CVE-2023-54107

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.