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net/sched: Abort __tc_modify_qdisc if parent class does not exist

Type: advisorySources: OSV

In the Linux kernel, the following vulnerability has been resolved: net/sched: Abort __tc_modify_qdisc if parent class does not exist Lion's patch [1] revealed an ancient bug in the qdisc API. Whenever a user creates/modifies a qdisc specifying as a parent another qdisc, the qdisc API will, during grafting, detect that the user is not trying to attach to a class and reject. However grafting is performed after qdisc_create (and thus the qdiscs' init callback) is executed. In qdiscs that eventually call qdisc_tree_reduce_backlog during init or change (such as fq, hhf, choke, etc), an issue arises. For example, executing the following commands: sudo tc qdisc add dev lo root handle a: htb default 2 sudo tc qdisc add dev lo parent a: handle beef fq Qdiscs such as fq, hhf, choke, etc unconditionally invoke qdisc_tree_reduce_backlog() in their control path init() or change() which then causes a failure to find the child class; however, that does not stop the unconditional invocation of the assumed child qdisc's qlen_notify with a null class. All these qdiscs make the assumption that class is non-null. The solution is ensure that qdisc_leaf() which looks up the parent class, and is invoked prior to qdisc_create(), should return failure on not finding the class. In this patch, we leverage qdisc_leaf to return ERR_PTRs whenever the parentid doesn't correspond to a class, so that we can detect it earlier on and abort before qdisc_create is called. [1] https://lore.kernel.org/netdev/d912

Products
Kernel
Affected versions
Not specified
Fixed versions
5.10.240, 5.15.189, 5.4.296, 6.1.146, 6.12.39, 6.15.7, 6.6.99
CVEs
CVE-2025-38457
Confirmed fixed CVEs
Not specified
Updated in this feed
2 Nov 2026, 21:46 UTC · Revision 1

Source evidence

net/sched: Abort __tc_modify_qdisc if parent class does not exist

core-osv · Original source

Published 25 Jul 2025, 15:27 UTC

In the Linux kernel, the following vulnerability has been resolved: net/sched: Abort __tc_modify_qdisc if parent class does not exist Lion's patch [1] revealed an ancient bug in the qdisc API. Whenever a user creates/modifies a qdisc specifying as a parent another qdisc, the qdisc API will, during grafting, detect that the user is not trying to attach to a class and reject. However grafting is performed after qdisc_create (and thus the qdiscs' init callback) is executed. In qdiscs that eventually call qdisc_tree_reduce_backlog during init or change (such as fq, hhf, choke, etc), an issue arises. For example, executing the following commands: sudo tc qdisc add dev lo root handle a: htb default 2 sudo tc qdisc add dev lo parent a: handle beef fq Qdiscs such as fq, hhf, choke, etc unconditionally invoke qdisc_tree_reduce_backlog() in their control path init() or change() which then causes a failure to find the child class; however, that does not stop the unconditional invocation of the assumed child qdisc's qlen_notify with a null class. All these qdiscs make the assumption that class is non-null. The solution is ensure that qdisc_leaf() which looks up the parent class, and is invoked prior to qdisc_create(), should return failure on not finding the class. In this patch, we leverage qdisc_leaf to return ERR_PTRs whenever the parentid doesn't correspond to a class, so that we can detect it earlier on and abort before qdisc_create is called. [1] https://lore.kernel.org/netdev/d912

osv:CVE-2025-38457

Kernel

Fixed versions: 5.4.296, 5.10.240, 5.15.189, 6.1.146, 6.6.99, 6.12.39, 6.15.7

Complete source record
{
  "id": "2256ec74a50ba6748f5b4b395b858612be7b1a3c3a39f81f0b90d353ea614ffa",
  "url": "https://osv.dev/vulnerability/CVE-2025-38457",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "net/sched: Abort __tc_modify_qdisc if parent class does not exist",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: net/sched: Abort __tc_modify_qdisc if parent class does not exist Lion's patch [1] revealed an ancient bug in the qdisc API. Whenever a user creates/modifies a qdisc specifying as a parent another qdisc, the qdisc API will, during grafting, detect that the user is not trying to attach to a class and reject. However grafting is performed after qdisc_create (and thus the qdiscs' init callback) is executed. In qdiscs that eventually call qdisc_tree_reduce_backlog during init or change (such as fq, hhf, choke, etc), an issue arises. For example, executing the following commands: sudo tc qdisc add dev lo root handle a: htb default 2 sudo tc qdisc add dev lo parent a: handle beef fq Qdiscs such as fq, hhf, choke, etc unconditionally invoke qdisc_tree_reduce_backlog() in their control path init() or change() which then causes a failure to find the child class; however, that does not stop the unconditional invocation of the assumed child qdisc's qlen_notify with a null class. All these qdiscs make the assumption that class is non-null. The solution is ensure that qdisc_leaf() which looks up the parent class, and is invoked prior to qdisc_create(), should return failure on not finding the class. In this patch, we leverage qdisc_leaf to return ERR_PTRs whenever the parentid doesn't correspond to a class, so that we can detect it earlier on and abort before qdisc_create is called. [1] https://lore.kernel.org/netdev/d912",
    "evidence": "osv:CVE-2025-38457",
    "products": [
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          {
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    "severity": "unknown",
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      ],
      "modified": "2026-10-28T02:30:23.895916264Z",
      "provider": "osv",
      "references": [
        "https://cert-portal.siemens.com/productcert/html/ssa-082556.html",
        "https://git.kernel.org/stable/c/23c165dde88eac405eebb59051ea1fe139a45803",
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        "https://git.kernel.org/stable/c/e28a383d6485c3bb51dc5953552f76c4dea33eea",
        "https://git.kernel.org/stable/c/ffdde7bf5a439aaa1955ebd581f5c64ab1533963",
        "https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html",
        "https://lists.debian.org/debian-lts-announce/2025/10/msg00008.html",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/38xxx/CVE-2025-38457.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2025-38457",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/38xxx/CVE-2025-38457.json"
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    "related_cves": [
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    "security_relevant": true,
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  "source_id": "core-osv",
  "document_id": 490344,
  "published_at": "2025-07-25T15:27:36.226+00:00"
}