CVE-2025-38499

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

cloneprivatemnt(): make sure that caller has CAPSYSADMIN in the right userns

What we want is to verify there is that clone won't expose something hidden by a mount we wouldn't be able to undo. "Wouldn't be able to undo" may be a result of MNT_LOCKED on a child, but it may also come from lacking admin rights in the userns of the namespace mount belongs to.

cloneprivatemnt() checks the former, but not the latter.

There's a number of rather confusing CAPSYSADMIN checks in various userns during the mount, especially with the new mount API; they serve different purposes and in case of cloneprivatemnt() they usually, but not always end up covering the missing check mentioned above.