ghsa-4mqg-q94g-wmfr
Vulnerability from github
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:
- htb_dequeue calls htb_dequeue_tree which calls the dequeue handler on the selected leaf qdisc
- netem_dequeue calls enqueue on the child qdisc
- blackhole_enqueue drops the packet and returns a value that is not just NET_XMIT_SUCCESS
- 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
- 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
- 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/
{
"affected": [],
"aliases": [
"CVE-2025-38468"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2025-07-28T12:15:28Z",
"severity": null
},
"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/",
"id": "GHSA-4mqg-q94g-wmfr",
"modified": "2025-07-28T12:30:34Z",
"published": "2025-07-28T12:30:34Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2025-38468"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/0e1d5d9b5c5966e2e42e298670808590db5ed628"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/3691f84269a23f7edd263e9b6edbc27b7ae332f4"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/7ff2d83ecf2619060f30ecf9fad4f2a700fca344"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/890a5d423ef0a7bd13447ceaffad21189f557301"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/e5c480dc62a3025b8428d4818e722da30ad6804f"
}
],
"schema_version": "1.4.0",
"severity": []
}
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