cgroup: fix use-after-free when umounting cgroupfs

Try:
  # mount -t cgroup xxx /cgroup
  # mkdir /cgroup/sub && rmdir /cgroup/sub && umount /cgroup

And you might see this:

ida_remove called for id=1 which is not allocated.

It's because cgroup_kill_sb() is called to destroy root->cgroup_ida
and free cgrp->root before ida_simple_removed() is called. What's
worse is we're accessing cgrp->root while it has been freed.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 4 insertions, 2 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 192d762ec1f4..bd4de465d5a9 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -848,9 +848,12 @@ static void cgroup_free_fn(struct work_struct *work)
          */
         dput(cgrp->parent->dentry);
 
+        ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
+
         /*
          * Drop the active superblock reference that we took when we
-         * created the cgroup
+         * created the cgroup. This will free cgrp->root, if we are
+         * holding the last reference to @sb.
          */
         deactivate_super(cgrp->root->sb);
 
@@ -862,7 +865,6 @@ static void cgroup_free_fn(struct work_struct *work)
 
         simple_xattrs_free(&cgrp->xattrs);
 
-        ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);
         kfree(rcu_dereference_raw(cgrp->name));
         kfree(cgrp);
 }