linux-kernel - [PATCH] Fix SLAB freelist randomization duplicate entries

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Message-Id: <20170103181908.143178-1-thgarnie@google.com>
Date:   Tue,  3 Jan 2017 10:19:08 -0800
From:   Thomas Garnier <thgarnie@...gle.com>
To:     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>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        jsperbeck@...gle.com, Thomas Garnier <thgarnie@...gle.com>
Subject: [PATCH] Fix SLAB freelist randomization duplicate entries

This patch fixes a bug in the freelist randomization code. When a high
random number is used, the freelist will contain duplicate entries. It
will result in different allocations sharing the same chunk.

Fixes: c7ce4f60ac19 ("mm: SLAB freelist randomization")
Signed-off-by: John Sperbeck <jsperbeck@...gle.com>
Reviewed-by: Thomas Garnier <thgarnie@...gle.com>
---
 mm/slab.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 29bc6c0dedd0..4f2ec6bb46eb 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2457,7 +2457,6 @@ union freelist_init_state {
 		unsigned int pos;
 		unsigned int *list;
 		unsigned int count;
-		unsigned int rand;
 	};
 	struct rnd_state rnd_state;
 };
@@ -2483,8 +2482,7 @@ static bool freelist_state_initialize(union freelist_init_state *state,
 	} else {
 		state->list = cachep->random_seq;
 		state->count = count;
-		state->pos = 0;
-		state->rand = rand;
+		state->pos = rand % count;
 		ret = true;
 	}
 	return ret;
@@ -2493,7 +2491,9 @@ 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)
 {
-	return (state->list[state->pos++] + state->rand) % state->count;
+	if (state->pos >= state->count)
+		state->pos = 0;
+	return state->list[state->pos++];
 }
 
 /* Swap two freelist entries */
-- 
2.11.0.390.gc69c2f50cf-goog