In the Linux kernel, the following vulnerability has been resolved:
blk-cgroup: dropping parent refcount after pd_free_fn() is done
Some cgroup policies will access parent pd through child pd even
after pd_offline_fn() is done. If pd_free_fn() for parent is called
before child, then UAF can be triggered. Hence it's better to guarantee
the order of pd_free_fn().
Currently refcount of parent blkg is dropped in __blkg_release(), which
is before pd_free_fn() is called in blkg_free_work_fn() while
blkg_free_work_fn() is called asynchronously.
This patch make sure pd_free_fn() called from removing cgroup is ordered
by delaying dropping parent refcount after calling pd_free_fn() for
child.
BTW, pd_free_fn() will also be called from blkcg_deactivate_policy()
from deleting device, and following patches will guarantee the order.
