Re: [RFC v1] mm: SLAB freelist randomization

[Thomas Garnier]

Thanks for the feedback Kees. I am preparing another RFC version.

For the config, I plan on creating an equivalent option for SLUB. Both
can benefit from randomizing their freelist order.

Thomas

On Wed, Apr 6, 2016 at 2:45 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
> > Provide an optional config (CONFIG_FREELIST_RANDOM) to randomize the
> > SLAB freelist.
>
> It may be useful to describe _how_ it randomizes it (i.e. a high-level
> description of what needed changing).
>
> > This security feature reduces the predictability of
> > the kernel slab allocator against heap overflows.
>
> I would add "... rendering attacks much less stable." And if you can
> find a specific example exploit that is foiled by this, I would refer
> to it.
>
> > Randomized lists are pre-computed using a Fisher-Yates shuffle and
>
> Should the use of Fisher-Yates (over other things) be justified?
>
> > re-used on slab creation for performance.
>
> I'd like to see some benchmark results for this so the Kconfig can
> include the performance characteristics. I recommend using hackbench
> and kernel build times with a before/after comparison.
>
> > ---
> > Based on next-20160405
> > ---
> >  init/Kconfig |   9 ++++
> >  mm/slab.c    | 155 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 164 insertions(+)
> >
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 0dfd09d..ee35418 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1742,6 +1742,15 @@ config SLOB
> >
> >  endchoice
> >
> > +config FREELIST_RANDOM
>
> I think I would name this "SLAB_FREELIST_RANDOM" since it's
> SLAB-specific, unless you think it could be extended to the other
> allocators in the future too? (If so, I'd mention the naming choice in
> the commit log.)
>
> > +       default n
> > +       depends on SLAB
> > +       bool "SLAB freelist randomization"
> > +       help
> > +         Randomizes the freelist order used on creating new SLABs. This
> > +         security feature reduces the predictability of the kernel slab
> > +         allocator against heap overflows.
> > +
> >  config SLUB_CPU_PARTIAL
> >         default y
> >         depends on SLUB && SMP
> > diff --git a/mm/slab.c b/mm/slab.c
> > index b70aabf..6f0d7be 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -1229,6 +1229,59 @@ static void __init set_up_node(struct kmem_cache *cachep, int index)
> >         }
> >  }
> >
> > +#ifdef CONFIG_FREELIST_RANDOM
> > +/*
> > + * Master lists are pre-computed random lists
> > + * Lists of different sizes are used to optimize performance on different
> > + * SLAB object sizes per pages.
> > + */
> > +static freelist_idx_t master_list_2[2];
> > +static freelist_idx_t master_list_4[4];
> > +static freelist_idx_t master_list_8[8];
> > +static freelist_idx_t master_list_16[16];
> > +static freelist_idx_t master_list_32[32];
> > +static freelist_idx_t master_list_64[64];
> > +static freelist_idx_t master_list_128[128];
> > +static freelist_idx_t master_list_256[256];
> > +static struct m_list {
> > +       size_t count;
> > +       freelist_idx_t *list;
> > +} master_lists[] = {
> > +       { ARRAY_SIZE(master_list_2), master_list_2 },
> > +       { ARRAY_SIZE(master_list_4), master_list_4 },
> > +       { ARRAY_SIZE(master_list_8), master_list_8 },
> > +       { ARRAY_SIZE(master_list_16), master_list_16 },
> > +       { ARRAY_SIZE(master_list_32), master_list_32 },
> > +       { ARRAY_SIZE(master_list_64), master_list_64 },
> > +       { ARRAY_SIZE(master_list_128), master_list_128 },
> > +       { ARRAY_SIZE(master_list_256), master_list_256 },
> > +};
> > +
> > +void __init freelist_random_init(void)
> > +{
> > +       unsigned int seed;
> > +       size_t z, i, rand;
> > +       struct rnd_state slab_rand;
> > +
> > +       get_random_bytes_arch(&seed, sizeof(seed));
> > +       prandom_seed_state(&slab_rand, seed);
> > +
> > +       for (z = 0; z < ARRAY_SIZE(master_lists); z++) {
> > +               for (i = 0; i < master_lists[z].count; i++)
> > +                       master_lists[z].list[i] = i;
> > +
> > +               /* Fisher-Yates shuffle */
> > +               for (i = master_lists[z].count - 1; i > 0; i--) {
> > +                       rand = prandom_u32_state(&slab_rand);
> > +                       rand %= (i + 1);
> > +                       swap(master_lists[z].list[i],
> > +                               master_lists[z].list[rand]);
> > +               }
> > +       }
> > +}
>
> For below...
>
> #else
> static inline freelist_random_init(void) { }
>
> > +#endif /* CONFIG_FREELIST_RANDOM */
> > +
> > +
> >  /*
> >   * Initialisation.  Called after the page allocator have been initialised and
> >   * before smp_init().
> > @@ -1255,6 +1308,10 @@ void __init kmem_cache_init(void)
