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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>
This commit is contained in:
Li Zefan 2013-03-05 10:57:03 +08:00 committed by Tejun Heo
parent f50daa704f
commit 7d8e0bf56a
2 changed files with 28 additions and 17 deletions
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17
include/linux/cgroup.h
View File

@ -44,14 +44,25 @@ extern void cgroup_unload_subsys(struct cgroup_subsys *ss);
extern const struct file_operations proc_cgroup_operations;
/* Define the enumeration of all builtin cgroup subsystems */
/*
* Define the enumeration of all cgroup subsystems.
*
* We define ids for builtin subsystems and then modular ones.
*/
#define SUBSYS(_x) _x ## _subsys_id,
#define IS_SUBSYS_ENABLED(option) IS_ENABLED(option)
enum cgroup_subsys_id {
#define IS_SUBSYS_ENABLED(option) IS_BUILTIN(option)
#include <linux/cgroup_subsys.h>
#undef IS_SUBSYS_ENABLED
CGROUP_BUILTIN_SUBSYS_COUNT,
__CGROUP_SUBSYS_TEMP_PLACEHOLDER = CGROUP_BUILTIN_SUBSYS_COUNT - 1,
#define IS_SUBSYS_ENABLED(option) IS_MODULE(option)
#include <linux/cgroup_subsys.h>
#undef IS_SUBSYS_ENABLED
CGROUP_SUBSYS_COUNT,
};
#undef IS_SUBSYS_ENABLED
#undef SUBSYS
/* Per-subsystem/per-cgroup state maintained by the system. */

28
kernel/cgroup.c
View File

@ -4940,17 +4940,17 @@ void cgroup_post_fork(struct task_struct *child)
* 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 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
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;