Re: [PATCH v2] mm: Add SLUB free list pointer obfuscation

[Andrew Morton]

On Thu, 22 Jun 2017 18:50:10 -0700 Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> 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 pointers. 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
> 
> This is based on patches by Daniel Micay, and refactored to avoid lots
> of #ifdef code.
> 
> ...
>
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1900,6 +1900,15 @@ 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.
> +

Well, it is optable-outable.

>  config SLUB_CPU_PARTIAL
>  	default y
>  	depends on SLUB && SMP
> diff --git a/mm/slub.c b/mm/slub.c
> index 57e5156f02be..590e7830aaed 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -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 +239,50 @@ static inline void stat(const struct kmem_cache *s, enum stat_item si)
>   * 			Core slab cache functions
>   *******************************************************************/
>  
> +#ifdef CONFIG_SLAB_FREELIST_HARDENED
> +# define initialize_random(s)					\
> +		do {						\
> +			s->random = get_random_long();		\
> +		} while (0)
> +# define FREEPTR_VAL(ptr, ptr_addr, s)	\
> +		(void *)((unsigned long)(ptr) ^ s->random ^ (ptr_addr))
> +#else
> +# define initialize_random(s)		do { } while (0)
> +# define FREEPTR_VAL(ptr, addr, s)	((void *)(ptr))
> +#endif
> +#define FREELIST_ENTRY(ptr_addr, s)				\
> +		FREEPTR_VAL(*(unsigned long *)(ptr_addr),	\
> +			    (unsigned long)ptr_addr, s)
> +

That's a bit of an eyesore.  Is there any reason why we cannot
implement all of the above in nice, conventional C functions?

>
> ...
>
> @@ -3536,6 +3557,7 @@ 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;
> +	initialize_random(s);
>  
>  	if (need_reserve_slab_rcu && (s->flags & SLAB_TYPESAFE_BY_RCU))
>  		s->reserved = sizeof(struct rcu_head);

We regularly have issues where the random system just isn't ready
(enough) for clients to use it.  Are you sure the above is actually
useful for the boot-time caches?