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

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.1% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-09-04 · updated 2026-10-30

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.

Affected products

Linux kernel: before 6.6.109; from 6.7 before 6.12.43; from 6.13 before 6.15.11; from 6.16 before 6.16.2 — fixed in 6.6.109, 6.12.43, 6.15.11, 6.16.2
Linux: 4.14 — fixed in 6.17

How to fix

Upgrade to a fixed version: Linux kernel 6.6.109, 6.12.43, 6.15.11, 6.16.2; Linux 6.17. Check the vendor advisory for mitigations.

References

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