On Sat, Apr 15, 2023 at 08:36:32PM +0300, david.keisarschm@xxxxxxxxxxxxxxx wrote:
> From: David Keisar Schmidt <david.keisarschm@xxxxxxxxxxxxxxx>
>
> The Slab allocator randomization uses the prandom_u32
> PRNG. That was added to prevent attackers to obtain information on the heap
> state, by randomizing the freelists state.
>
> However, this PRNG turned out to be weak, as noted in commit c51f8f88d705
> To fix it, we have changed the invocation of prandom_u32_state to get_random_u32
> to ensure the PRNG is strong. Since a modulo operation is applied right after that,
> we used get_random_u32_below, to achieve uniformity.
>
> In addition, we changed the freelist_init_state union to struct,
> since the rnd_state inside which is used to store the state of prandom_u32,
> is not needed anymore, since get_random_u32 maintains its own state.
makes sense to me, but some nits:
I think the subject should start with "mm/slab:" for this patch
and "mm/slab_common:" for the next patch.
>
> Signed-off-by: David Keisar Schmidt <david.keisarschm@xxxxxxxxxxxxxxx>
> ---
> This fifth series changes only the arch/x86/mm/kaslr patch.
>
> Changes since v3:
>
> * edited commit message.
>
> Changes since v2:
>
> * replaced instances of get_random_u32 with get_random_u32_below
> in mm/slab.c.
>
> mm/slab.c | 25 ++++++++++---------------
> 1 file changed, 10 insertions(+), 15 deletions(-)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index edbe722fb..7c8bb4a8f 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -2360,20 +2360,17 @@ static void cache_init_objs_debug(struct kmem_cache *cachep, struct slab *slab)
>
> #ifdef CONFIG_SLAB_FREELIST_RANDOM
> /* Hold information during a freelist initialization */
> -union freelist_init_state {
> - struct {
> - unsigned int pos;
> - unsigned int *list;
> - unsigned int count;
> - };
> - struct rnd_state rnd_state;
> +struct freelist_init_state {
> + unsigned int pos;
> + unsigned int *list;
> + unsigned int count;
> };
>
> /*
> * Initialize the state based on the randomization method available.
> * return true if the pre-computed list is available, false otherwise.
> */
> -static bool freelist_state_initialize(union freelist_init_state *state,
> +static bool freelist_state_initialize(struct freelist_init_state *state,
> struct kmem_cache *cachep,
> unsigned int count)
> {
> @@ -2381,23 +2378,22 @@ static bool freelist_state_initialize(union freelist_init_state *state,
> unsigned int rand;
>
> /* Use best entropy available to define a random shift */
> - rand = get_random_u32();
> + rand = get_random_u32_below(count);
>
> /* Use a random state if the pre-computed list is not available */
This comment should be removed too.
> if (!cachep->random_seq) {
> - prandom_seed_state(&state->rnd_state, rand);
> ret = false;
> } else {
> state->list = cachep->random_seq;
> state->count = count;
> - state->pos = rand % count;
> + state->pos = rand;
this could be:
state->pos = get_random_u32_below(count);
without defining the variable rand.
> ret = true;
> }
> return ret;
> }
>
> /* Get the next entry on the list and randomize it using a random shift */
> -static freelist_idx_t next_random_slot(union freelist_init_state *state)
> +static freelist_idx_t next_random_slot(struct freelist_init_state *state)
> {
> if (state->pos >= state->count)
> state->pos = 0;
> @@ -2418,7 +2414,7 @@ static void swap_free_obj(struct slab *slab, unsigned int a, unsigned int b)
> static bool shuffle_freelist(struct kmem_cache *cachep, struct slab *slab)
> {
> unsigned int objfreelist = 0, i, rand, count = cachep->num;
> - union freelist_init_state state;
> + struct freelist_init_state state;
> bool precomputed;
>
> if (count < 2)
> @@ -2447,8 +2443,7 @@ static bool shuffle_freelist(struct kmem_cache *cachep, struct slab *slab)
>
> /* Fisher-Yates shuffle */
> for (i = count - 1; i > 0; i--) {
> - rand = prandom_u32_state(&state.rnd_state);
> - rand %= (i + 1);
> + rand = get_random_u32_below(i+1);
per the coding standard this should be
rand = get_random_u32_below(i +1);
> swap_free_obj(slab, i, rand);
> }
> } else {
> --
> 2.37.3
>
