GHSA-QPFV-2QF2-XR5W
Vulnerability from github – Published: 2026-09-11 21:31 – Updated: 2026-09-13 09:32In 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.
{
"affected": [],
"aliases": [
"CVE-2026-89678"
],
"database_specific": {
"cwe_ids": [],
"github_reviewed": false,
"github_reviewed_at": null,
"nvd_published_at": "2026-09-11T20:19:54Z",
"severity": "HIGH"
},
"details": "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.",
"id": "GHSA-qpfv-2qf2-xr5w",
"modified": "2026-09-13T09:32:27Z",
"published": "2026-09-11T21:31:36Z",
"references": [
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2026-89678"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/250ec14932d5cfe102f68a57892bb566eee7f83e"
},
{
"type": "WEB",
"url": "https://git.kernel.org/stable/c/fa6590dfd16ab55f03b658b079072ace3504825e"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
"type": "CVSS_V3"
}
]
}
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