CVE-2026-89678

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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/fa6590dfd16ab55f03b658b079072ace3504825e
• https://git.kernel.org/stable/c/250ec14932d5cfe102f68a57892bb566eee7f83e

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