{"id":"AZL-65910","summary":"CVE-2025-38468 affecting package kernel for versions less than 6.6.104.2-1","details":"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 -\u003e htb_deactivate -\u003e\n   htb_deactiviate_prios -\u003e htb_remove_class_from_row -\u003e 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/","modified":"2026-04-21T04:37:43.736105Z","published":"2025-07-28T12:15:28Z","upstream":["CVE-2025-38468"],"references":[{"type":"WEB","url":"https://nvd.nist.gov/vuln/detail/CVE-2025-38468"}],"affected":[{"package":{"name":"kernel","ecosystem":"Azure Linux:3","purl":"pkg:rpm/azure-linux/kernel"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"6.6.104.2-1"}]}],"database_specific":{"source":"https://github.com/microsoft/AzureLinuxVulnerabilityData/blob/main/osv/AZL-65910.json"}}],"schema_version":"1.7.5"}