From c4e490cf148e85ead0d1b1c2caaba833f1d5b29f Mon Sep 17 00:00:00 2001 From: John Sperbeck Date: Tue, 10 Jan 2017 16:58:24 -0800 Subject: [PATCH] mm/slab.c: 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. It will result in odd behaviours and crashes. It should be uncommon but it depends on the machines. We saw it happening more often on some machines (every few hours of running tests). Fixes: c7ce4f60ac19 ("mm: SLAB freelist randomization") Link: http://lkml.kernel.org/r/20170103181908.143178-1-thgarnie@google.com Signed-off-by: John Sperbeck Signed-off-by: Thomas Garnier Cc: Christoph Lameter Cc: Pekka Enberg Cc: David Rientjes Cc: Joonsoo Kim Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- 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 */ -- 2.20.1