net/sched: Always pass notifications when child class becomes empty

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

net/sched: Always pass notifications when child class becomes empty

Certain classful qdiscs may invoke their classes' dequeue handler on an

enqueue operation. This may unexpectedly empty the child qdisc and thus

make an in-flight class passive via qlen_notify(). Most qdiscs do not

expect such behaviour at this point in time and may re-activate the

class eventually anyways which will lead to a use-after-free.

The referenced fix commit attempted to fix this behavior for the HFSC

case by moving the backlog accounting around, though this turned out to

be incomplete since the parent's parent may run into the issue too.

The following reproducer demonstrates this use-after-free:

tc qdisc add dev lo root handle 1: drr

tc filter add dev lo parent 1: basic classid 1:1

tc class add dev lo parent 1: classid 1:1 drr

tc qdisc add dev lo parent 1:1 handle 2: hfsc def 1

tc class add dev lo parent 2: classid 2:1 hfsc rt m1 8 d 1 m2 0

tc qdisc add dev lo parent 2:1 handle 3: netem

tc qdisc add dev lo parent 3:1 handle 4: blackhole

echo 1 | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888

tc class delete dev lo classid 1:1

echo 1 | socat -u STDIN UDP4-DATAGRAM:127.0.0.1:8888

Since backlog accounting issues leading to a use-after-frees on stale

class pointers is a recurring pattern at this point, this patch takes

a different approach. Instead of trying to fix the accounting, the patch

ensures that qdisc_tree_reduce_backlog always calls qlen_notify when

the child qdisc is empty. This solves the problem because deletion of

qdiscs always involves a call to qdisc_reset() and / or

qdisc_purge_queue() which ultimately resets its qlen to 0 thus causing

the following qdisc_tree_reduce_backlog() to report to the parent. Note

that this may call qlen_notify on passive classes multiple times. This

is not a problem after the recent patch series that made all the

classful qdiscs qlen_notify() handlers idempotent.