linux-kernel - [PATCH v2 1/3] Replace invocation of weak PRNG in mm/slab.c

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Message-Id: <4925f0fe15e9cda5f9d90d75c6e15acdc413ef9e.1670890407.git.david.keisarschm@mail.huji.ac.il>
Date:   Tue, 13 Dec 2022 12:34:57 +0200
From:   david.keisarschm@...l.huji.ac.il
To:     linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>
Cc:     David <david.keisarschm@...l.huji.ac.il>, aksecurity@...il.com,
        ilay.bahat1@...il.com, linux-mm@...ck.org
Subject: [PATCH v2 1/3] Replace invocation of weak PRNG in mm/slab.c

From: David <david.keisarschm@...l.huji.ac.il>

We changed the invocation
 of prandom_u32_state to get_random_u32.
 We also changed the freelist_init_state
 to struct instead of a union,
 since the rnd_state is not needed anymore
- get_random_u32 maintains its own state.
This change it important since it
is make the slab allocator randomization
stronger.

Signed-off-by: David <david.keisarschm@...l.huji.ac.il>
---
 mm/slab.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 92d6b1d48..1476104f4 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)
 {
@@ -2385,7 +2382,6 @@ static bool freelist_state_initialize(union freelist_init_state *state,
 
 	/* Use a random state if the pre-computed list is not available */
 	if (!cachep->random_seq) {
-		prandom_seed_state(&state->rnd_state, rand);
 		ret = false;
 	} else {
 		state->list = cachep->random_seq;
@@ -2397,7 +2393,7 @@ static bool freelist_state_initialize(union freelist_init_state *state,
 }
 
 /* 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)
@@ -2447,7 +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 = get_random_u32();
 			rand %= (i + 1);
 			swap_free_obj(slab, i, rand);
 		}
-- 
2.38.0