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CVE-2026-89708 (GCVE-0-2026-89708)

Vulnerability from cvelistv5 – Published: 2026-09-11 19:46 – Updated: 2026-09-13 06:33
VLAI
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 working backchannel exists. Clear cl_cb_session on the error return in nfsd4_process_cb_update(). Both stores use rcu_assign_pointer(). - Annotate cl_cb_session with __rcu. All rpciod-side readers use rcu_read_lock()/rcu_dereference() and check for NULL, bailing to the appropriate error or requeue path: encode_cb_sequence4args(), decode_cb_sequence4resok(), nfsd41_cb_get_slot(), nfsd41_cb_release_slot(), nfsd4_cb_prepare(), and nfsd4_cb_sequence_done(). - Switch __free_session() from kfree() to kfree_rcu() so the session slab is not reclaimed until after an RCU grace period, guaranteeing that rpciod readers inside rcu_read_lock() never dereference freed memory. - Pass the session pointer to the nfsd_cb_seq_status and nfsd_cb_free_slot tracepoints instead of having them re-read cl_cb_session. - nfsd4_cb_prepare() calls rpc_exit() when the session is NULL, routing through the done/release path to requeue the callback.
Severity
9.8 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: dcbeaa68dbbdacbbb330a86c7fc95a28473fc209 , < f164eb52b6f3cbf40f07fe379f9f421f88e02f76 (git)
Affected: dcbeaa68dbbdacbbb330a86c7fc95a28473fc209 , < 13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25 (git)
Affected: dcbeaa68dbbdacbbb330a86c7fc95a28473fc209 , < 01c5d5f58a5db9b0ee5afba2e49d3157788687b2 (git)
guessed Create a notification for this product.
Linux Linux Affected: 2.6.38
Unaffected: 0 , < 2.6.38 (semver)
Unaffected: 6.18.50 , ≤ 6.18.* (semver)
Unaffected: 7.2.4 , ≤ 7.2.* (semver)
Unaffected: 7.3-rc1 , ≤ * (original_commit_for_fix)
guessed Create a notification for this product.
Show details on NVD website
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/nfsd/nfs4callback.c",
            "fs/nfsd/nfs4state.c",
            "fs/nfsd/state.h",
            "fs/nfsd/trace.h"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "status": "affected",
              "version": "dcbeaa68dbbdacbbb330a86c7fc95a28473fc209",
              "versionType": "git"
            },
            {
              "lessThan": "13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25",
              "status": "affected",
              "version": "dcbeaa68dbbdacbbb330a86c7fc95a28473fc209",
              "versionType": "git"
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            "fs/nfsd/state.h",
            "fs/nfsd/trace.h"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "status": "affected",
              "version": "2.6.38"
            },
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              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
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            },
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              "status": "unaffected",
              "version": "7.2.4",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.3-rc1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
              "cpeMatch": [
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "6.18.50",
                  "versionStartIncluding": "2.6.38",
                  "vulnerable": true
                },
                {
                  "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
                  "versionEndExcluding": "7.2.4",
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                  "vulnerable": true
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                  "vulnerable": true
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              "operator": "OR"
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      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown\n\nAfter a DESTROY_SESSION the per-session teardown path can free a\nsession while rpciod still holds an inflight callback rpc_task that\ndereferences clp-\u003ecl_cb_session.  nfsd4_probe_callback_sync() flushes\ncl_callback_wq, but once nfsd4_run_cb_work() has called\nrpc_call_async() the rpc_task lives on rpciod; flushing the workqueue\ndoes not wait for it.  rpc_shutdown_client() does drain rpciod tasks,\nbut uses a 1-second wait_event_timeout \u2014 tasks stuck in rpc_delay()\n(e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain.