CVE-2026-89678
요약
\~에 의해 MITRE • 2026. 09. 11.
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.
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