[PATCH v3 2/3] Replace invocation of weak PRNG inside mm/slab_common.c

[david.keisarschm@xxxxxxxxxxxxxxx]

From: David Keisar Schmidt <david.keisarschm@xxxxxxxxxxxxxxx>

This third series add some changes to the commit messages,
and also replaces get_random_u32 with get_random_u32_below,
in a case a modulo operation is done on the result.

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.

# Changes since v2

* edited commit message.
* replaced instances of get_random_u32 with get_random_u32_below
    in mm/slab_common.c.


Signed-off-by: David Keisar Schmidt <david.keisarschm@xxxxxxxxxxxxxxx>
---
 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 0042fb273..e254b2f55 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1130,7 +1130,7 @@ 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;
@@ -1141,8 +1141,7 @@ 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]);
 	}
 }
@@ -1151,7 +1150,6 @@ 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;
@@ -1160,10 +1158,7 @@ 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;
 }
 
-- 
2.38.0