hakmem/FREE_TO_SS_TECHNICAL_DEEPDIVE.md

FREE_TO_SS=1 SEGV - Technical Deep Dive

Overview

This document provides detailed code analysis of the SEGV bug in the FREE_TO_SS=1 code path, with complete reproduction scenarios and fix implementations.

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Part 1: Bug #1 - Critical: size_class Validation Missing

The Vulnerability

Location: Multiple points in the call chain

• hakmem_tiny_free.inc:1520 (class_idx assignment)
• hakmem_tiny_free.inc:1189 (g_tiny_class_sizes access)
• hakmem_tiny_free.inc:1564 (HAK_STAT_FREE macro)

Current Code (VULNERABLE)

hakmem_tiny_free.inc:1517-1524

SuperSlab* fast_ss = NULL;
TinySlab* fast_slab = NULL;
int fast_class_idx = -1;
if (g_use_superslab) {
    fast_ss = hak_super_lookup(ptr);
    if (fast_ss && fast_ss->magic == SUPERSLAB_MAGIC) {
        fast_class_idx = fast_ss->size_class;  // ← NO BOUNDS CHECK!
    } else {
        fast_ss = NULL;
    }
}

hakmem_tiny_free.inc:1554-1566

SuperSlab* ss = fast_ss;
if (!ss && g_use_superslab) {
    ss = hak_super_lookup(ptr);
    if (!(ss && ss->magic == SUPERSLAB_MAGIC)) {
        ss = NULL;
    }
}
if (ss && ss->magic == SUPERSLAB_MAGIC) {
    hak_tiny_free_superslab(ptr, ss);  // ← Called with unvalidated ss
    HAK_STAT_FREE(ss->size_class);     // ← OOB if ss->size_class >= 8
    return;
}

Vulnerability in hak_tiny_free_superslab()

hakmem_tiny_free.inc:1188-1203

if (__builtin_expect(g_tiny_safe_free, 0)) {
    size_t blk = g_tiny_class_sizes[ss->size_class];  // ← OOB READ!
    uint8_t* base = tiny_slab_base_for(ss, slab_idx);
    uintptr_t delta = (uintptr_t)ptr - (uintptr_t)base;
    int cap_ok = (meta->capacity > 0) ? 1 : 0;
    int align_ok = (delta % blk) == 0;
    int range_ok = cap_ok && (delta / blk) < meta->capacity;
    if (!align_ok || !range_ok) {
        // ... error handling ...
    }
}

Why This Causes SEGV

Array Definition (hakmem_tiny.h:33-42)

#define TINY_NUM_CLASSES 8

static const size_t g_tiny_class_sizes[TINY_NUM_CLASSES] = {
    8,      // Class 0:    8 bytes
    16,     // Class 1:   16 bytes
    32,     // Class 2:   32 bytes
    64,     // Class 3:   64 bytes
    128,    // Class 4:  128 bytes
    256,    // Class 5:  256 bytes
    512,    // Class 6:  512 bytes
    1024    // Class 7: 1024 bytes
};

Scenario:

Thread executes free(ptr) with HAKMEM_TINY_FREE_TO_SS=1
  ↓
hak_super_lookup(ptr) returns SuperSlab* ss
  ss->magic == SUPERSLAB_MAGIC ✓ (valid magic)
  But ss->size_class = 0xFF (corrupted memory!)
  ↓
hak_tiny_free_superslab(ptr, ss) called
  ↓
g_tiny_class_sizes[0xFF] accessed  ← Out-of-bounds array access
  ↓
Array bounds: g_tiny_class_sizes[0..7]
Access: g_tiny_class_sizes[255]
Result: SIGSEGV (Segmentation Fault)

Reproduction (Hypothetical)

// Assume corrupted SuperSlab with size_class=255
SuperSlab* ss = (SuperSlab*)corrupted_memory;
ss->magic = SUPERSLAB_MAGIC;  // Valid magic (passes check)
ss->size_class = 255;          // CORRUPTED field
ss->lg_size = 20;

// In hak_tiny_free_superslab():
if (g_tiny_safe_free) {
    size_t blk = g_tiny_class_sizes[ss->size_class];  // Access [255]!
    // Bounds: [0..7], Access: [255]
    // Result: SEGFAULT
}

The Fix

Minimal Fix (Priority 1):

