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

[Kees Cook]

On Wed, Jul 5, 2017 at 4:39 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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?

I could rework it using static inlines. I was mainly avoiding #ifdef
blocks in the freelist manipulation functions, but I could push them
up to these functions instead. I'll send a v2.

>> ...
>>
>> @@ -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?

IMO, this isn't reason enough to block this since we have similar
problems in other places (e.g. the stack canary itself). The random
infrastructure is aware of these problems and is continually improving
(e.g. on x86 without enough pool entropy, this will fall back to
RDRAND, IIUC). Additionally for systems that are chronically short on
entropy they can build with the latent_entropy GCC plugin to at least
bump the seeding around a bit. So, it _can_ be low entropy, but adding
this still improves the situation since the address of the freelist
pointer itself is part of the XORing, so even if the random number was
static, it would still require info exposures to work around the
obfuscation.

Shorter version: yeah, it'll still be useful. :)

-Kees

-- 
Kees Cook
Pixel Security