Patch "mm/slab.c: fix SLAB freelist randomization duplicate entries" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

mm/slab.c: fix SLAB freelist randomization duplicate entries

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-slab.c-fix-slab-freelist-randomization-duplicate-entries.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: John Sperbeck <[email protected]>
Date: Tue, 10 Jan 2017 16:58:24 -0800
Subject: mm/slab.c: fix SLAB freelist randomization duplicate entries

From: John Sperbeck <[email protected]>

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).

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

@@ -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 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/mm-slab.c-fix-slab-freelist-randomization-duplicate-entries.patch