Commit 7d8e0bf5 authored by Li Zefan's avatar Li Zefan Committed by Tejun Heo
Browse files

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: default avatarLi Zefan <lizefan@huawei.com>
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
parent f50daa70

include/linux/cgroup.h

+14 −3
Original line number Diff line number Diff line
@@ -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. */

kernel/cgroup.c

+14 −14
Original line number Diff line number Diff line
@@ -4940,16 +4940,16 @@ 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++) {

			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.

		 * 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.

		 */

			if (!ss || ss->module)

				continue;

		for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {

			struct cgroup_subsys *ss = subsys[i];



			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;

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