Linux Kernel CVE-2025-38468

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

1. netem_dequeue calls enqueue on the child qdisc
2. blackhole_enqueue drops the packet and returns a value that is not

just NET_XMIT_SUCCESS

1. 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

1. 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

1. 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:

1. | htb_enqueue() {
2. + 13.635 us | netem_enqueue();
3. 4.719 us | htb_activate_prios();

0)

2249.199 us | }

1. | htb_dequeue() {
2. 2.355 us | htb_lookup_leaf();
3. | netem_dequeue() {
4. + 11.061 us | blackhole_enqueue();
5. | qdisc_tree_reduce_backlog() {
6. | qdisc_lookup_rcu() {
7. 1.873 us | qdisc_match_from_root();
8. 6.292 us | }
9. 1.894 us | htb_search();
10. | htb_qlen_notify() {
11. 2.655 us | htb_deactivate_prios();
12. 6.933 us | }
13. + 25.227 us | }
14. 1.983 us | blackhole_dequeue();
15. + 86.553 us | }

0)

2932.761 us | qdisc_warn_nonwc();

1. | htb_lookup_leaf() {
2. | 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/