Commit 2482ddec authored by Kees Cook's avatar Kees Cook Committed by Linus Torvalds
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mm: add SLUB free list pointer obfuscation

This SLUB free list pointer obfuscation code is modified from Brad
Spengler/PaX Team's code in the last public patch of grsecurity/PaX
based on my understanding of the code.  Changes or omissions from the
original code are mine and don't reflect the original grsecurity/PaX

This adds a per-cache random value to SLUB caches that is XORed with
their freelist pointer address and value.  This adds nearly zero
overhead and frustrates the very common heap overflow exploitation
method of overwriting freelist pointers.

A recent example of the attack is written up here:

and there is a section dedicated to the technique the book "A Guide to
Kernel Exploitation: Attacking the Core".

This is based on patches by Daniel Micay, and refactored to minimize the
use of #ifdef.

With 200-count cycles of "hackbench -g 20 -l 1000" I saw the following
run times:

 	mean 10.11882499999999999995
	variance .03320378329145728642
	stdev .18221905304181911048

	mean 10.12654000000000000014
	variance .04700556623115577889
	stdev .21680767106160192064

The difference gets lost in the noise, but if the above is to be taken
literally, using CONFIG_FREELIST_HARDENED is 0.07% slower.


Signed-off-by: default avatarKees Cook <>
Suggested-by: default avatarDaniel Micay <>
Cc: Rik van Riel <>
Cc: Tycho Andersen <>
Cc: Alexander Popov <>
Cc: Christoph Lameter <>
Cc: Pekka Enberg <>
Cc: David Rientjes <>
Cc: Joonsoo Kim <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent ea37df54
......@@ -115,6 +115,10 @@ struct kmem_cache {
unsigned long random;
* Defragmentation by allocating from a remote node.
......@@ -1576,6 +1576,15 @@ config SLAB_FREELIST_RANDOM
security feature reduces the predictability of the kernel slab
allocator against heap overflows.
bool "Harden slab freelist metadata"
depends on SLUB
Many kernel heap attacks try to target slab cache metadata and
other infrastructure. This options makes minor performance
sacrifies to harden the kernel slab allocator against common
freelist exploit methods.
default y
depends on SLUB && SMP
......@@ -34,6 +34,7 @@
#include <linux/stacktrace.h>
#include <linux/prefetch.h>
#include <linux/memcontrol.h>
#include <linux/random.h>
#include <trace/events/kmem.h>
......@@ -238,30 +239,58 @@ static inline void stat(const struct kmem_cache *s, enum stat_item si)
* Core slab cache functions
* Returns freelist pointer (ptr). With hardening, this is obfuscated
* with an XOR of the address where the pointer is held and a per-cache
* random number.
static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
unsigned long ptr_addr)
return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);
return ptr;
/* Returns the freelist pointer recorded at location ptr_addr. */
static inline void *freelist_dereference(const struct kmem_cache *s,
void *ptr_addr)
return freelist_ptr(s, (void *)*(unsigned long *)(ptr_addr),
(unsigned long)ptr_addr);
static inline void *get_freepointer(struct kmem_cache *s, void *object)
return *(void **)(object + s->offset);
return freelist_dereference(s, object + s->offset);
static void prefetch_freepointer(const struct kmem_cache *s, void *object)
prefetch(object + s->offset);
if (object)
prefetch(freelist_dereference(s, object + s->offset));
static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
unsigned long freepointer_addr;
void *p;
if (!debug_pagealloc_enabled())
return get_freepointer(s, object);
probe_kernel_read(&p, (void **)(object + s->offset), sizeof(p));
return p;
freepointer_addr = (unsigned long)object + s->offset;
probe_kernel_read(&p, (void **)freepointer_addr, sizeof(p));
return freelist_ptr(s, p, freepointer_addr);
static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)
*(void **)(object + s->offset) = fp;
unsigned long freeptr_addr = (unsigned long)object + s->offset;
*(void **)freeptr_addr = freelist_ptr(s, fp, freeptr_addr);
/* Loop over all objects in a slab */
......@@ -3563,6 +3592,9 @@ static int kmem_cache_open(struct kmem_cache *s, unsigned long flags)
s->flags = kmem_cache_flags(s->size, flags, s->name, s->ctor);
s->reserved = 0;
s->random = get_random_long();
if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU))
s->reserved = sizeof(struct rcu_head);
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