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CVE-2026-63918 (GCVE-0-2026-63918)

Vulnerability from cvelistv5 – Published: 2026-07-19 14:55 – Updated: 2026-08-05 12:37
VLAI
Title
l2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname
Summary
In the Linux kernel, the following vulnerability has been resolved: l2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname A reader in l2tp_session_get_by_ifname() can return a pointer to a session whose refcount has reached zero. The getter takes its reference with plain refcount_inc(), but every other session getter in the same file (l2tp_v2_session_get, l2tp_v3_session_get, and the corresponding _get_next variants) uses refcount_inc_not_zero() because the IDR/RCU lookup can race with refcount_dec_and_test() -> l2tp_session_free() -> kfree_rcu(). The ifname getter is the only outlier; the inconsistency was raised on-list after 979c017803c4 ("l2tp: use list_del_rcu in l2tp_session_unhash"). A reader inside rcu_read_lock_bh() that matches session->ifname can be preempted between the strcmp() and the refcount_inc(). If the last reference drops on another CPU in that window, the reader's refcount_inc() runs on a counter that has reached zero. refcount_t catches the addition-on-zero, prints "refcount_t: addition on 0; use-after-free", saturates the counter, and returns the saturated pointer to the caller. Session memory is held live by the in-flight RCU read section, but the kfree_rcu() callback queued from l2tp_session_free() will free it once the grace period closes; a caller that dereferences the returned session past that point hits a slab-use-after-free. On PREEMPT_RT local_bh_disable() is a per-CPU sleeping lock and the preemption window is real; on stock PREEMPT kernels local_bh_disable() is a preempt_count increment that closes the cross-CPU race in practice (see below). Use refcount_inc_not_zero() and continue the list walk on failure, matching the other session getters in the file. The ifname getter is the only session getter in net/l2tp/ that still uses the bare refcount_inc() pattern; this change restores file-internal consistency. The success path is unchanged.
Severity
7.8 (High)
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: abe7a1a7d0b69e63b1bca5f9531023a52336784f , < ee80455feffb9cb62b5b58715cabeff495e666b2 (git)
Affected: abe7a1a7d0b69e63b1bca5f9531023a52336784f , < 947013fd7c8c35dd5856557b215840098a3f67f8 (git)
Affected: abe7a1a7d0b69e63b1bca5f9531023a52336784f , < 782d60a6596aee9b29c2eecfa70033899278bf65 (git)
Affected: abe7a1a7d0b69e63b1bca5f9531023a52336784f , < 05f95729ca844704d15e49ce14868af4b403b32b (git)
guessed Create a notification for this product.
Linux Linux Affected: 6.12
Unaffected: 0 , < 6.12 (semver)
Unaffected: 6.12.93 , ≤ 6.12.* (semver)
Unaffected: 6.18.35 , ≤ 6.18.* (semver)
Unaffected: 7.0.12 , ≤ 7.0.* (semver)
Unaffected: 7.1 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
Show details on NVD website
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{
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          "product": "Linux",
          "programFiles": [
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                  "vulnerable": true
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                {
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          "lang": "en",
          "value": "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() -\u003e\nl2tp_session_free() -\u003e 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-\u003eifname can\nbe preempted between the strcmp() and the refcount_inc(). If the\nlast reference drops on another CPU in that window, the reader\u0027s\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."
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            "version": "3.1"
          },
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              "value": "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."
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        "shortName": "Linux"
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-63918",
    "datePublished": "2026-07-19T14:55:21.414Z",
    "dateReserved": "2026-07-19T07:54:57.020Z",
    "dateUpdated": "2026-08-05T12:37:12.656Z",
    "state": "PUBLISHED"
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    "redhat_vex": {
      "aggregate_severity": "Moderate",
      "current_release_date": "2026-07-21T21:23:08+00:00",
      "cve": "CVE-2026-63918",
      "id": "CVE-2026-63918",
      "initial_release_date": "2026-07-19T00:00:00+00:00",
      "product_status:known_affected": "76",
      "product_status:known_not_affected": "198",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: l2tp: use refcount_inc_not_zero in l2tp_session_get_by_ifname",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-63918.json",
      "version": "3"
    }
  }
}



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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
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