nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown

{
  "id": "15ac3583ba68bcbae15bc061b5af235c5320be234a97cc3c5f3a07c5f2828824",
  "url": "https://osv.dev/vulnerability/CVE-2026-89708",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown",
    "summary": "In the Linux kernel, the following vulnerability has been resolved: nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown After a DESTROY_SESSION the per-session teardown path can free a session while rpciod still holds an inflight callback rpc_task that dereferences clp->cl_cb_session. nfsd4_probe_callback_sync() flushes cl_callback_wq, but once nfsd4_run_cb_work() has called rpc_call_async() the rpc_task lives on rpciod; flushing the workqueue does not wait for it. rpc_shutdown_client() does drain rpciod tasks, but uses a 1-second wait_event_timeout — tasks stuck in rpc_delay() (e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain. destroy path rpciod ------------ ------ unhash_session(ses) nfsd4_probe_callback_sync(clp) flush_workqueue(cl_callback_wq) /* returns; rpc_task still live */ nfsd4_put_session_locked(ses) free_session(ses) -> kfree(ses) nfsd4_cb_sequence_done() reads cb_clp->cl_cb_session /* freed slab */ A second window exists in nfsd4_process_cb_update(). When __nfsd4_find_backchannel() returns NULL because unhash_session() has already removed the destroyed session from cl_sessions, setup_callback_client() takes the v4.1 early return so clp->cl_cb_session = ses never fires and the field retains a pointer to the about-to-be-freed session. Fix both by converting cl_cb_session to an RCU-protected pointer: - Move the cl_cb_session = ses assignment in setup_callback_client() to after rpc_create() succeeds, so it is only published when a wor",
    "evidence": "osv:CVE-2026-89708",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "6.12.111",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "6.18.50",
            "evidence": "/affected/1/ranges"
          },
          {
            "value": "7.2.4",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "critical",
    "structured": {
      "id": "CVE-2026-89708",
      "aliases": [
        "CVE-2026-89708"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
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                  "introduced": "dcbeaa68dbbdacbbb330a86c7fc95a28473fc209"
                },
                {
                  "fixed": "2775ec1d617e665eab71ce0f13ab1f7959713a2b"
                },
                {
                  "fixed": "f164eb52b6f3cbf40f07fe379f9f421f88e02f76"
                },
                {
                  "fixed": "13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25"
                },
                {
                  "fixed": "01c5d5f58a5db9b0ee5afba2e49d3157788687b2"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89708.json"
          }
        },
        {
          "ranges": [
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "2.6.38"
                },
                {
                  "fixed": "6.12.111"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.13.0"
                },
                {
                  "fixed": "6.18.50"
                }
              ]
            },
            {
              "type": "ECOSYSTEM",
              "events": [
                {
                  "introduced": "6.19.0"
                },
                {
                  "fixed": "7.2.4"
                }
              ]
            }
          ],
          "package": {
            "name": "Kernel",
            "ecosystem": "Linux"
          },
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89708.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:52.657394028Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/01c5d5f58a5db9b0ee5afba2e49d3157788687b2",
        "https://git.kernel.org/stable/c/13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25",
        "https://git.kernel.org/stable/c/2775ec1d617e665eab71ce0f13ab1f7959713a2b",
        "https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89708.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-89708",
        "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/2026/89xxx/CVE-2026-89708.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-89708",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 521474,
  "published_at": "2026-09-11T19:46:24.061+00:00"
}