\n\n    destroy path                       rpciod\n    ------------                       ------\n    unhash_session(ses)\n    nfsd4_probe_callback_sync(clp)\n      flush_workqueue(cl_callback_wq)\n      /* returns; rpc_task still live */\n    nfsd4_put_session_locked(ses)\n    free_session(ses) -\u003e kfree(ses)\n                                       nfsd4_cb_sequence_done()\n                                         reads cb_clp-\u003ecl_cb_session\n                                         /* freed slab */\n\nA second window exists in nfsd4_process_cb_update().  When\n__nfsd4_find_backchannel() returns NULL because unhash_session() has\nalready removed the destroyed session from cl_sessions,\nsetup_callback_client() takes the v4.1 early return so\nclp-\u003ecl_cb_session = ses never fires and the field retains a pointer\nto the about-to-be-freed session.\n\nFix both by converting cl_cb_session to an RCU-protected pointer:\n\n  - Move the cl_cb_session = ses assignment in setup_callback_client()\n    to after rpc_create() succeeds, so it is only published when a\n    working backchannel exists.  Clear cl_cb_session on the error\n    return in nfsd4_process_cb_update().  Both stores use\n    rcu_assign_pointer().\n\n  - Annotate cl_cb_session with __rcu.  All rpciod-side readers use\n    rcu_read_lock()/rcu_dereference() and check for NULL, bailing to\n    the appropriate error or requeue path:\n    encode_cb_sequence4args(), decode_cb_sequence4resok(),\n    nfsd41_cb_get_slot(), nfsd41_cb_release_slot(),\n    nfsd4_cb_prepare(), and nfsd4_cb_sequence_done().\n\n  - Switch __free_session() from kfree() to kfree_rcu() so the\n    session slab is not reclaimed until after an RCU grace period,\n    guaranteeing that rpciod readers inside rcu_read_lock() never\n    dereference freed memory.\n\n  - Pass the session pointer to the nfsd_cb_seq_status and\n    nfsd_cb_free_slot tracepoints instead of having them re-read\n    cl_cb_session.\n\n  - nfsd4_cb_prepare() calls rpc_exit() when the session is NULL,\n    routing through the done/release path to requeue the callback."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 9.8,
            "baseSeverity": "CRITICAL",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - The UAF is in nfsd NFSv4.1 session teardown (nfsd4_destroy_session) and the rpciod callback path (nfsd4_cb_sequence_done). DESTROY_SESSION and the inflight CB_COMPOUND run over TCP/2049, so a remote NFS client reaches the bug with no local access.\nAC:L - The attacker controls both sides: they keep a callback rpc_task on rpciod (e.g. NFS4ERR_DELAY so rpc_delay is 2s) and send DESTROY_SESSION. flush_workqueue does not wait for rpciod, and rpc_shutdown_client\u0027s 1s drain can expire first, so the window is attacker-driven and retryable.\nPR:N - Typical nfsd exports use AUTH_SYS/AUTH_NULL, so RPC identities are self-asserted. EXCHANGE_ID, CREATE_SESSION with a backchannel, OPEN, and DESTROY_SESSION succeed for any host allowed by the export; nfsd4_mach_creds_match is a no-op under default SP4_NONE. No local account is required.\nUI:N - The attacking NFS client issues CREATE_SESSION, OPEN/CB_RECALL traffic, NFS4ERR_DELAY replies, and DESTROY_SESSION itself. No victim on the server must mount a filesystem, open a file, or take any other action.\nS:U - The use-after-free corrupts a kmalloc\u0027d nfsd4_session in the host nfsd kernel. Impact stays in that kernel security authority and does not cross a VM, IOMMU, or sandbox boundary.\nC:H - After free_session kfree\u0027s the session, rpciod still reads se_sessionid and se_cb_seq_nr in encode_cb_sequence4args and decode_cb_sequence4resok. Reclaim of that slab lets the attacker disclose kernel memory via the callback they receive. Per kernel guidance a UAF is C:H.\nI:H - The same dangling cl_cb_session is written by update_cb_slot_table (attacker-controlled target_highest_slotid), grab_slot/nfsd41_cb_release_slot (se_cb_slot_avail and se_lock), and nfsd4_cb_sequence_done (se_cb_seq_nr++). That is a UAF write primitive for heap corruption and control-flow hijack.\nA:H - Use-after-free of struct nfsd4_session from an rpciod callback oopses or panics the NFS server even without full exploitation, and the attacker can repeat DESTROY_SESSION against inflight callbacks to deny service."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-09-13T06:33:31.107Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76"
        },
        {