> >         if (!slab_max_order_set && totalram_pages > (32 << 20) >> PAGE_SHIFT)
> >                 slab_max_order = SLAB_MAX_ORDER_HI;
> >
> > +#ifdef CONFIG_FREELIST_RANDOM
> > +       freelist_random_init();
> > +#endif /* CONFIG_FREELIST_RANDOM */
>
> Rather than these embedded ifdefs, I would create stub function at the
> top, as above.
>
> > +
> >         /* Bootstrap is tricky, because several objects are allocated
> >          * from caches that do not exist yet:
> >          * 1) initialize the kmem_cache cache: it contains the struct
> > @@ -2442,6 +2499,98 @@ static void cache_init_objs_debug(struct kmem_cache *cachep, struct page *page)
> >  #endif
> >  }
> >
> > +#ifdef CONFIG_FREELIST_RANDOM
> > +enum master_type {
> > +       match,
> > +       less,
> > +       more
> > +};
> > +
> > +struct random_mng {
> > +       unsigned int padding;
> > +       unsigned int pos;
> > +       unsigned int count;
> > +       struct m_list master_list;
> > +       unsigned int master_count;
> > +       enum master_type type;
> > +};
> > +
> > +static void random_mng_initialize(struct random_mng *mng, unsigned int count)
> > +{
> > +       unsigned int idx;
> > +       const unsigned int last_idx = ARRAY_SIZE(master_lists) - 1;
> > +
> > +       memset(mng, 0, sizeof(*mng));
> > +       mng->count = count;
> > +       mng->pos = 0;
> > +       /* count is >= 2 */
> > +       idx = ilog2(count) - 1;
> > +       if (idx >= last_idx)
> > +               idx = last_idx;
> > +       else if (roundup_pow_of_two(idx + 1) != count)
> > +               idx++;
> > +       mng->master_list = master_lists[idx];
> > +       if (mng->master_list.count == mng->count)
> > +               mng->type = match;
> > +       else if (mng->master_list.count > mng->count)
> > +               mng->type = more;
> > +       else
> > +               mng->type = less;
> > +}
> > +
> > +static freelist_idx_t get_next_entry(struct random_mng *mng)
> > +{
> > +       if (mng->type == less && mng->pos == mng->master_list.count) {
> > +               mng->padding += mng->pos;
> > +               mng->pos = 0;
> > +       }
> > +       BUG_ON(mng->pos >= mng->master_list.count);
> > +       return mng->master_list.list[mng->pos++];
> > +}
> > +
> > +static freelist_idx_t next_random_slot(struct random_mng *mng)
> > +{
> > +       freelist_idx_t cur, entry;
> > +
> > +       entry = get_next_entry(mng);
> > +
> > +       if (mng->type != match) {
> > +               while ((entry + mng->padding) >= mng->count)
> > +                       entry = get_next_entry(mng);
> > +               cur = entry + mng->padding;
> > +               BUG_ON(cur >= mng->count);
> > +       } else {
> > +               cur = entry;
> > +       }
> > +
> > +       return cur;
> > +}
> > +
> > +static void shuffle_freelist(struct kmem_cache *cachep, struct page *page,
> > +                            unsigned int count)
> > +{
> > +       unsigned int i;
> > +       struct random_mng mng;
> > +
> > +       if (count < 2) {
> > +               for (i = 0; i < count; i++)
> > +                       set_free_obj(page, i, i);
> > +               return;
> > +       }
> > +
> > +       /* Last chunk is used already in this case */
> > +       if (OBJFREELIST_SLAB(cachep))
> > +               count--;
> > +
> > +       random_mng_initialize(&mng, count);
> > +       for (i = 0; i < count; i++)
> > +               set_free_obj(page, i, next_random_slot(&mng));
> > +
> > +       if (OBJFREELIST_SLAB(cachep))
> > +               set_free_obj(page, i, i);
> > +}
>
> Same thing here...
>
> #else
> static inline void set_free_obj(...) { }
> static inline void shuffle_freelist(struct kmem_cache *cachep,
>                               struct page *page, unsigned int count) { }
>
> > +#endif /* CONFIG_FREELIST_RANDOM */
> > +
> >  static void cache_init_objs(struct kmem_cache *cachep,
> >                             struct page *page)
> >  {
> > @@ -2464,8 +2613,14 @@ static void cache_init_objs(struct kmem_cache *cachep,
> >                         kasan_poison_object_data(cachep, objp);
> >                 }
> >
> > +#ifndef CONFIG_FREELIST_RANDOM
> >                 set_free_obj(page, i, i);
> > +#endif /* CONFIG_FREELIST_RANDOM */
>
> For this one, I'd use:
>
>                    if (config_enabled(CONFIG_FREELIST_RANDOM))
>                            set_free_obj(page, i, i);
>
> >         }
> > +
> > +#ifdef CONFIG_FREELIST_RANDOM
> > +       shuffle_freelist(cachep, page, cachep->num);
> > +#endif /* CONFIG_FREELIST_RANDOM */
>
> This one can drop the ifdef in favor of using the stub function too.
>
> >  }
> >
> >  static void kmem_flagcheck(struct kmem_cache *cachep, gfp_t flags)
> > --
> > 2.8.0.rc3.226.g39d4020
> >
>
> Exciting!
>
> -Kees
>
> --
> Kees Cook
> Chrome OS & Brillo Security