CVE-2021-47125

In the Linux kernel, the following vulnerability has been resolved:
sch_htb: fix refcount leak in htb_parent_to_leaf_offload The commit
ae81feb7338c (“sch_htb: fix null pointer dereference on a null new_q”)
fixes a NULL pointer dereference bug, but it is not correct. Because
htb_graft_helper properly handles the case when new_q is NULL, and after
the previous patch by skipping this call which creates an inconsistency :
dev_queue->qdisc will still point to the old qdisc, but cl->parent->leaf.q
will point to the new one (which will be noop_qdisc, because new_q was
NULL). The code is based on an assumption that these two pointers are the
same, so it can lead to refcount leaks. The correct fix is to add a NULL
pointer check to protect qdisc_refcount_inc inside
htb_parent_to_leaf_offload.