{
  "schema_version": "1.6.1",
  "id": "RPI-CVE-2026-63918",
  "modified": "2026-10-09T11:39:44Z",
  "upstream": [
    "CVE-2026-63918"
  ],
  "published": "2026-07-19T14:56:29Z",
  "summary": "l2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname",
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nl2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname\n\nA reader in l2tp_session_get_by_ifname() can return a pointer to a\nsession whose refcount has reached zero. The getter takes its\nreference with plain refcount_inc(), but every other session getter\nin the same file (l2tp_v2_session_get, l2tp_v3_session_get, and the\ncorresponding _get_next variants) uses refcount_inc_not_zero()\nbecause the IDR/RCU lookup can race with refcount_dec_and_test() ->\nl2tp_session_free() -> kfree_rcu(). The ifname getter is the only\noutlier; the inconsistency was raised on-list after 979c017803c4\n(\"l2tp: use list_del_rcu in l2tp_session_unhash\").\n\nA reader inside rcu_read_lock_bh() that matches session->ifname can\nbe preempted between the strcmp() and the refcount_inc(). If the\nlast reference drops on another CPU in that window, the reader's\nrefcount_inc() runs on a counter that has reached zero. refcount_t\ncatches the addition-on-zero, prints \"refcount_t: addition on 0;\nuse-after-free\", saturates the counter, and returns the saturated\npointer to the caller. Session memory is held live by the in-flight\nRCU read section, but the kfree_rcu() callback queued from\nl2tp_session_free() will free it once the grace period closes; a\ncaller that dereferences the returned session past that point hits\na slab-use-after-free. On PREEMPT_RT local_bh_disable() is a per-CPU\nsleeping lock and the preemption window is real; on stock PREEMPT\nkernels local_bh_disable() is a preempt_count increment that closes\nthe cross-CPU race in practice (see below).\n\nUse refcount_inc_not_zero() and continue the list walk on failure,\nmatching the other session getters in the file. The ifname getter\nis the only session getter in net/l2tp/ that still uses the bare\nrefcount_inc() pattern; this change restores file-internal\nconsistency. The success path is unchanged.",
  "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-2026-63918"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/ee80455feffb9cb62b5b58715cabeff495e666b2"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/947013fd7c8c35dd5856557b215840098a3f67f8"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/782d60a6596aee9b29c2eecfa70033899278bf65"
    },
    {
      "type": "FIX",
      "url": "https://git.kernel.org/stable/c/05f95729ca844704d15e49ce14868af4b403b32b"
    }
  ],
  "database_specific": {
    "source": "https://git.kernel.org/pub/scm/linux/security/vulns.git",
    "upstream_modified": "2026-08-05T13:16:28Z",
    "severity": {
      "source": "kernel CNA",
      "cvss_v3_score": 7.8,
      "cvss_v3_severity": "High",
      "scenario": "AV:L - The vulnerable code is only reached via the L2TP genetlink management API (netlink syscall), not from the L2TP data-plane packet receive path. Attackers must issue local netlink commands such as SESSION_GET, SESSION_DELETE, or SESSION_MODIFY with L2TP_ATTR_IFNAME.\nAC:L - Exploitation requires racing session lookup-by-ifname against concurrent session deletion, and an attacker with CAP_NET_ADMIN can control both sides by issuing parallel netlink operations. On PREEMPT_RT kernels the preemption window between strcmp() and refcount_inc() is real and reliably winnable.\nPR:L - All affected genetlink commands are gated by GENL_UNS_ADMIN_PERM, requiring CAP_NET_ADMIN in the network namespace. That capability is obtainable by unprivileged users via user/network namespaces (unshare -Urn), not only by init-namespace root.\nUI:N - No victim interaction is required; the attacker triggers the race directly through netlink management commands without needing another user to mount, open, or configure anything.\nS:U - Impact is confined to kernel memory corruption and privilege escalation within the same kernel security domain. This is not a VM escape, sandbox boundary cross, or IOMMU bypass.\nC:H - The bug is a slab use-after-free on struct l2tp_session; returning and later dereferencing a session whose refcount reached zero enables reading freed kernel heap memory and leaking kernel pointers or other sensitive data.\nI:H - UAF on struct l2tp_session corrupts a heap object that contains function pointers (recv_skb, session_close) and is subsequently used by session_delete/modify/notify handlers, enabling arbitrary kernel write or code execution via heap grooming.\nA:H - Dereferencing the freed l2tp_session after the RCU grace period causes kernel oops or panic. Even failed exploitation attempts trigger refcount_t \"addition on 0; use-after-free\" warnings and can crash the system under load."
    }
  }
}