CVE-2026-89696 (GCVE-0-2026-89696)
Vulnerability from cvelistv5
Published
2026-09-11 19:46
Modified
2026-10-19 08:51
Severity ?
7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Summary
In the Linux kernel, the following vulnerability has been resolved: nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref When CONFIG_NFSD_V4_2_INTER_SSC is enabled, nfsd4_putfh() can return success with fh_dentry and fh_export both NULL if fh_verify() returns nfserr_stale and putfh->no_verify is true. The NFSD4_FH_FOREIGN flag is set, but the compound dispatch loop only uses this flag to bypass the nfserr_nofilehandle check -- it does not prevent subsequent ops from running with a NULL fh_dentry. A remote client can exploit this by crafting a COMPOUND that includes an inter-SSC COPY (which causes check_if_stalefh_allowed() to set no_verify=true on the saved PUTFH) with an additional op inserted between the source PUTFH and SAVEFH. For example, SETATTR calls fh_want_write() which dereferences fh_export->ex_path.mnt without calling fh_verify() first, causing a NULL pointer dereference in the nfsd kthread. Fix this by gating the dispatch loop: when NFSD4_FH_FOREIGN is set and fh_dentry is NULL, only OP_SAVEFH (needed for the inter-SSC flow) and ops with ALLOWED_WITHOUT_FH (which don't need a resolved filehandle) may proceed. All other ops receive nfserr_stale, per RFC 7862 Section 15.2.3 which specifies that foreign filehandle validation is deferred to the consuming operation and NFS4ERR_STALE returned at that point.
References
Impacted products
Vendor Product Version
Linux Linux Version: b9e8638e3d9ed8334f1f7071e081860aac37e83e
Version: b9e8638e3d9ed8334f1f7071e081860aac37e83e
Version: b9e8638e3d9ed8334f1f7071e081860aac37e83e
Version: b9e8638e3d9ed8334f1f7071e081860aac37e83e
Version: b9e8638e3d9ed8334f1f7071e081860aac37e83e
Version: b9e8638e3d9ed8334f1f7071e081860aac37e83e
Version: b9e8638e3d9ed8334f1f7071e081860aac37e83e
Version: b9e8638e3d9ed8334f1f7071e081860aac37e83e
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/nfsd/nfs4proc.c"
          ],
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          "vendor": "Linux",
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          "product": "Linux",
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          "vendor": "Linux",
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              "version": "0",
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              "version": "5.10.270",
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              "status": "unaffected",
              "version": "5.15.221",
              "versionType": "semver"
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              "lessThanOrEqual": "6.1.*",
              "status": "unaffected",
              "version": "6.1.188",
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              "version": "6.6.157",
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              "lessThanOrEqual": "6.12.*",
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              "version": "7.2.4",
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              "status": "unaffected",
              "version": "7.3",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "cpeApplicability": [
        {
          "nodes": [
            {
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                  "versionEndExcluding": "5.10.270",
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                  "vulnerable": true
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                  "vulnerable": true
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              ],
              "negate": false,
              "operator": "OR"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref\n\nWhen CONFIG_NFSD_V4_2_INTER_SSC is enabled, nfsd4_putfh() can return\nsuccess with fh_dentry and fh_export both NULL if fh_verify() returns\nnfserr_stale and putfh-\u003eno_verify is true. The NFSD4_FH_FOREIGN flag\nis set, but the compound dispatch loop only uses this flag to bypass\nthe nfserr_nofilehandle check -- it does not prevent subsequent ops\nfrom running with a NULL fh_dentry.\n\nA remote client can exploit this by crafting a COMPOUND that includes\nan inter-SSC COPY (which causes check_if_stalefh_allowed() to set\nno_verify=true on the saved PUTFH) with an additional op inserted\nbetween the source PUTFH and SAVEFH. For example, SETATTR calls\nfh_want_write() which dereferences fh_export-\u003eex_path.mnt without\ncalling fh_verify() first, causing a NULL pointer dereference in the\nnfsd kthread.\n\nFix this by gating the dispatch loop: when NFSD4_FH_FOREIGN is set\nand fh_dentry is NULL, only OP_SAVEFH (needed for the inter-SSC flow)\nand ops with ALLOWED_WITHOUT_FH (which don\u0027t need a resolved\nfilehandle) may proceed. All other ops receive nfserr_stale, per\nRFC 7862 Section 15.2.3 which specifies that foreign filehandle\nvalidation is deferred to the consuming operation and NFS4ERR_STALE\nreturned at that point."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "baseScore": 7.5,
            "baseSeverity": "HIGH",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - The bug is in nfsd NFSv4 COMPOUND dispatch and is triggered by a crafted NFSv4.2 COMPOUND over TCP/2049 (PUTFH of a stale fh, then SETATTR, with an inter-SSC COPY in the same compound). No local access is required.\nAC:L - After EXCHANGE_ID/CREATE_SESSION, the attacker fully controls the compound that marks the source PUTFH no_verify and inserts SETATTR before SAVEFH, so fh_want_write hits NULL fh_export deterministically with no race or layout dependency.\nPR:N - AUTH_SYS (the usual nfsd configuration) verifies no secret; a client just asserts a UID to create a session. SETATTR then runs on the FOREIGN fh without fh_verify, so no export access check or other credential gate is reached.\nUI:N - The attacker sends the COMPOUND directly to nfsd; no administrator or victim user action is required at exploitation time.\nS:U - The NULL dereference oopses the nfsd kthread in the host kernel\u0027s own authority and does not cross a VM, IOMMU, or sandbox boundary.\nC:N - After the stale FOREIGN PUTFH, fh_export and fh_dentry are NULL rather than dangling or attacker-controlled, so the fault is on the first load of fh_export-\u003eex_path.mnt with no disclosure.\nI:N - This is a pure NULL pointer dereference in fh_want_write with no write to attacker-influenced memory and no control-flow hijack primitive.\nA:H - The NULL dereference oopses the nfsd kthread and can be replayed to exhaust nfsd threads, denying NFS service; with panic_on_oops it panics the host."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-10-19T08:51:09.479Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/ffdc844e673d2bcb2af8c8c3eef8cdea59d0bf32"
        },
        {
          "url": "https://git.kernel.org/stable/c/00c84f4eec715e501efc080a5670114189e42507"
        },
        {
          "url": "https://git.kernel.org/stable/c/2d9846fd1c767920fddd4bde30eb35cd2969df13"
        },
        {
          "url": "https://git.kernel.org/stable/c/977e6f006a7a3ffc4216ae6034f768f8de6fd138"
        },
        {
          "url": "https://git.kernel.org/stable/c/35f248bd40b47b229d4999581df45b97daadd977"
        },
        {
          "url": "https://git.kernel.org/stable/c/311f7d926630940650447cbd1c932b076b40a6c4"
        },
        {
          "url": "https://git.kernel.org/stable/c/bf4d338dc8625d70c7f2cb0657d66851a7ac9154"
        },
        {
          "url": "https://git.kernel.org/stable/c/c59738a00aa51b16adc1b5ceb7c80877168efb4d"
        }
      ],
      "title": "nfsd: block non-SAVEFH ops after FOREIGN PUTFH to prevent NULL deref",
      "x_generator": {
        "engine": "bippy-1.2.0"
      }
    }
  },
  "cveMetadata": {
    "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
    "assignerShortName": "Linux",
    "cveId": "CVE-2026-89696",
    "datePublished": "2026-09-11T19:46:15.030Z",
    "dateReserved": "2026-09-11T19:38:34.750Z",
    "dateUpdated": "2026-10-19T08:51:09.479Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}


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