GHSA-4MQG-Q94G-WMFR

Vulnerability from github – Published: 2025-07-28 12:30 – Updated: 2025-12-22 21:30
VLAI?
Details

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 ?
5.5 (Medium)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Show details on source website
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{
  "affected": [],
  "aliases": [
    "CVE-2025-38468"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-476"
    ],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2025-07-28T12:15:28Z",
    "severity": "MODERATE"
  },
  "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": "2025-12-22T21:30:30Z",
  "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/5c0506cd1b1a3b145bda2612bbf7fe78d186c355"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/7ff2d83ecf2619060f30ecf9fad4f2a700fca344"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/850226aef8d28a00cf966ef26d2f8f2bff344535"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/890a5d423ef0a7bd13447ceaffad21189f557301"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/e5c480dc62a3025b8428d4818e722da30ad6804f"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/fed3570e548a6c9f95c5f4c9e1a7afc1679fd90d"
    },
    {
      "type": "WEB",
      "url": "https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html"
    },
    {
      "type": "WEB",
      "url": "https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ]
}


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