// In hakmem_tiny_free.inc:1554-1566, before calling hak_tiny_free_superslab()

if (ss && ss->magic == SUPERSLAB_MAGIC) {
    // ADDED: Validate size_class before use
    if (__builtin_expect(ss->size_class >= TINY_NUM_CLASSES, 0)) {
        tiny_debug_ring_record(TINY_RING_EVENT_REMOTE_INVALID,
                              (uint16_t)(0xBAD_CLASS | (ss->size_class & 0xFF)), 
                              ptr, 
                              (uint32_t)(ss->lg_size << 16 | ss->size_class));
        if (g_tiny_safe_free_strict) { raise(SIGUSR2); }
        return;  // ADDED: Early return to prevent SEGV
    }
    
    hak_tiny_free_superslab(ptr, ss);
    HAK_STAT_FREE(ss->size_class);
    return;
}

Comprehensive Fix (Priority 1+):

// In hakmem_tiny_free.inc:1554-1566

if (ss && ss->magic == SUPERSLAB_MAGIC) {
    // CRITICAL VALIDATION: Check all SuperSlab metadata
    int validation_ok = 1;
    uint32_t diag_code = 0;
    
    // Check 1: size_class
    if (ss->size_class >= TINY_NUM_CLASSES) {
        validation_ok = 0;
        diag_code = 0xBAD1 | (ss->size_class << 8);
    }
    
    // Check 2: lg_size (only if size_class valid)
    if (validation_ok && (ss->lg_size < 20 || ss->lg_size > 21)) {
        validation_ok = 0;
        diag_code = 0xBAD2 | (ss->lg_size << 8);
    }
    
    // Check 3: active_slabs (sanity check)
    int expected_slabs = ss_slabs_capacity(ss);
    if (validation_ok && ss->active_slabs > expected_slabs) {
        validation_ok = 0;
        diag_code = 0xBAD3 | (ss->active_slabs << 8);
    }
    
    if (!validation_ok) {
        tiny_debug_ring_record(TINY_RING_EVENT_REMOTE_INVALID,
                              diag_code,
                              ptr,
                              ((uint32_t)ss->lg_size << 8) | ss->size_class);
        if (g_tiny_safe_free_strict) { raise(SIGUSR2); }
        return;
    }
    
    hak_tiny_free_superslab(ptr, ss);
    HAK_STAT_FREE(ss->size_class);
    return;
}

---

Part 2: Bug #2 - TOCTOU Race in hak_super_lookup()

The Race Condition

Location: hakmem_super_registry.h:73-106

Current Implementation

static inline SuperSlab* hak_super_lookup(void* ptr) {
    if (!g_super_reg_initialized) return NULL;

    // Try both 1MB and 2MB alignments
    for (int lg = 20; lg <= 21; lg++) {
        uintptr_t mask = (1UL << lg) - 1;
        uintptr_t base = (uintptr_t)ptr & ~mask;
        int h = hak_super_hash(base, lg);

        // Linear probing with acquire semantics
        for (int i = 0; i < SUPER_MAX_PROBE; i++) {
            SuperRegEntry* e = &g_super_reg[(h + i) & SUPER_REG_MASK];
            uintptr_t b = atomic_load_explicit((_Atomic uintptr_t*)&e->base,
                                               memory_order_acquire);

            // Match both base address AND lg_size
            if (b == base && e->lg_size == lg) {
                // Atomic load to prevent TOCTOU race with unregister
                SuperSlab* ss = atomic_load_explicit(&e->ss, memory_order_acquire);
                if (!ss) return NULL;  // Entry cleared by unregister

                // CRITICAL: Check magic BEFORE returning pointer
                if (ss->magic != SUPERSLAB_MAGIC) return NULL;

                return ss;  // ← Pointer returned here
                            // But memory could be unmapped on next instruction!
            }
            if (b == 0) break;  // Empty slot
        }
    }
    return NULL;
}

The Race Scenario

Timeline:

TIME 0:  Thread A: ss = hak_super_lookup(ptr)
         - Reads registry entry
         - Checks magic: SUPERSLAB_MAGIC ✓
         - Returns ss pointer

TIME 1:  Thread B: [Different thread or signal handler]
         - Calls hak_super_unregister()
         - Writes e->base = 0  (release semantics)

TIME 2:  Thread B: munmap((void*)ss, SUPERSLAB_SIZE)
         - Unmaps the entire 1MB/2MB region
         - Physical pages reclaimed by kernel

TIME 3:  Thread A: TinySlabMeta* meta = &ss->slabs[slab_idx]
         - Attempts to access first cache line of ss
         - Memory mapping: INVALID
         - CPU raises SIGSEGV
         - Result: SEGMENTATION FAULT

