Re: [PATCH v5 1/3] Replace invocation of weak PRNG

[Hyeonggon Yoo]

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
>