cgroup: fix RCU accesses around task->cgroups

There are several places in kernel/cgroup.c where task->cgroups is
accessed and modified without going through proper RCU accessors.
None is broken as they're all lock protected accesses; however, this
still triggers sparse RCU address space warnings.

* Consistently use task_css_set() for task->cgroups dereferencing.

* Use RCU_INIT_POINTER() to clear task->cgroups to &init_css_set on
  exit.

* Remove unnecessary rcu_dereference_raw() from cset->subsys[]
  dereference in cgroup_exit().

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 13 insertions, 11 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 2d3a132e881d..ee9f0c1c8bff 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -724,7 +724,7 @@ static struct cgroup *task_cgroup_from_root(struct task_struct *task,
          * task can't change groups, so the only thing that can happen
          * is that it exits and its css is set back to init_css_set.
          */
-        cset = task->cgroups;
+        cset = task_css_set(task);
         if (cset == &init_css_set) {
                 res = &root->top_cgroup;
         } else {
@@ -1971,7 +1971,7 @@ static void cgroup_task_migrate(struct cgroup *old_cgrp,
          * css_set to init_css_set and dropping the old one.
          */
         WARN_ON_ONCE(tsk->flags & PF_EXITING);
-        old_cset = tsk->cgroups;
+        old_cset = task_css_set(tsk);
 
         task_lock(tsk);
         rcu_assign_pointer(tsk->cgroups, new_cset);
@@ -2094,8 +2094,11 @@ static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
          * we use find_css_set, which allocates a new one if necessary.
          */
         for (i = 0; i < group_size; i++) {
+                struct css_set *old_cset;
+
                 tc = flex_array_get(group, i);
-                tc->cg = find_css_set(tc->task->cgroups, cgrp);
+                old_cset = task_css_set(tc->task);
+                tc->cg = find_css_set(old_cset, cgrp);
                 if (!tc->cg) {
                         retval = -ENOMEM;
                         goto out_put_css_set_refs;
@@ -3012,7 +3015,7 @@ static void cgroup_enable_task_cg_lists(void)
                  * entry won't be deleted though the process has exited.
                  */
                 if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list))
-                        list_add(&p->cg_list, &p->cgroups->tasks);
+                        list_add(&p->cg_list, &task_css_set(p)->tasks);
                 task_unlock(p);
         } while_each_thread(g, p);
         read_unlock(&tasklist_lock);
@@ -5061,8 +5064,8 @@ static const struct file_operations proc_cgroupstats_operations = {
 void cgroup_fork(struct task_struct *child)
 {
         task_lock(current);
+        get_css_set(task_css_set(current));
         child->cgroups = current->cgroups;
-        get_css_set(child->cgroups);
         task_unlock(current);
         INIT_LIST_HEAD(&child->cg_list);
 }
@@ -5097,7 +5100,7 @@ void cgroup_post_fork(struct task_struct *child)
                 write_lock(&css_set_lock);
                 task_lock(child);
                 if (list_empty(&child->cg_list))
-                        list_add(&child->cg_list, &child->cgroups->tasks);
+                        list_add(&child->cg_list, &task_css_set(child)->tasks);
                 task_unlock(child);
                 write_unlock(&css_set_lock);
         }
@@ -5177,8 +5180,8 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
 
         /* Reassign the task to the init_css_set. */
         task_lock(tsk);
-        cset = tsk->cgroups;
-        tsk->cgroups = &init_css_set;
+        cset = task_css_set(tsk);
+        RCU_INIT_POINTER(tsk->cgroups, &init_css_set);
 
         if (run_callbacks && need_forkexit_callback) {
                 /*
@@ -5187,8 +5190,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
                  */
                 for_each_builtin_subsys(ss, i) {
                         if (ss->exit) {
-                                struct cgroup *old_cgrp =
-                                        rcu_dereference_raw(cset->subsys[i])->cgroup;
+                                struct cgroup *old_cgrp = cset->subsys[i]->cgroup;
                                 struct cgroup *cgrp = task_cgroup(tsk, i);
 
                                 ss->exit(cgrp, old_cgrp, tsk);
@@ -5555,7 +5557,7 @@ static u64 current_css_set_refcount_read(struct cgroup *cgrp,
         u64 count;
 
         rcu_read_lock();
-        count = atomic_read(&current->cgroups->refcount);
+        count = atomic_read(&task_css_set(current)->refcount);
         rcu_read_unlock();
         return count;
 }