{"cve":"CVE-2025-38468","nvd":{"published":"2025-07-28T12:15:28.503","modified":"2026-06-17T09:16:55.220","status":"Modified","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nnet/sched: Return NULL when htb_lookup_leaf encounters an empty rbtree\n\nhtb_lookup_leaf has a BUG_ON that can trigger with the following:\n\ntc qdisc del dev lo root\ntc qdisc add dev lo root handle 1: htb default 1\ntc class add dev lo parent 1: classid 1:1 htb rate 64bit\ntc qdisc add dev lo parent 1:1 handle 2: netem\ntc qdisc add dev lo parent 2:1 handle 3: blackhole\nping -I lo -c1 -W0.001 127.0.0.1\n\nThe root cause is the following:\n\n1. htb_dequeue calls htb_dequeue_tree which calls the dequeue handler on\n   the selected leaf qdisc\n2. netem_dequeue calls enqueue on the child qdisc\n3. blackhole_enqueue drops the packet and returns a value that is not\n   just NET_XMIT_SUCCESS\n4. Because of this, netem_dequeue calls qdisc_tree_reduce_backlog, and\n   since qlen is now 0, it calls htb_qlen_notify -> htb_deactivate ->\n   htb_deactiviate_prios -> htb_remove_class_from_row -> htb_safe_rb_erase\n5. As this is the only class in the selected hprio rbtree,\n   __rb_change_child in __rb_erase_augmented sets the rb_root pointer to\n   NULL\n6. Because blackhole_dequeue returns NULL, netem_dequeue returns NULL,\n   which causes htb_dequeue_tree to call htb_lookup_leaf with the same\n   hprio rbtree, and fail the BUG_ON\n\nThe function graph for this scenario is shown here:\n 0)               |  htb_enqueue() {\n 0) + 13.635 us   |    netem_enqueue();\n 0)   4.719 us    |    htb_activate_prios();\n 0) # 2249.199 us |  }\n 0)               |  htb_dequeue() {\n 0)   2.355 us    |    htb_lookup_leaf();\n 0)               |    netem_dequeue() {\n 0) + 11.061 us   |      blackhole_enqueue();\n 0)               |      qdisc_tree_reduce_backlog() {\n 0)               |        qdisc_lookup_rcu() {\n 0)   1.873 us    |          qdisc_match_from_root();\n 0)   6.292 us    |        }\n 0)   1.894 us    |        htb_search();\n 0)               |        htb_qlen_notify() {\n 0)   2.655 us    |          htb_deactivate_prios();\n 0)   6.933 us    |        }\n 0) + 25.227 us   |      }\n 0)   1.983 us    |      blackhole_dequeue();\n 0) + 86.553 us   |    }\n 0) # 2932.761 us |    qdisc_warn_nonwc();\n 0)               |    htb_lookup_leaf() {\n 0)               |      BUG_ON();\n ------------------------------------------\n\nThe full original bug report can be seen here [1].\n\nWe can fix this just by returning NULL instead of the BUG_ON,\nas htb_dequeue_tree returns NULL when htb_lookup_leaf returns\nNULL.\n\n[1] https://lore.kernel.org/netdev/pF5XOOIim0IuEfhI-SOxTgRvNoDwuux7UHKnE_Y5-zVd4wmGvNk2ceHjKb8ORnzw0cGwfmVu42g9dL7XyJLf1NEzaztboTWcm0Ogxuojoeo=@willsroot.io/","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","score":5.5},"cwes":["CWE-476"],"references":["https://git.kernel.org/stable/c/0e1d5d9b5c5966e2e42e298670808590db5ed628","https://git.kernel.org/stable/c/3691f84269a23f7edd263e9b6edbc27b7ae332f4","https://git.kernel.org/stable/c/5c0506cd1b1a3b145bda2612bbf7fe78d186c355","https://git.kernel.org/stable/c/7ff2d83ecf2619060f30ecf9fad4f2a700fca344","https://git.kernel.org/stable/c/850226aef8d28a00cf966ef26d2f8f2bff344535"],"products":["debian:debian_linux","linux:linux_kernel"],"source":"nvd","synced_at":"2026-10-19T22:05:46.678065081+00:00"},"epss":{"cve":"CVE-2025-38468","score":0.00196,"percentile":0.09384,"date":"2026-10-31"},"kev":null,"osv":[],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}