FKIE_CVE-2026-63918

Vulnerability from fkie_nvd - Published: 2026-07-19 16:17 - Updated: 2026-07-20 15:16
Severity
7.8 (High) - CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
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.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/05f95729ca844704d15e49ce14868af4b403b32b
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/782d60a6596aee9b29c2eecfa70033899278bf65
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/947013fd7c8c35dd5856557b215840098a3f67f8
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/ee80455feffb9cb62b5b58715cabeff495e666b2
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "net/l2tp/l2tp_core.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "ee80455feffb9cb62b5b58715cabeff495e666b2",
              "status": "affected",
              "version": "abe7a1a7d0b69e63b1bca5f9531023a52336784f",
              "versionType": "git"
            },
            {
              "lessThan": "947013fd7c8c35dd5856557b215840098a3f67f8",
              "status": "affected",
              "version": "abe7a1a7d0b69e63b1bca5f9531023a52336784f",
              "versionType": "git"
            },
            {
              "lessThan": "782d60a6596aee9b29c2eecfa70033899278bf65",
              "status": "affected",
              "version": "abe7a1a7d0b69e63b1bca5f9531023a52336784f",
              "versionType": "git"
            },
            {
              "lessThan": "05f95729ca844704d15e49ce14868af4b403b32b",
              "status": "affected",
              "version": "abe7a1a7d0b69e63b1bca5f9531023a52336784f",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "net/l2tp/l2tp_core.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.12"
            },
            {
              "lessThan": "6.12",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
              "version": "6.12.93",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.35",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.0.*",
              "status": "unaffected",
              "version": "7.0.12",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "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."
    }
  ],
  "id": "CVE-2026-63918",
  "lastModified": "2026-07-20T15:16:56.090",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "LOCAL",
          "availabilityImpact": "HIGH",
          "baseScore": 7.8,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "HIGH",
          "integrityImpact": "HIGH",
          "privilegesRequired": "LOW",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 1.8,
        "impactScore": 5.9,
        "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-07-19T16:17:09.917",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/05f95729ca844704d15e49ce14868af4b403b32b"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/782d60a6596aee9b29c2eecfa70033899278bf65"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/947013fd7c8c35dd5856557b215840098a3f67f8"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/ee80455feffb9cb62b5b58715cabeff495e666b2"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



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