CVE-2026-89704 nfsd: sample writeback error cursor before async COPY loop
A race condition in the Linux kernel NFS server allows silent data loss during async COPY operations due to incorrect writeback error cursor sampling.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-89704?
CVE-2026-89704 is a high-severity vulnerability in Linux, published on 11 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
nfsd: sample writeback error cursor before async COPY loop
_nfsd_copy_file_range() samples dst->f_wb_err into "since" after the copy loop, then uses it to detect writeback errors via filemap_check_wb_err() once vfs_fsync_range() returns. Because the nfsd_file cache reuses a single struct file across requests targeting the same inode, a concurrent COMMIT or stable WRITE on dst advances dst->f_wb_err to the current mapping->wb_err via file_check_and_advance_wb_err() during its own vfs_fsync_range(). If that advancement lands between the writeback error appearing in mapping->wb_err and the COPY worker sampling "since", the worker captures the already-advanced cursor, errseq_check() sees cur == since and returns zero, and NFSD4_COPY_F_COMMITTED is set even though writeback failed. CB_OFFLOAD then encodes wr_stable_how = FILE_SYNC4, the client treats the copied data as durable, and the failure becomes silent data loss.
Sample since once at the start of the function. The cursor then reflects state in effect before this COPY issues any writes, and filemap_check_wb_err() detects any error that occurs during the copy regardless of which thread first observes it. This matches the pattern used by nfsd_vfs_write() and nfsd4_clone_file_range().
Which versions are affected by CVE-2026-89704?
How do I fix CVE-2026-89704?
Apply the vendor's patch.
Is CVE-2026-89704 being exploited?
There is no known exploitation of CVE-2026-89704 in the wild and no public exploit code.
Sources
Also known as AZL-100820, BELL-CVE-2026-89704, DEBIAN-CVE-2026-89704, GHSA-f634-v549-95r8, UBUNTU-CVE-2026-89704.