[PATCH 4.9 021/120] mm/slab.c: fix SLAB freelist randomization duplicate entries

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: John Sperbeck <jsperbeck@xxxxxxxxxx>

commit c4e490cf148e85ead0d1b1c2caaba833f1d5b29f upstream.

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@xxxxxxxxxx
Signed-off-by: John Sperbeck <jsperbeck@xxxxxxxxxx>
Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 mm/slab.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2475,7 +2475,6 @@ union freelist_init_state {
 		unsigned int pos;
 		unsigned int *list;
 		unsigned int count;
-		unsigned int rand;
 	};
 	struct rnd_state rnd_state;
 };
@@ -2501,8 +2500,7 @@ static bool freelist_state_initialize(un
 	} else {
 		state->list = cachep->random_seq;
 		state->count = count;
-		state->pos = 0;
-		state->rand = rand;
+		state->pos = rand % count;
 		ret = true;
 	}
 	return ret;
@@ -2511,7 +2509,9 @@ static bool freelist_state_initialize(un
 /* 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 */