CVE-2025-38058

In the Linux kernel, the following vulnerability has been resolved:

_legitimizemnt(): check for MNTSYNCUMOUNT should be under mount_lock

... or we risk stealing final mntput from sync umount - raising mntcount after umount(2) has verified that victim is not busy, but before it has set MNTSYNCUMOUNT; in that case _legitimizemnt() doesn't see that it's safe to quietly undo mntcount increment and leaves dropping the reference to caller, where it'll be a full-blown mntput().

Check under mount_lock is needed; leaving the current one done before taking that makes no sense - it's nowhere near common enough to bother with.