[kernel-hardening] [RFC v1] mm: SLAB freelist randomization

[kernel-hardening] [RFC v1] mm: SLAB freelist randomization

[Thomas Garnier]

Provide an optional config (CONFIG_FREELIST_RANDOM) to randomize the
SLAB freelist. This security feature reduces the predictability of
the kernel slab allocator against heap overflows.

Randomized lists are pre-computed using a Fisher-Yates shuffle and
re-used on slab creation for performance.
---
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
+	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]);
+		}
+	}
+}
+#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 */
+
 	/* 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);
+}
+#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 */
 	}
+
+#ifdef CONFIG_FREELIST_RANDOM
+	shuffle_freelist(cachep, page, cachep->num);
+#endif /* CONFIG_FREELIST_RANDOM */
 }
 
 static void kmem_flagcheck(struct kmem_cache *cachep, gfp_t flags)
-- 
2.8.0.rc3.226.g39d4020

Re: [kernel-hardening] [RFC v1] mm: SLAB freelist randomization

[Greg KH]

On Wed, Apr 06, 2016 at 12:35:48PM -0700, Thomas Garnier wrote:
> Provide an optional config (CONFIG_FREELIST_RANDOM) to randomize the
> SLAB freelist. This security feature reduces the predictability of
> the kernel slab allocator against heap overflows.
> 
> Randomized lists are pre-computed using a Fisher-Yates shuffle and
> re-used on slab creation for performance.
> ---
> Based on next-20160405
> ---

No signed-off-by:?

Re: [kernel-hardening] [RFC v1] mm: SLAB freelist randomization

[Thomas Garnier]

Yes, sorry about that. It will be in the next RFC or PATCH.

On Wed, Apr 6, 2016 at 1:54 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
> On Wed, Apr 06, 2016 at 12:35:48PM -0700, Thomas Garnier wrote:
>> Provide an optional config (CONFIG_FREELIST_RANDOM) to randomize the
>> SLAB freelist. This security feature reduces the predictability of
>> the kernel slab allocator against heap overflows.
>>
>> Randomized lists are pre-computed using a Fisher-Yates shuffle and
>> re-used on slab creation for performance.
>> ---
>> Based on next-20160405
>> ---
>
> No signed-off-by:?
>

[kernel-hardening] Re: [RFC v1] mm: SLAB freelist randomization

[Kees Cook]

On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <thgarnie@google.com> 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

[kernel-hardening] 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@chromium.org> wrote:
>
> On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <thgarnie@google.com> 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

Re: [kernel-hardening] Re: [RFC v1] mm: SLAB freelist randomization

[Yves-Alexis Perez]

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On mer., 2016-04-06 at 14:45 -0700, Kees Cook wrote:
> > 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.

One good example might (or might not) be the keyring issue from earlier this
year (CVE-2016-0728):

http://perception-point.io/2016/01/14/analysis-and-exploitation-of-a-linux-ker
nel-vulnerability-cve-2016-0728/

Regards,
-- 
Yves-Alexis


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Re: [kernel-hardening] Re: [RFC v1] mm: SLAB freelist randomization

[Thomas Garnier]

That's a use after free. The randomization of the freelist should not
have much effect on that. I was going to quote this exploit that is
applicable to SLAB as well:
https://jon.oberheide.org/blog/2010/09/10/linux-kernel-can-slub-overflow

Regards.
Thomas

On Thu, Apr 7, 2016 at 9:17 AM, Yves-Alexis Perez <corsac@debian.org> wrote:
> On mer., 2016-04-06 at 14:45 -0700, Kees Cook wrote:
>> > 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.
>
> One good example might (or might not) be the keyring issue from earlier this
> year (CVE-2016-0728):
>
> http://perception-point.io/2016/01/14/analysis-and-exploitation-of-a-linux-ker
> nel-vulnerability-cve-2016-0728/
>
> Regards,
> --
> Yves-Alexis
>

[kernel-hardening] Re: [RFC v1] mm: SLAB freelist randomization

[Jesper Dangaard Brouer]

On Wed, 6 Apr 2016 14:45:30 -0700 Kees Cook <keescook@chromium.org> wrote:

> On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <thgarnie@google.com> wrote:
[...]
> > 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.
> 

It looks like it only happens on init, right? (Thus must bench tools
might not be the right choice).

My slab tools for benchmarking the fastpath is here:
 https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/slab_bulk_test01.c

And I also carry a version of Christoph's slab bench tool:
 https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/slab_test.c

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer

[kernel-hardening] Re: [RFC v1] mm: SLAB freelist randomization

[Kees Cook]

On Thu, Apr 7, 2016 at 2:14 PM, Jesper Dangaard Brouer
<brouer@redhat.com> wrote:
>
> On Wed, 6 Apr 2016 14:45:30 -0700 Kees Cook <keescook@chromium.org> wrote:
>
>> On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <thgarnie@google.com> wrote:
> [...]
>> > 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.
>>
>
> It looks like it only happens on init, right? (Thus must bench tools
> might not be the right choice).

Oh! Yes, you're right. I entirely missed that detail. :) 0-cost
randomization! Sounds good to me. :)

-Kees

>
> My slab tools for benchmarking the fastpath is here:
>  https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/slab_bulk_test01.c
>
> And I also carry a version of Christoph's slab bench tool:
>  https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/mm/slab_test.c
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   Author of http://www.iptv-analyzer.org
>   LinkedIn: http://www.linkedin.com/in/brouer



-- 
Kees Cook
Chrome OS & Brillo Security