On Sat, Apr 15, 2023 at 08:36:49PM +0300, david.keisarschm@xxxxxxxxxxxxxxx wrote:
> From: David Keisar Schmidt <david.keisarschm@xxxxxxxxxxxxxxx>
>
> The Slab allocator randomization inside slab_common.c
> uses the prandom_u32 PRNG. That was added to prevent attackers to obtain
> information on the heap 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,
> in the Fisher-Yates shuffle, we used get_random_u32_below, to achieve uniformity.
>
> Signed-off-by: David Keisar Schmidt <david.keisarschm@xxxxxxxxxxxxxxx>
> ---
same comment for the subject line.
>
> 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_common.c.
>
> mm/slab_common.c | 11 +++--------
> 1 file changed, 3 insertions(+), 8 deletions(-)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index bf4e777cf..361da2191 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1146,7 +1146,7 @@ EXPORT_SYMBOL(kmalloc_large_node);
>
> #ifdef CONFIG_SLAB_FREELIST_RANDOM
> /* Randomize a generic freelist */
> -static void freelist_randomize(struct rnd_state *state, unsigned int *list,
> +static void freelist_randomize(unsigned int *list,
> unsigned int count)
> {
> unsigned int rand;
> @@ -1157,8 +1157,7 @@ static void freelist_randomize(struct rnd_state *state, unsigned int *list,
>
> /* Fisher-Yates shuffle */
> for (i = count - 1; i > 0; i--) {
> - rand = prandom_u32_state(state);
> - rand %= (i + 1);
> + rand = get_random_u32_below(i+1);
same here.
otherwise looks good to me.
> swap(list[i], list[rand]);
> }
> }
> @@ -1167,7 +1166,6 @@ static void freelist_randomize(struct rnd_state *state, unsigned int *list,
> int cache_random_seq_create(struct kmem_cache *cachep, unsigned int count,
> gfp_t gfp)
> {
> - struct rnd_state state;
>
> if (count < 2 || cachep->random_seq)
> return 0;
> @@ -1176,10 +1174,7 @@ int cache_random_seq_create(struct kmem_cache *cachep, unsigned int count,
> if (!cachep->random_seq)
> return -ENOMEM;
>
> - /* Get best entropy at this stage of boot */
> - prandom_seed_state(&state, get_random_long());
> -
> - freelist_randomize(&state, cachep->random_seq, count);
> + freelist_randomize(cachep->random_seq, count);
> return 0;
> }
>
> --
> 2.37.3
>
