From 2482ddec670fb83717d129012bc558777cb159f7 Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Wed, 6 Sep 2017 16:19:18 -0700 Subject: [PATCH] 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 code. 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: http://cyseclabs.com/blog/cve-2016-6187-heap-off-by-one-exploit 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: before: mean 10.11882499999999999995 variance .03320378329145728642 stdev .18221905304181911048 after: 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. Link: http://lkml.kernel.org/r/20170802180609.GA66807@beast Signed-off-by: Kees Cook Suggested-by: Daniel 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: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/slub_def.h | 4 ++++ init/Kconfig | 9 +++++++++ mm/slub.c | 42 +++++++++++++++++++++++++++++++++++----- 3 files changed, 50 insertions(+), 5 deletions(-) diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h index cc0faf3a90be..0783b622311e 100644 --- a/include/linux/slub_def.h +++ b/include/linux/slub_def.h @@ -115,6 +115,10 @@ struct kmem_cache { #endif #endif +#ifdef CONFIG_SLAB_FREELIST_HARDENED + unsigned long random; +#endif + #ifdef CONFIG_NUMA /* * Defragmentation by allocating from a remote node. diff --git a/init/Kconfig b/init/Kconfig index 5f0ef850e808..78cb2461012e 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1576,6 +1576,15 @@ config SLAB_FREELIST_RANDOM security feature reduces the predictability of the kernel slab allocator against heap overflows. +config SLAB_FREELIST_HARDENED + bool "Harden slab freelist metadata" + depends on SLUB + help + 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. + config SLUB_CPU_PARTIAL default y depends on SLUB && SMP diff --git a/mm/slub.c b/mm/slub.c index 3e90d791dd41..6c87c2c6af24 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -34,6 +34,7 @@ #include #include #include +#include #include @@ -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) +{ +#ifdef CONFIG_SLAB_FREELIST_HARDENED + return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr); +#else + return ptr; +#endif +} + +/* 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; +#ifdef CONFIG_SLAB_FREELIST_HARDENED + s->random = get_random_long(); +#endif if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU)) s->reserved = sizeof(struct rcu_head);