# CVE-2026-89678

## Summary

- **CVE ID:** CVE-2026-89678
- **Severity:** HIGH
- **CVSS Score:** 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N)
- **CWE:** N/A
- **Published:** Sep 11, 2026
- **Last Modified:** Sep 13, 2026

## Description

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.

## Affected Products

- Linux — Linux (06c5c97293e3fca99ce15da157068edf45a7c6e4)
- Linux — Linux (6.19)
- Linux — Linux (0)
- Linux — Linux (7.2.4)
- Linux — Linux (7.3-rc1)

## References

- [CNA](https://git.kernel.org/stable/c/fa6590dfd16ab55f03b658b079072ace3504825e)
- [CNA](https://git.kernel.org/stable/c/250ec14932d5cfe102f68a57892bb566eee7f83e)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.35%
- **EPSS Percentile:** 28.9

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_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-18._