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
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:?
Yes, sorry about that. It will be in the next RFC or PATCH.
On Wed, Apr 6, 2016 at 1:54 PM, Greg KH <[email protected]> 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:?
>
On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <[email protected]> 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
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 <[email protected]> wrote:
>
> On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <[email protected]> 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
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
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 <[email protected]> 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
>
On Wed, 6 Apr 2016 14:45:30 -0700 Kees Cook <[email protected]> wrote:
> On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <[email protected]> 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
On Thu, Apr 7, 2016 at 2:14 PM, Jesper Dangaard Brouer
<[email protected]> wrote:
>
> On Wed, 6 Apr 2016 14:45:30 -0700 Kees Cook <[email protected]> wrote:
>
>> On Wed, Apr 6, 2016 at 12:35 PM, Thomas Garnier <[email protected]> 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