CVE-2025-38709

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.