Re: [PATCH v5 2/3] Replace invocation of weak PRNG

[Hyeonggon Yoo]

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
>