CVE-2025-38468 Information

Jul 29, 2025 cve

Description

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/

Reference

https://git.kernel.org/stable/c/0e1d5d9b5c5966e2e42e298670808590db5ed628 https://git.kernel.org/stable/c/3691f84269a23f7edd263e9b6edbc27b7ae332f4 https://git.kernel.org/stable/c/7ff2d83ecf2619060f30ecf9fad4f2a700fca344 https://git.kernel.org/stable/c/890a5d423ef0a7bd13447ceaffad21189f557301 https://git.kernel.org/stable/c/e5c480dc62a3025b8428d4818e722da30ad6804f

CNNVD-202507-3458 (Published: 2025-07-28)

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