{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2025-21920",
  "modified": "2026-10-07T17:22:01Z",
  "upstream": [
    "CVE-2025-21920"
  ],
  "published": "2025-04-01T15:40:16Z",
  "summary": "vlan: enforce underlying device type",
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nvlan: enforce underlying device type\n\nCurrently, VLAN devices can be created on top of non-ethernet devices.\n\nBesides the fact that it doesn't make much sense, this also causes a\nbug which leaks the address of a kernel function to usermode.\n\nWhen creating a VLAN device, we initialize GARP (garp_init_applicant)\nand MRP (mrp_init_applicant) for the underlying device.\n\nAs part of the initialization process, we add the multicast address of\neach applicant to the underlying device, by calling dev_mc_add.\n\n__dev_mc_add uses dev->addr_len to determine the length of the new\nmulticast address.\n\nThis causes an out-of-bounds read if dev->addr_len is greater than 6,\nsince the multicast addresses provided by GARP and MRP are only 6\nbytes long.\n\nThis behaviour can be reproduced using the following commands:\n\nip tunnel add gretest mode ip6gre local ::1 remote ::2 dev lo\nip l set up dev gretest\nip link add link gretest name vlantest type vlan id 100\n\nThen, the following command will display the address of garp_pdu_rcv:\n\nip maddr show | grep 01:80:c2:00:00:21\n\nFix the bug by enforcing the type of the underlying device during VLAN\ndevice initialization.",
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  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2025-21920"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/7f1564b2b2072b7aa1ac75350e9560a07c7a44fd"
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    },
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    "source": "https://git.kernel.org/pub/scm/linux/security/vulns.git",
    "upstream_modified": "2026-05-12T04:54:46Z"
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}