How does the Linux SLUB allocator organize kernel heap memory?

The Linux kernel manages object allocations (via kmalloc and dedicated caches like task_struct or cred) using the SLUB allocator (the default slab allocator since Linux 2.6.23, superseding legacy SLAB and SLOB). SLUB organizes memory into three caching tiers: 1) Per-CPU Cache (kmem_cache_cpu): Allows CPU cores to allocate and free objects locklessly via per-CPU pointers (freelist); 2) Per-CPU Partial Lists: Pre-allocated slabs kept warm on the local CPU; 3) Per-Node Partial Lists (kmem_cache_node): Slabs shared across cores within a NUMA node protected by spinlocks. If all partial slabs are exhausted, SLUB falls back to the buddy page allocator (alloc_pages).

What is freelist pointer mangling (CONFIG_SLAB_FREELIST_HARDENED) and how does it stop heap attacks?

In a standard freelist, each unallocated object stores a raw 64-bit virtual memory pointer to the next free object inside its payload (usually at offset 0). In a heap overflow or use-after-free (UAF) exploit, an attacker overwrites this freelist pointer to point to arbitrary memory (e.g. credentials or function pointers), causing the next kmalloc to return an attacker-controlled target address (freelist hijacking). With CONFIG_SLAB_FREELIST_HARDENED, the kernel encrypts each freelist pointer: ptr = ptr ^ cache->random ^ swab64(addr). Because the pointer is XORed with a 64-bit boot-time secret random cookie and the object's own virtual address, blind pointer overwrites result in a corrupted address that triggers a kernel panic (BUG_ON) rather than arbitrary code execution.

How does freelist randomization (CONFIG_SLAB_FREELIST_RANDOM) defeat heap feng-shui?

When a new slab page is initialized, objects are traditionally chained into the freelist in linear order (object 0 -> object 1 -> object 2). Attackers exploit this deterministic layout using "heap feng-shui", allocating vulnerable objects adjacent to target high-value structs to guarantee reliable out-of-bounds corruption. Freelist randomization uses a Fisher-Yates shuffle at slab initialization time to scramble the order of free objects, preventing attackers from predicting relative spatial offsets between sequential allocations.

What are the trade-offs of init_on_alloc and init_on_free kernel boot parameters?

init_on_alloc=1 zeroes out memory immediately upon allocation, preventing uninitialized kernel stack/heap infoleaks. init_on_free=1 zeroes memory immediately when kfree() is called, scrubbing sensitive crypto keys or passwords and disrupting use-after-free exploits that rely on reading or hijacking stale data structures. While highly effective for defense-in-depth, enabling memory zeroing introduces a measurable CPU overhead (~2-7% depending on memory allocation churn and hardware zero-page hardware offload).

Linux Kernel Slab Allocator & Free List Hardening Studio | Heap Exploitation Mitigations

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Linux 6.x SLUB Kernel ABI Heap Security Hardening Anti-UAF • Freelist Obfuscation

Linux Kernel Slab Allocator & Free List Hardening Studio

Simulate Linux kernel SLUB heap allocation, per-CPU cache hierarchy, and modern exploit mitigations. Explore freelist pointer encryption (XOR cookies), freelist randomization, redzoning, and memory zeroing defenses against Use-After-Free (UAF).

1. Slab Cache & Object Sizing

Kernel Slab Cache (kmem_cache): Slab Order & Page Count: Allocation Allocation Path Simulation:

2. Active Security Mitigations

CONFIG_SLAB_FREELIST_HARDENED (Pointer XOR Cookie)

Encrypts freelist pointer: ptr ^ cache->random ^ swab64(addr).

CONFIG_SLAB_FREELIST_RANDOM (Fisher-Yates Shuffle)

Scrambles allocation order to prevent heap layout prediction.

init_on_alloc=1 / init_on_free=1 (Zero Memory Scrubbing)

Clears memory buffers to prevent infoleaks and stale pointer UAFs.

Hardened Freelist Active: Arbitrary pointer overwrite will panic kernel.

SLUB Cache Architecture & Object Layout

Latency: 12 ns (Fastpath)

Objects per Slab Page

32 objects

4,096 bytes (Order 0)

Object Packing Efficiency

100%

0 bytes internal waste

Exploit Difficulty Score

Very High

Cookie + Randomization

CPU Hardening Overhead

~0.5%

Negligible bitwise XOR

Production Implementation & Inspection Code

Architectural Comparison: SLUB Hardening Features

Mitigation Feature	Mechanism	Threat Mitigated	CPU Overhead
CONFIG_SLAB_FREELIST_HARDENED	ptr ^ cache->random ^ swab64(addr)	Freelist pointer hijacking, arbitrary kmalloc	< 0.5%
CONFIG_SLAB_FREELIST_RANDOM	Fisher-Yates shuffle at slab init	Heap feng-shui layout predictability	0.0% (One-time at slab creation)
init_on_alloc / init_on_free	memset(0) on kalloc and kfree	Uninitialized memory leaks & stale pointer UAF	~2 - 7%
SLAB_RED_ZONE	Canary padding bytes around object	Out-of-bounds heap buffer overwrite	~1 - 3% (Memory slack)

Linux Kernel Slab Allocator & Free List Hardening Studio

1. Slab Cache & Object Sizing

2. Active Security Mitigations

SLUB Cache Architecture & Object Layout

Production Implementation & Inspection Code

Architectural Comparison: SLUB Hardening Features

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