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authorMikulas Patocka <mpatocka@redhat.com>2014-04-14 16:58:55 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2014-04-26 17:19:02 -0700
commit99c3556cdd0d809f4082e790a3df0ab86ded5913 (patch)
tree12085e518bde779dce50256db39e30bf03b1f8f2
parent387df1bd3fc46bc695b317dda38b3254f4409036 (diff)
user namespace: fix incorrect memory barriers
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@redhat.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--kernel/user_namespace.c11
1 files changed, 5 insertions, 6 deletions
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index dd06439b9c84..80a57afd8647 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -152,7 +152,7 @@ static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
/* 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_map *map, u32 id)
/* 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 *map, u32 id)
/* 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 *file, const char __user *buf,
* 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);