cgroup: avoid accessing modular cgroup subsys structure without locking
subsys[i] is set to NULL in cgroup_unload_subsys() at modular unload,
and that's protected by cgroup_mutex, and then the memory *subsys[i]
resides will be freed.
So this is unsafe without any locking:
if (!ss || ss->module)
...
v2:
- add a comment for enum cgroup_subsys_id
- simplify the comment in cgroup_exit()
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 9df799d..7a6c4c7 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4940,17 +4940,17 @@
* and addition to css_set.
*/
if (need_forkexit_callback) {
- for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+ /*
+ * fork/exit callbacks are supported only for builtin
+ * subsystems, and the builtin section of the subsys
+ * array is immutable, so we don't need to lock the
+ * subsys array here. On the other hand, modular section
+ * of the array can be freed at module unload, so we
+ * can't touch that.
+ */
+ for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
struct cgroup_subsys *ss = subsys[i];
- /*
- * fork/exit callbacks are supported only for
- * builtin subsystems and we don't need further
- * synchronization as they never go away.
- */
- if (!ss || ss->module)
- continue;
-
if (ss->fork)
ss->fork(child);
}
@@ -5015,13 +5015,13 @@
tsk->cgroups = &init_css_set;
if (run_callbacks && need_forkexit_callback) {
- for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+ /*
+ * fork/exit callbacks are supported only for builtin
+ * subsystems, see cgroup_post_fork() for details.
+ */
+ for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
struct cgroup_subsys *ss = subsys[i];
- /* modular subsystems can't use callbacks */
- if (!ss || ss->module)
- continue;
-
if (ss->exit) {
struct cgroup *old_cgrp =
rcu_dereference_raw(cg->subsys[i])->cgroup;