Commit ffe4dfe0 authored by David Keisar Schmidt's avatar David Keisar Schmidt Committed by Vlastimil Babka
Browse files

mm/slab_common: Replace invocation of weak PRNG 



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 c51f8f88
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: default avatarDavid Keisar Schmidt <david.keisarschm@mail.huji.ac.il>
Signed-off-by: default avatarVlastimil Babka <vbabka@suse.cz>
parent f7e466e9

mm/slab_common.c

+3 −8
Original line number Diff line number Diff line
@@ -1141,7 +1141,7 @@ 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;
@@ -1152,8 +1152,7 @@ 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);

		swap(list[i], list[rand]);

	}

}
@@ -1162,7 +1161,6 @@ 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;
@@ -1171,10 +1169,7 @@ 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;

}