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CVE-2026-89678 (GCVE-0-2026-89678)

Vulnerability from cvelistv5 – Published: 2026-09-11 19:46 – Updated: 2026-09-13 06:33
VLAI
Title
nfsd: fix partial-write detection in nfsd_direct_write
Summary
In the Linux kernel, the following vulnerability has been resolved: nfsd: fix partial-write detection in nfsd_direct_write nfsd_direct_write() walks a list of write segments and, after each vfs_iocb_iter_write(), tries to detect a short write so the loop can stop before placing the next segment at a wrong file offset: host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter); if (host_err < 0) return host_err; *cnt += host_err; if (host_err < segments[i].iter.count) break; /* partial write */ vfs_iocb_iter_write() runs the iter through ->write_iter(), which advances the iter by the number of bytes written. By the time the check runs, segments[i].iter.count is the residual, not the original request length: before write_iter: iter.count == original_len after write_iter: iter.count == original_len - host_err The condition then reduces to host_err < original_len - host_err, so the break fires only when less than half of the segment was written. Any short write completing between 50% and 99% of the segment slips through; the loop advances to the next segment with kiocb->ki_pos only bumped by the short amount, writing the next segment's payload at the wrong offset and over-reporting *cnt to the NFS client. Snapshot the segment's byte count before the write and compare host_err against that snapshot so any short write breaks the loop.
Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Assigner
References
Impacted products
Vendor Product Version CPE status
Linux Linux Affected: 06c5c97293e3fca99ce15da157068edf45a7c6e4 , < fa6590dfd16ab55f03b658b079072ace3504825e (git)
Affected: 06c5c97293e3fca99ce15da157068edf45a7c6e4 , < 250ec14932d5cfe102f68a57892bb566eee7f83e (git)
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Linux Linux Affected: 6.19
Unaffected: 0 , < 6.19 (semver)
Unaffected: 7.2.4 , ≤ 7.2.* (semver)
Unaffected: 7.3-rc1 , ≤ * (original_commit_for_fix)
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{
  "containers": {
    "cna": {
      "affected": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "fs/nfsd/vfs.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
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              "status": "affected",
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                  "vulnerable": true
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          "lang": "en",
          "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: fix partial-write detection in nfsd_direct_write\n\nnfsd_direct_write() walks a list of write segments and, after each\nvfs_iocb_iter_write(), tries to detect a short write so the loop can\nstop before placing the next segment at a wrong file offset:\n\n    host_err = vfs_iocb_iter_write(file, kiocb, \u0026segments[i].iter);\n    if (host_err \u003c 0)\n            return host_err;\n    *cnt += host_err;\n    if (host_err \u003c segments[i].iter.count)\n            break;\t/* partial write */\n\nvfs_iocb_iter_write() runs the iter through -\u003ewrite_iter(), which\nadvances the iter by the number of bytes written. By the time the\ncheck runs, segments[i].iter.count is the residual, not the original\nrequest length:\n\n    before write_iter: iter.count == original_len\n    after  write_iter: iter.count == original_len - host_err\n\nThe condition then reduces to host_err \u003c original_len - host_err, so\nthe break fires only when less than half of the segment was written.\nAny short write completing between 50% and 99% of the segment slips\nthrough; the loop advances to the next segment with kiocb-\u003eki_pos\nonly bumped by the short amount, writing the next segment\u0027s payload\nat the wrong offset and over-reporting *cnt to the NFS client.\n\nSnapshot the segment\u0027s byte count before the write and compare\nhost_err against that snapshot so any short write breaks the loop."
        }
      ],
      "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:H/A:N",
            "version": "3.1"
          },
          "scenarios": [
            {
              "lang": "en",
              "value": "AV:N - nfsd_direct_write() is reached from NFSv2/v3 WRITE (nfsd_proc_write/nfsd3_proc_write) and NFSv4 WRITE (nfsd4_write) in the in-kernel NFS server via svc_recv/svc_process over TCP/UDP 2049, so a remote client supplies the payload, offset, and length on the wire.\nAC:L - On nfsd hosts with io_cache_write=DIRECT, the attacker chooses WRITE offset and size to force a prefix/middle/suffix split (nsegs\u003e=2) and can induce a 50-99% short write via quota, ENOSPC, or a concurrent truncate they also send; no race or layout outside their control is required.\nPR:N - Typical nfsd deployments accept AUTH_SYS/AUTH_NULL with IP-based export ACLs, so a network peer can MOUNT/LOOKUP, spoof an owner UID, and issue WRITE without cryptographic authentication or a local account on the NFS server.\nUI:N - The attacking NFS client sends the WRITE RPC itself; no separate victim user or administrator action on the server is required at attack time beyond nfsd already exporting a writable filesystem with direct writes enabled.\nS:U - Misplaced writes and the over-reported WRITE count affect only file data on the host running nfsd, remaining in that kernel/filesystem security authority without crossing a VM, IOMMU, or sandbox boundary.\nC:N - The bug only writes the client\u0027s own payload at a shifted file offset and over-reports the WRITE byte count; it does not read kernel memory or other inodes and is not a use-after-free or out-of-bounds read.\nI:H - After a 50-99% short write the loop continues and places the next segment at ki_pos advanced only by the short count, silently storing payload at the wrong file offset and over-reporting *cnt so clients treat corrupted server data as fully committed; when uncertain versus Low, High is required.\nA:N - The defect corrupts file contents and the NFS WRITE count without oops, panic, BUG_ON, deadlock, or hang; files remain accessible and nfsd workers continue, so there is no kernel or service crash."
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2026-09-13T06:33:01.153Z",
        "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
        "shortName": "Linux"
      },
      "references": [
        {
          "url": "https://git.kernel.org/stable/c/fa6590dfd16ab55f03b658b079072ace3504825e"
        },
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          "url": "https://git.kernel.org/stable/c/250ec14932d5cfe102f68a57892bb566eee7f83e"
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    "assignerShortName": "Linux",
    "cveId": "CVE-2026-89678",
    "datePublished": "2026-09-11T19:46:02.184Z",
    "dateReserved": "2026-09-11T19:38:34.747Z",
    "dateUpdated": "2026-09-13T06:33:01.153Z",
    "state": "PUBLISHED"
  },
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  "dataVersion": "5.2",
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      "percentile": "0.28942"
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    "redhat_vex": {
      "aggregate_severity": "Important",
      "current_release_date": "2026-09-13T06:51:52+00:00",
      "cve": "CVE-2026-89678",
      "id": "CVE-2026-89678",
      "initial_release_date": "2026-09-11T19:46:02.184000+00:00",
      "product_status:known_affected": "2",
      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "kernel: Kernel: NFS daemon (nfsd) data corruption due to incorrect partial-write detection",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-89678.json",
      "version": "3"
    },
    "suse_vex": {
      "aggregate_severity": "moderate",
      "current_release_date": "2026-09-16T00:00:26Z",
      "cve": "CVE-2026-89678",
      "id": "CVE-2026-89678",
      "initial_release_date": "2026-09-12T16:23:49Z",
      "product_status:known_not_affected": "353",
      "source": "SUSE CSAF VEX",
      "status": "interim",
      "title": "SUSE CVE CVE-2026-89678",
      "url": "https://ftp.suse.com/pub/projects/security/csaf-vex/cve-2026-89678.json",
      "version": "5"
    }
  }
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

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Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
  • 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.