# 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.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-100820, BELL-CVE-2026-89704, DEBIAN-CVE-2026-89704, GHSA-f634-v549-95r8, UBUNTU-CVE-2026-89704

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-89704 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-89704 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-89704/ (Markdown: https://insights.sbomify.com/CVE-2026-89704/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-89704
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-89704
- OSV: https://osv.dev/vulnerability/CVE-2026-89704

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-89704/
Summaries are generated and may contain mistakes; the official description above is the record.