nfsd: fix partial-write detection in nfsd_direct_write

{
  "id": "a8e6cbe543108e0648491a90e0ced13546d6a93356e805ab6db86cc61f2a0a90",
  "url": "https://osv.dev/vulnerability/CVE-2026-89678",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "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.",
    "evidence": "osv:CVE-2026-89678",
    "products": [
      {
        "name": {
          "value": "Kernel",
          "evidence": "/affected/1/package/name"
        },
        "prerequisites": [],
        "fixed_versions": [
          {
            "value": "7.2.4",
            "evidence": "/affected/1/ranges"
          }
        ],
        "affected_versions": []
      }
    ],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-89678",
      "aliases": [
        "CVE-2026-89678"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
              "type": "GIT",
              "events": [
                {
                  "introduced": "06c5c97293e3fca99ce15da157068edf45a7c6e4"
                },
                {
                  "fixed": "fa6590dfd16ab55f03b658b079072ace3504825e"
                },
                {
                  "fixed": "250ec14932d5cfe102f68a57892bb566eee7f83e"
                }
              ]
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-89678.json"
          }
        },
        {
          "ranges": [
            {
              "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-2026-89678.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:25.081217966Z",
      "provider": "osv",
      "references": [
        "https://git.kernel.org/stable/c/250ec14932d5cfe102f68a57892bb566eee7f83e",
        "https://git.kernel.org/stable/c/fa6590dfd16ab55f03b658b079072ace3504825e",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/89xxx/CVE-2026-89678.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-89678",
        "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-2026-89678.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-89678",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 491680,
  "published_at": "2026-09-11T19:46:02.184+00:00"
}