net/sched: Abort __tc_modify_qdisc if parent class does not exist

In the Linux kernel, the following vulnerability has been resolved:

net/sched: Abort __tc_modify_qdisc if parent class does not exist

Lion's patch [1] revealed an ancient bug in the qdisc API.

Whenever a user creates/modifies a qdisc specifying as a parent another

qdisc, the qdisc API will, during grafting, detect that the user is

not trying to attach to a class and reject. However grafting is

performed after qdisc_create (and thus the qdiscs' init callback) is

executed. In qdiscs that eventually call qdisc_tree_reduce_backlog

during init or change (such as fq, hhf, choke, etc), an issue

arises. For example, executing the following commands:

sudo tc qdisc add dev lo root handle a: htb default 2

sudo tc qdisc add dev lo parent a: handle beef fq

Qdiscs such as fq, hhf, choke, etc unconditionally invoke

qdisc_tree_reduce_backlog() in their control path init() or change() which

then causes a failure to find the child class; however, that does not stop

the unconditional invocation of the assumed child qdisc's qlen_notify with

a null class. All these qdiscs make the assumption that class is non-null.

The solution is ensure that qdisc_leaf() which looks up the parent

class, and is invoked prior to qdisc_create(), should return failure on

not finding the class.

In this patch, we leverage qdisc_leaf to return ERR_PTRs whenever the

parentid doesn't correspond to a class, so that we can detect it

earlier on and abort before qdisc_create is called.

[1] https://lore.kernel.org/netdev/[email protected]/