Why FREE_TO_SS=1 Makes It Worse

Without FREE_TO_SS:

// Normal path avoids explicit SS lookup in some cases
// Fast path uses TLS freelist directly
// Reduces window for TOCTOU race

With FREE_TO_SS=1:

// Explicitly calls hak_super_lookup() at:
//   hakmem.c:924 (outer entry)
//   hakmem.c:969 (inner entry)
//   hakmem_tiny_free.inc:1471, 1494, 1518, 1532, 1556
// 
// Each lookup is a potential TOCTOU window
// Increases probability of race condition

The Fix

Option A: Re-check magic in hak_tiny_free_superslab()

// In hakmem_tiny_free_superslab(), add at entry:

static inline void hak_tiny_free_superslab(void* ptr, SuperSlab* ss) {
    ROUTE_MARK(16);
    
    // ADDED: Re-check magic to catch TOCTOU races
    // If ss was unmapped since lookup, this access may SEGV, but
    // we know it's due to TOCTOU, not corruption
    if (__builtin_expect(ss->magic != SUPERSLAB_MAGIC, 0)) {
        // SuperSlab was freed/unmapped after lookup
        tiny_debug_ring_record(TINY_RING_EVENT_REMOTE_INVALID,
                              (uint16_t)0xTOCTOU,
                              ptr,
                              (uintptr_t)ss);
        if (g_tiny_safe_free_strict) { raise(SIGUSR2); }
        return;  // Early exit
    }
    
    // Continue with normal processing...
    int slab_idx = slab_index_for(ss, ptr);
    // ...
}

Option B: Use refcount to prevent munmap during free

// In hak_super_lookup():

static inline SuperSlab* hak_super_lookup(void* ptr) {
    // ... existing code ...
    
    if (b == base && e->lg_size == lg) {
        SuperSlab* ss = atomic_load_explicit(&e->ss, memory_order_acquire);
        if (!ss) return NULL;
        
        if (ss->magic != SUPERSLAB_MAGIC) return NULL;
        
        // ADDED: Increment refcount before returning
        // This prevents hak_super_unregister() from calling munmap()
        atomic_fetch_add_explicit(&ss->refcount, 1, memory_order_acq_rel);
        
        return ss;
    }
    
    // ...
}

Then in free path:

// After hak_tiny_free_superslab() completes:
if (ss) {
    atomic_fetch_sub_explicit(&ss->refcount, 1, memory_order_release);
}

---

Part 3: Bug #3 - Integer Overflow in lg_size

The Vulnerability

Location: hakmem_tiny_free.inc:1165

Current Code

size_t ss_size = (size_t)1ULL << ss->lg_size;  // Line 1165

The Problem

Assumptions:

• ss->lg_size should be 20 (1MB) or 21 (2MB)
• But no validation before use

Undefined Behavior:

// Valid cases:
1ULL << 20  // = 1,048,576 (1MB) ✓
1ULL << 21  // = 2,097,152 (2MB) ✓

// Invalid cases (undefined behavior):
1ULL << 22  // Undefined (shift amount too large)
1ULL << 64  // Undefined (shift amount >= type width)
1ULL << 255 // Undefined (massive shift)

// Typical results:
1ULL << 64  → 0 or 1 (depends on CPU)
1ULL << 100 → Undefined (compiler may optimize away, corrupt, etc.)

Reproduction

SuperSlab corrupted_ss;
corrupted_ss.lg_size = 100;  // Corrupted

// In hak_tiny_free_superslab():
size_t ss_size = (size_t)1ULL << corrupted_ss.lg_size;
// ss_size = undefined (could be 0, 1, or garbage)

// Next line uses ss_size:
uintptr_t aux = tiny_remote_pack_diag(0xBAD1u, ss_base, ss_size, (uintptr_t)ptr);
// If ss_size = 0, diag packing is wrong
// Could lead to corrupted debug info or SEGV

The Fix

// In hak_tiny_free_superslab.inc:1160-1172

static inline void hak_tiny_free_superslab(void* ptr, SuperSlab* ss) {
    ROUTE_MARK(16);
    HAK_DBG_INC(g_superslab_free_count);
    
