{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2024-49940",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2024-49940"
  ],
  "published": "2024-10-21T18:02:47Z",
  "summary": "l2tp: prevent possible tunnel refcount underflow",
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nl2tp: prevent possible tunnel refcount underflow\n\nWhen a session is created, it sets a backpointer to its tunnel. When\nthe session refcount drops to 0, l2tp_session_free drops the tunnel\nrefcount if session->tunnel is non-NULL. However, session->tunnel is\nset in l2tp_session_create, before the tunnel refcount is incremented\nby l2tp_session_register, which leaves a small window where\nsession->tunnel is non-NULL when the tunnel refcount hasn't been\nbumped.\n\nMoving the assignment to l2tp_session_register is trivial but\nl2tp_session_create calls l2tp_session_set_header_len which uses\nsession->tunnel to get the tunnel's encap. Add an encap arg to\nl2tp_session_set_header_len to avoid using session->tunnel.\n\nIf l2tpv3 sessions have colliding IDs, it is possible for\nl2tp_v3_session_get to race with l2tp_session_register and fetch a\nsession which doesn't yet have session->tunnel set. Add a check for\nthis case.",
  "severity": [
    {
      "type": "CVSS_V3",
      "score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H"
    }
  ],
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  "references": [
    {
      "type": "ADVISORY",
      "url": "https://www.cve.org/CVERecord?id=CVE-2024-49940"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/f7415e60c25a6108cd7955a20b2e66b6251ffe02"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/24256415d18695b46da06c93135f5b51c548b950"
    }
  ],
  "database_specific": {
    "source": "https://git.kernel.org/pub/scm/linux/security/vulns.git",
    "upstream_modified": "2026-08-05T11:52:36Z",
    "severity": {
      "source": "kernel CNA",
      "cvss_v3_score": 7.8,
      "cvss_v3_severity": "High",
      "scenario": "AV:L - The refcount underflow is triggered by local session creation \u2014 either the L2TP generic-netlink `L2TP_CMD_SESSION_CREATE` command or a `connect()` on an `AF_PPPOX`/`PX_PROTO_OL2TP` socket. A remote peer cannot itself drive `l2tp_session_create()`/`l2tp_session_register()`, though remote packets in `l2tp_udp_encap_recv()` are what subsequently dereference the prematurely freed tunnel.\nAC:L - The attacker fully controls both sides: they choose the session ID, so they can deterministically force `l2tp_session_register()` to fail (`-EEXIST` on a colliding L2TPv3 ID, or `-ENODEV`), and they can run concurrent create/teardown threads to widen the window between `l2tp_session_create()` and the tunnel refcount bump. No condition outside attacker influence is required, and L2TP is a stock distro module.\nPR:L - `pppol2tp_create()`/`pppox_create()` perform no capability check, so a plain unprivileged local user can reach `l2tp_session_create()` and `l2tp_session_register()` through `connect()`. The netlink path is `GENL_UNS_ADMIN_PERM` (CAP_NET_ADMIN against the netns user_ns), which any unprivileged user obtains via `unshare -Urn`.\nUI:N - The attacker performs the entire sequence \u2014 tunnel setup, session creation with a colliding ID, and session teardown \u2014 from their own process. No victim action or cooperating user is involved.\nS:U - The underflow and the resulting use-after-free are confined to the kernel's own l2tp objects within the same security authority; no VM, IOMMU, or sandbox boundary is crossed.\nC:H - The dropped-too-early tunnel reference frees `struct l2tp_tunnel` while `session->tunnel` and the datapath still reference it, giving a use-after-free on a kmalloc'd object that can be reclaimed with attacker-sprayed data and then read back (e.g. `tunnel->sock`, `tunnel->encap`, `tunnel->version` used in `l2tp_recv_common()`/`l2tp_xmit_skb()`), enabling kernel memory disclosure.\nI:H - A use-after-free on the tunnel object lets an attacker reoccupy the freed slab and control fields such as `tunnel->sock`, which the transmit and receive paths dereference and pass to socket operations, yielding a write/control-flow-hijack primitive. Additionally the extra `l2tp_tunnel_dec_refcount()` corrupts kernel reference-counting state itself.\nA:H - Premature tunnel free reliably produces an oops/panic when the still-live session or a received L2TP packet dereferences the dangling `session->tunnel`, and the `refcount_t` underflow itself raises a WARN that panics on `panic_on_warn` systems."
    }
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}