          "url": "https://git.kernel.org/stable/c/13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25"
        },
        {
          "url": "https://git.kernel.org/stable/c/01c5d5f58a5db9b0ee5afba2e49d3157788687b2"
        }
      ],
      "title": "nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown",
      "x_generator": {
        "engine": "bippy-1.2.0"
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    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-89708",
    "datePublished": "2026-09-11T19:46:24.061Z",
    "dateReserved": "2026-09-11T19:38:34.751Z",
    "dateUpdated": "2026-09-13T06:33:31.107Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
    "epss": {
      "cve": "CVE-2026-89708",
      "date": "2026-09-17",
      "epss": "0.00435",
      "percentile": "0.37146"
    },
    "microsoft_vex": {
      "current_release_date": "2026-09-14T14:49:36.000Z",
      "cve": "CVE-2026-89708",
      "id": "msrc_CVE-2026-89708",
      "initial_release_date": "2026-09-13T01:03:39.000Z",
      "product_status:known_affected": "1",
      "source": "Microsoft CSAF VEX",
      "status": "final",
      "title": "nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown",
      "url": "https://msrc.microsoft.com/csaf/vex/2026/msrc_cve-2026-89708.json",
      "version": "2"
    },
    "nvd": {
      "cve": {
        "affected": [
          {
            "affectedData": [
              {
                "defaultStatus": "unaffected",
                "product": "Linux",
                "programFiles": [
                  "fs/nfsd/nfs4callback.c",
                  "fs/nfsd/nfs4state.c",
                  "fs/nfsd/state.h",
                  "fs/nfsd/trace.h"
                ],
                "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                "vendor": "Linux",
                "versions": [
                  {
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                    "versionType": "git"
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                    "versionType": "git"
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                    "versionType": "git"
                  }
                ]
              },
              {
                "defaultStatus": "affected",
                "product": "Linux",
                "programFiles": [
                  "fs/nfsd/nfs4callback.c",
                  "fs/nfsd/nfs4state.c",
                  "fs/nfsd/state.h",
                  "fs/nfsd/trace.h"
                ],
                "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                "vendor": "Linux",
                "versions": [
                  {
                    "status": "affected",
                    "version": "2.6.38"
                  },
                  {
                    "lessThan": "2.6.38",
                    "status": "unaffected",
                    "version": "0",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "6.18.*",
                    "status": "unaffected",
                    "version": "6.18.50",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "7.2.*",
                    "status": "unaffected",
                    "version": "7.2.4",
                    "versionType": "semver"
                  },
                  {
                    "lessThanOrEqual": "*",
                    "status": "unaffected",
                    "version": "7.3-rc1",
                    "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\nnfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown\n\nAfter a DESTROY_SESSION the per-session teardown path can free a\nsession while rpciod still holds an inflight callback rpc_task that\ndereferences clp-\u003ecl_cb_session.  nfsd4_probe_callback_sync() flushes\ncl_callback_wq, but once nfsd4_run_cb_work() has called\nrpc_call_async() the rpc_task lives on rpciod; flushing the workqueue\ndoes not wait for it.  rpc_shutdown_client() does drain rpciod tasks,\nbut uses a 1-second wait_event_timeout \u2014 tasks stuck in rpc_delay()\n(e.g. 2-second NFS4ERR_DELAY retries) can outlive the drain.\n\n    destroy path                       rpciod\n    ------------                       ------\n    unhash_session(ses)\n    nfsd4_probe_callback_sync(clp)\n      flush_workqueue(cl_callback_wq)\n      /* returns; rpc_task still live */\n    nfsd4_put_session_locked(ses)\n    free_session(ses) -\u003e kfree(ses)\n                                       nfsd4_cb_sequence_done()\n                                         reads cb_clp-\u003ecl_cb_session\n                                         /* freed slab */\n\nA second window exists in nfsd4_process_cb_update().  