CVE-2025-38709 loop: Avoid updating block size under exclusive owner
A race condition in the Linux kernel loop device allows block size changes during mount, causing filesystem confusion and potential denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-38709?
CVE-2025-38709 is a medium-severity vulnerability in Linux kernel, published on 4 Sep 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official 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.
Which versions are affected by CVE-2025-38709?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-38709?
Apply the vendor's patch.
Is CVE-2025-38709 being exploited?
There is no known exploitation of CVE-2025-38709 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-66812, AZL-71257, BELL-CVE-2025-38709, DEBIAN-CVE-2025-38709, GHSA-4rvm-hh4p-2jfj, ROOT-OS-DEBIAN-11-CVE-2025-38709 and 4 more.