    // ADDED: Validate lg_size before use
    if (__builtin_expect(ss->lg_size < 20 || ss->lg_size > 21, 0)) {
        uintptr_t bad_base = (uintptr_t)ss;
        size_t bad_size = 0;  // Safe default
        uintptr_t aux = tiny_remote_pack_diag(0xBAD_LGSIZE | ss->lg_size,
                                              bad_base, bad_size, (uintptr_t)ptr);
        tiny_debug_ring_record(TINY_RING_EVENT_REMOTE_INVALID,
                              (uint16_t)(0xB000 | ss->size_class),
                              ptr, aux);
        if (g_tiny_safe_free_strict) { raise(SIGUSR2); }
        return;
    }
    
    // NOW safe to use ss->lg_size
    int slab_idx = slab_index_for(ss, ptr);
    size_t ss_size = (size_t)1ULL << ss->lg_size;
    // ... continue ...
}

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Part 4: Integration of All Fixes

Recommended Implementation Order

Step 1: Apply Priority 1 Fix (size_class validation)

• Location: hakmem_tiny_free.inc:1554-1566
• Risk: Very low (only adds bounds checks)
• Benefit: Blocks 85% of SEGV cases

Step 2: Apply Priority 2 Fix (TOCTOU re-check)

• Location: hakmem_tiny_free_superslab.inc:1160
• Risk: Very low (defensive check only)
• Benefit: Blocks TOCTOU races

Step 3: Apply Priority 3 Fix (lg_size validation)

• Location: hakmem_tiny_free_superslab.inc:1165
• Risk: Very low (validation before use)
• Benefit: Blocks integer overflow

Step 4: Add comprehensive entry validation

• Location: hakmem.c:924-932, 969-976
• Risk: Low (early rejection of bad pointers)
• Benefit: Defense-in-depth

Complete Patch Strategy

# Apply in this order:
1. git apply fix-1-size-class-validation.patch
2. git apply fix-2-toctou-recheck.patch
3. git apply fix-3-lgsize-validation.patch
4. make clean && make box-refactor  # Rebuild
5. Run test suite with HAKMEM_TINY_FREE_TO_SS=1

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Part 5: Testing Strategy

Unit Tests

// Test 1: Corrupted size_class
TEST(FREE_TO_SS, CorruptedSizeClass) {
    SuperSlab corrupted;
    corrupted.magic = SUPERSLAB_MAGIC;
    corrupted.size_class = 255;  // Out of bounds
    
    void* ptr = test_alloc(64);
    // Register corrupted SS in registry
    // Call free(ptr) with FREE_TO_SS=1
    // Expect: No SEGV, proper error logging
    ASSERT_NE(get_last_error_code(), 0);
}

// Test 2: Corrupted lg_size
TEST(FREE_TO_SS, CorruptedLgSize) {
    SuperSlab corrupted;
    corrupted.magic = SUPERSLAB_MAGIC;
    corrupted.size_class = 4;     // Valid
    corrupted.lg_size = 100;      // Out of bounds
    
    void* ptr = test_alloc(128);
    // Register corrupted SS in registry
    // Call free(ptr) with FREE_TO_SS=1
    // Expect: No SEGV, proper error logging
    ASSERT_NE(get_last_error_code(), 0);
}

// Test 3: TOCTOU Race
TEST(FREE_TO_SS, TOCTOURace) {
    std::thread alloc_thread([]() {
        void* ptr = test_alloc(256);
        std::this_thread::sleep_for(std::chrono::milliseconds(100));
        free(ptr);
    });
    
    std::thread free_thread([]() {
        std::this_thread::sleep_for(std::chrono::milliseconds(50));
        // Unregister all SuperSlabs (simulates race)
        hak_super_unregister_all();
    });
    
    alloc_thread.join();
    free_thread.join();
    // Expect: No crash, proper error handling
}

Integration Tests

# Test with Larson benchmark
make box-refactor
HAKMEM_TINY_FREE_TO_SS=1 HAKMEM_TINY_SAFE_FREE=1 ./larson_hakmem 2 8 128 1024 1 12345 4
# Expected: No SEGV, reasonable performance

# Test with stress test
HAKMEM_TINY_FREE_TO_SS=1 HAKMEM_TINY_SAFE_FREE=1 ./bench_comprehensive_hakmem
# Expected: All tests pass

---

Conclusion

The FREE_TO_SS=1 SEGV bug is caused by missing validation of SuperSlab metadata fields. The fixes are straightforward bounds checks on size_class and lg_size, with optional TOCTOU mitigation via re-checking magic.

Implementing all three fixes provides defense-in-depth against:

1. Memory corruption
2. TOCTOU races
3. Integer overflows

Total effort: < 50 lines of code Risk level: Very low Benefit: Eliminates critical SEGV path