CVE-2025-38468
Full vulnerability record from the local CVENow database.
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/
Severity
MEDIUM
CVSS score
5.5
(v3.1)
Vulnerability status
Modified
Source identifier
416baaa9-dc9f-4396-8d5f-8c081fb06d67
Published
2025-07-28 12:15:28.503000
Last modified
2026-11-05 21:18:33.203000
View original CVE JSON
{
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"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"published": "2025-07-28T12:15:28.503",
"lastModified": "2026-11-05T21:18:33.203",
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{
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"value": "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/"
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"value": "En el kernel de Linux, se ha resuelto la siguiente vulnerabilidad: net/sched: Devuelve NULL cuando htb_lookup_leaf encuentra un rbtree vacío htb_lookup_leaf tiene un BUG_ON que puede activarse con lo siguiente: 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 La causa raíz es la siguiente: 1. htb_dequeue llama a htb_dequeue_tree, que llama al controlador de dequeue en el qdisc de hoja seleccionado 2. netem_dequeue llama a enqueue en el hijo qdisc 3. blackhole_enqueue descarta el paquete y devuelve un valor que no es solo NET_XMIT_SUCCESS 4. Debido a esto, netem_dequeue llama a qdisc_tree_reduce_backlog y, dado que qlen ahora es 0, llama a htb_qlen_notify -> htb_deactivate -> htb_deactiviate_prios -> htb_remove_class_from_row -> htb_safe_rb_erase 5. Como esta es la única clase en el rbtree hprio seleccionado, __rb_change_child en __rb_erase_augmented establece el puntero rb_root en NULL 6. Debido a que blackhole_dequeue devuelve NULL, netem_dequeue devuelve NULL, lo que hace que htb_dequeue_tree llame a htb_lookup_leaf con el mismo rbtree hprio y falle el BUG_ON Se muestra el gráfico de funciones para este escenario aquí: 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/"
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