net/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree

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

net/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree

htb_lookup_leaf has a BUG_ON that can trigger with the following:

tc qdisc del dev lo root

tc qdisc add dev lo root handle 1: htb default 1

tc class add dev lo parent 1: classid 1:1 htb rate 64bit

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

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

ping -I lo -c1 -W0.001 127.0.0.1

The root cause is the following:

1. htb_dequeue calls htb_dequeue_tree which calls the dequeue handler on

the selected leaf qdisc

2. netem_dequeue calls enqueue on the child qdisc

3. blackhole_enqueue drops the packet and returns a value that is not

just NET_XMIT_SUCCESS

4. Because of this, netem_dequeue calls qdisc_tree_reduce_backlog, and

since qlen is now 0, it calls htb_qlen_notify -> htb_deactivate ->

htb_deactiviate_prios -> htb_remove_class_from_row -> htb_safe_rb_erase

5. As this is the only class in the selected hprio rbtree,

__rb_change_child in __rb_erase_augmented sets the rb_root pointer to

NULL

6. Because blackhole_dequeue returns NULL, netem_dequeue returns NULL,

which causes htb_dequeue_tree to call htb_lookup_leaf with the same

hprio rbtree, and fail the BUG_ON

The function graph for this scenario is shown here:

0) | htb_enqueue() {

0) + 13.635 us | netem_enqueue();

0) 4.719 us | htb_activate_prios();

0) # 2249.199 us | }

0) | htb_dequeue() {

0) 2.355 us | htb_lookup_leaf();

0) | netem_dequeue() {

0) + 11.061 us | blackhole_enqueue();

0) | qdisc_tree_reduce_backlog() {

0) | qdisc_lookup_rcu() {

0) 1.873 us | qdisc_match_from_root();

0) 6.292 us | }

0) 1.894 us | htb_search();

0) | htb_qlen_notify() {

0) 2.655 us | htb_deactivate_prios();

0) 6.933 us | }

0) + 25.227 us | }

0) 1.983 us | blackhole_dequeue();

0) + 86.553 us | }

0) # 2932.761 us | qdisc_warn_nonwc();

0) | htb_lookup_leaf() {

0) | BUG_ON();

------------------------------------------

The full original bug report can be seen here [1].

We can fix this just by returning NULL instead of the BUG_ON,

as htb_dequeue_tree returns NULL when htb_lookup_leaf returns

NULL.

[1] https://lore.kernel.org/netdev/pF5XOOIim0IuEfhI-SOxTgRvNoDwuux7UHKnE_Y5-zVd4wmGvNk2ceHjKb8ORnzw0cGwfmVu42g9dL7XyJLf1NEzaztboTWcm0Ogxuojoeo=@willsroot.io/