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

Summary

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/

Timeline

1. Jul 28, 2025, 11:12 AM UTCCVE published
2. Oct 15, 2026, 1:07 AM UTCRecord last updated

Affected versions

• Linux Linux

  From 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 · fixed in fed3570e548a6c9f95c5f4c9e1a7afc1679fd90d; From 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 · fixed in 5c0506cd1b1a3b145bda2612bbf7fe78d186c355; From 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 · fixed in 850226aef8d28a00cf966ef26d2f8f2bff344535; From 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 · fixed in 890a5d423ef0a7bd13447ceaffad21189f557301; From 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 · fixed in 7ff2d83ecf2619060f30ecf9fad4f2a700fca344; From 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 · fixed in e5c480dc62a3025b8428d4818e722da30ad6804f; From 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 · fixed in 3691f84269a23f7edd263e9b6edbc27b7ae332f4; From 512bb43eb5422ee69a1be05ea0d89dc074fac9a2 · fixed in 0e1d5d9b5c5966e2e42e298670808590db5ed628

• Linux Linux

  2.6.29

• KernelLinux

  From 2.6.29 · fixed in 5.4.297; From 5.5.0 · fixed in 5.10.241; From 5.11.0 · fixed in 5.15.190; From 5.16.0 · fixed in 6.1.147; From 6.2.0 · fixed in 6.6.100; From 6.7.0 · fixed in 6.12.40; From 6.13.0 · fixed in 6.15.8

References 8