When\n__nfsd4_find_backchannel() returns NULL because unhash_session() has\nalready removed the destroyed session from cl_sessions,\nsetup_callback_client() takes the v4.1 early return so\nclp-\u003ecl_cb_session = ses never fires and the field retains a pointer\nto the about-to-be-freed session.\n\nFix both by converting cl_cb_session to an RCU-protected pointer:\n\n  - Move the cl_cb_session = ses assignment in setup_callback_client()\n    to after rpc_create() succeeds, so it is only published when a\n    working backchannel exists.  Clear cl_cb_session on the error\n    return in nfsd4_process_cb_update().  Both stores use\n    rcu_assign_pointer().\n\n  - Annotate cl_cb_session with __rcu.  All rpciod-side readers use\n    rcu_read_lock()/rcu_dereference() and check for NULL, bailing to\n    the appropriate error or requeue path:\n    encode_cb_sequence4args(), decode_cb_sequence4resok(),\n    nfsd41_cb_get_slot(), nfsd41_cb_release_slot(),\n    nfsd4_cb_prepare(), and nfsd4_cb_sequence_done().\n\n  - Switch __free_session() from kfree() to kfree_rcu() so the\n    session slab is not reclaimed until after an RCU grace period,\n    guaranteeing that rpciod readers inside rcu_read_lock() never\n    dereference freed memory.\n\n  - Pass the session pointer to the nfsd_cb_seq_status and\n    nfsd_cb_free_slot tracepoints instead of having them re-read\n    cl_cb_session.\n\n  - nfsd4_cb_prepare() calls rpc_exit() when the session is NULL,\n    routing through the done/release path to requeue the callback."
          }
        ],
        "id": "CVE-2026-89708",
        "lastModified": "2026-09-13T07:17:36.477",
        "metrics": {
          "cvssMetricV31": [
            {
              "cvssData": {
                "attackComplexity": "LOW",
                "attackVector": "NETWORK",
                "availabilityImpact": "HIGH",
                "baseScore": 9.8,
                "baseSeverity": "CRITICAL",
                "confidentialityImpact": "HIGH",
                "integrityImpact": "HIGH",
                "privilegesRequired": "NONE",
                "scope": "UNCHANGED",
                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
                "version": "3.1"
              },
              "exploitabilityScore": 3.9,
              "impactScore": 5.9,
              "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
              "type": "Secondary"
            }
          ]
        },
        "published": "2026-09-11T20:19:58.140",
        "references": [
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/01c5d5f58a5db9b0ee5afba2e49d3157788687b2"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/13bdd486c3aad4fc19e6d8b9c3556a4b4c190b25"
          },
          {
            "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
            "url": "https://git.kernel.org/stable/c/f164eb52b6f3cbf40f07fe379f9f421f88e02f76"
          }
        ],
        "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "vulnStatus": "Received"
      }
    },
    "redhat_vex": {
      "aggregate_severity": "Important",
      "current_release_date": "2026-09-15T13:22:55+00:00",
      "cve": "CVE-2026-89708",
      "id": "CVE-2026-89708",
      "initial_release_date": "2026-09-11T00:00:00+00:00",
      "product_status:known_affected": "262",
      "product_status:under_investigation": "14",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: nfsd: RCU-protect cl_cb_session to fix use-after-free on session teardown",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89708.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "important",
      "current_release_date": "2026-09-15T23:59:43Z",
      "cve": "CVE-2026-89708",
      "id": "CVE-2026-89708",
      "initial_release_date": "2026-09-12T16:23:15Z",
      "product_status:known_affected": "353",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-89708",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-89708.json",
      "version": "4"
    }
  }
}



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  • Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


Detection rules are retrieved from Rulezet.


Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.