This is a note to let you know that I've just added the patch titled
user namespace: fix incorrect memory barriers
to the 3.13-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:
user-namespace-fix-incorrect-memory-barriers.patch
and it can be found in the queue-3.13 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From e79323bd87808fdfbc68ce6c5371bd224d9672ee Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Date: Mon, 14 Apr 2014 16:58:55 -0400
Subject: user namespace: fix incorrect memory barriers
From: Mikulas Patocka <mpatocka@xxxxxxxxxx>
commit e79323bd87808fdfbc68ce6c5371bd224d9672ee upstream.
smp_read_barrier_depends() can be used if there is data dependency between
the readers - i.e. if the read operation after the barrier uses address
that was obtained from the read operation before the barrier.
In this file, there is only control dependency, no data dependecy, so the
use of smp_read_barrier_depends() is incorrect. The code could fail in the
following way:
* the cpu predicts that idx < entries is true and starts executing the
body of the for loop
* the cpu fetches map->extent[0].first and map->extent[0].count
* the cpu fetches map->nr_extents
* the cpu verifies that idx < extents is true, so it commits the
instructions in the body of the for loop
The problem is that in this scenario, the cpu read map->extent[0].first
and map->nr_extents in the wrong order. We need a full read memory barrier
to prevent it.
Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/user_namespace.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -152,7 +152,7 @@ static u32 map_id_range_down(struct uid_
/* Find the matching extent */
extents = map->nr_extents;
- smp_read_barrier_depends();
+ smp_rmb();
for (idx = 0; idx < extents; idx++) {
first = map->extent[idx].first;
last = first + map->extent[idx].count - 1;
@@ -176,7 +176,7 @@ static u32 map_id_down(struct uid_gid_ma
/* Find the matching extent */
extents = map->nr_extents;
- smp_read_barrier_depends();
+ smp_rmb();
for (idx = 0; idx < extents; idx++) {
first = map->extent[idx].first;
last = first + map->extent[idx].count - 1;
@@ -199,7 +199,7 @@ static u32 map_id_up(struct uid_gid_map
/* Find the matching extent */
extents = map->nr_extents;
- smp_read_barrier_depends();
+ smp_rmb();
for (idx = 0; idx < extents; idx++) {
first = map->extent[idx].lower_first;
last = first + map->extent[idx].count - 1;
@@ -615,9 +615,8 @@ static ssize_t map_write(struct file *fi
* were written before the count of the extents.
*
* To achieve this smp_wmb() is used on guarantee the write
- * order and smp_read_barrier_depends() is guaranteed that we
- * don't have crazy architectures returning stale data.
- *
+ * order and smp_rmb() is guaranteed that we don't have crazy
+ * architectures returning stale data.
*/
mutex_lock(&id_map_mutex);
Patches currently in stable-queue which might be from mpatocka@xxxxxxxxxx are
queue-3.13/user-namespace-fix-incorrect-memory-barriers.patch
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