cgroup: fix exit() vsrmdir() race

In cgroup_exit() put_css_set_taskexit() is called without any lock, which might lead to accessing a freed cgroup:

thread1 thread2
---------------------------------------------
exit()
cgroup_exit()

    put_css_set_taskexit()
      atomic_dec(cgrp->count);
                                   rmdir();
      /* not safe !! */
      check_for_release(cgrp);

rcu_read_lock() can be used to make sure the cgroup is alive.

---
 kernel/cgroup.c | 8 ++++++++
 1 file changed, 8 insertions(+)

index 368cff1..09c5869 100644
@@ -422,12 +422,20 @@ static void __put_css_set(struct css_set *cg, int taskexit)
 		struct cgroup *cgrp = link->cgrp;
 		list_del(&link->cg_link_list);
 		list_del(&link->cgrp_link_list);
+
+		/*
+		 * We may not be holding cgroup_mutex, and if cgrp->count is
+		 * dropped to 0 the cgroup can be destroyed at any time, hence
+		 * rcu_read_lock is used to keep it alive.
+		 */
+		rcu_read_lock();
 		if (atomic_dec_and_test(&cgrp->count) &&
 		    notify_on_release(cgrp)) {
 			if (taskexit)
 				set_bit(CGRP_RELEASABLE, &cgrp->flags);
 			check_for_release(cgrp);
 		}
+		rcu_read_unlock();
 
 		kfree(link);
 	}

1.8.0.2

init_task.cgroups is initialized at boot phase, and whenver a ask is forked, it's cgroups pointer is inherited from its parent, and it's never set to NULL afterwards.

---
 kernel/cgroup.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index 09c5869..5d4c92e 100644
@@ -5011,8 +5011,7 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
 	}
 	task_unlock(tsk);
 
-	if (cg)
-		put_css_set_taskexit(cg);
+	put_css_set_taskexit(cg);
 }
 
 /**

1.8.0.2

Applied to cgroup/for-3.9.

Thanks.

tejun

Applying to cgroup/for-3.8-fixes w/ stable cc'd.

* Can you please add WARN_ON_ONCE(!rcu_read_lock_held()) in check_for_release()?

* cgroup_release_agent() seems broken too. It's accessing cgrp after removing it from release_list. Can you please fix it too?

Thanks.

tejun

Oh, we should be okay as we're holding cgroup_mutex there. Please forget about the above.

Thanks.

tejun