perf: Fix refcount warning on event->mmap_count increment

{
  "id": "20a60b814fcd31b48ee71afaa7ae345eb9cc45040cafc27e80684b003c1556b0",
  "url": "https://osv.dev/vulnerability/CVE-2026-23127",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "perf: Fix refcount warning on event->mmap_count increment",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: perf: Fix refcount warning on event->mmap_count increment When calling refcount_inc(&event->mmap_count) inside perf_mmap_rb(), the following warning is triggered: refcount_t: addition on 0; use-after-free. WARNING: lib/refcount.c:25 PoC: struct perf_event_attr attr = {0}; int fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0); mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); int victim = syscall(__NR_perf_event_open, &attr, 0, -1, fd, PERF_FLAG_FD_OUTPUT); mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, victim, 0); This occurs when creating a group member event with the flag PERF_FLAG_FD_OUTPUT. The group leader should be mmap-ed and then mmap-ing the event triggers the warning. Since the event has copied the output_event in perf_event_set_output(), event->rb is set. As a result, perf_mmap_rb() calls refcount_inc(&event->mmap_count) when event->mmap_count = 0. Disallow the case when event->mmap_count = 0. This also prevents two events from updating the same user_page.",
    "evidence": "osv:CVE-2026-23127",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "6.18.8",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "unknown",
    "structured": {
      "id": "CVE-2026-23127",
      "aliases": [
        "CVE-2026-23127"
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "448f97fba9013ffa13f5dd82febd18836b189499"
                },
                {
                  "fixed": "23c0e4bd93d0b250775162faf456470485ac9fc7"
                },
                {
                  "fixed": "d06bf78e55d5159c1b00072e606ab924ffbbad35"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-23127.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.18.0"
                },
                {
                  "fixed": "6.18.8"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-23127.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:24.307713933Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/23c0e4bd93d0b250775162faf456470485ac9fc7",
        "https://git.kernel.org/stable/c/d06bf78e55d5159c1b00072e606ab924ffbbad35",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/23xxx/CVE-2026-23127.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-23127",
        "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"
      ],
      "provider_metadata": {
        "cna_assigner": "Linux",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/23xxx/CVE-2026-23127.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-23127",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": null
  },
  "source_id": "core-osv",
  "document_id": 490826,
  "published_at": "2026-02-14T15:09:56.237+00:00"
}