cgroup: fix exit() vs rmdir() 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.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 8008522..5d4c92e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -422,12 +422,20 @@
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);
}