CVE-2026-98989: namespace: check a recursive bind mount for mount namespace loops

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

namespace: check a recursive bind mount for mount namespace loops

doloopback() refuses to bind mount the file of a mount namespace that is as old as the caller's or older because a mount namespace that holds a mount of its own file, or of an ancestor's, would cause a cycle. But it only checks the dentry the bind mount starts from. With MSREC everything below it is copied and doloopback() passes CLCOPYMNTNSFILE so mount namespace files below the source are copied.

That's fine for a source in the caller's own mount namespace. Every mount namespace file in there passed the same check when it was bind-mounted. It isn't fine for a source in another mount namespace. maycopytree() accepts a bind mount of any nsfs or pidfs file no matter what mount namespace it lives in so that /proc/<pid>/ns/<ns> and pidfds can be bind mounted. If we create a bind-mount stack of mount namespaces file descriptors the kernel will copy them irrespective of their ancestoral relationship to the mount namespace in question:

151 146 0:7 net:[4026531833] /tmp/nrc/y rw - nsfs nsfs rw 152 151 0:7 mnt:[4026532293] /tmp/nrc/y rw - nsfs nsfs rw

Mount 152 is a mount of the mount namespace file of mount namespace 4026532293 inside mount namespace 4026532293. That namespace and every mount in it are leaked... All it takes is a process in an older mount namespace that stacks the file and repeating it leaks without limit:

after control: Shmem: 380 kB child: mount(/proc/self/fd/6, MSBIND|MSREC): ok after cycle: Shmem: 65916 kB

domovemount() runs checkfornsfsmounts() over a detached tree for exactly that reason. Do the same for the copy before it is grafted.