.

## 💻 패턴

### Manual poison after free (C)
```c
#include <stdlib.h>
#include <stdint.h>

#define POISON_PTR ((void *)(uintptr_t)0xDEADBEEFDEADBEEFULL)

void safe_free(void **p) {
    if (!p || !*p) return;
    free(*p);
    *p = POISON_PTR; // dereference → fault, NULL check 도 통과
}

// Usage
void example(void) {
    char *buf = malloc(64);
    safe_free((void **)&buf);
    // *buf = 'x'; // ← SIGSEGV at 0xDEADBEEFDEADBEEF
}
```

### Linux kernel list poison
```c
// include/linux/poison.h
#define LIST_POISON1  ((void *) 0xdead000000000100)
#define LIST_POISON2  ((void *) 0xdead000000000122)

static inline void __list_del_entry(struct list_head *entry) {
    __list_del(entry->prev, entry->next);
    entry->next = LIST_POISON1; // dereference → kernel oops at distinct addr
    entry->prev = LIST_POISON2;
}
```

### Stack canary (poison-ish)
```c
#define CANARY 0xCAFEBABEDEADBEEFULL

void critical_fn(void) {
    volatile uint64_t canary = CANARY;
    char buf[64];
    // ... do work, possibly overflowing buf ...
    if (canary != CANARY) {
        __builtin_trap(); // overflow detected
    }
}
```

### MALLOC_PERTURB_ (glibc env)
```bash
# Fill freed memory with 0x42, alloc memory with ~0x42
MALLOC_PERTURB_=66 ./my_program
```

### Safe-linking (glibc 2.32+ tcache pointer obfuscation)
```c
// Conceptual: actual implementation in malloc.c
#define PROTECT_PTR(pos, ptr) \
    ((__typeof(ptr))((((size_t)(pos)) >> 12) ^ ((size_t)(ptr))))
#define REVEAL_PTR(ptr) PROTECT_PTR(&(ptr), ptr)

// Storing freed chunk's next pointer obfuscated → attacker-controlled
// linked-list overwrite no longer trivially redirects allocator.
```

### C++ smart pointer reset (매 modern equivalent)
```cpp
#include <memory>

void modern() {
    auto p = std::make_unique<Widget>();
    p->use();
    p.reset(); // p == nullptr now; UAF 매 not possible via p
    // p->use(); // SIGSEGV (NULL deref)
}
```

### Sanitizer-driven (no manual poison)
```bash
# AddressSanitizer poisons freed memory + redzones automatically
clang -fsanitize=address -g -O1 prog.c -o prog
./prog # use-after-free → ASAN report with stack
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| Modern C++ | smart pointer + reset (no manual poison) |
| Modern systems | Rust ownership (compile-time prevent) |
| C kernel / embedded | manual poison (`LIST_POISON*`, magic) |
| Debug build | `-fsanitize=address` |
| Production heap | safe-linking (glibc 2.32+ default) |
| Stack overflow detect | canary + `-fstack-protector-strong` |

**기본값**: 매 new code 매 Rust / smart pointer. C 의 legacy 만 manual poison.

## 🔗 Graph

## 🤖 LLM 활용
**언제**: legacy C/C++ codebase 의 UAF debug, kernel module dev, embedded firmware hardening.
**언제 X**: managed language (JS/TS/Python/Java) — GC 가 매 처리. 매 over-engineering.

## ❌ 안티패턴
- **Free without poison + raw pointer 재사용**: 매 silent UAF — heap reuse 시 detection X.
- **Poison value 가 valid mappable address**: 매 0x1 매 page 0 의 일부 — distinct unmapped 사용.
- **Poison only after free, not in struct invalidation**: 매 list, tree node 의 stale pointer 미처리.
- **Trust poison without ASLR**: 매 attacker 매 정확한 poison value 알아도 OK — poison 매 detection only, not mitigation alone.
- **Production with ASAN**: 매 2-3x slowdown, memory 2x — 매 staging only.

## 🧪 검증 / 중복
- Verified (Linux kernel `include/linux/poison.h`, glibc malloc.c safe-linking, Google AddressSanitizer paper, OWASP Memory Corruption).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — manual poison, LIST_POISON, canary, safe-linking, ASAN 패턴 |