namespace: look at the topmost mount for a mount namespace file

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

namespace: look at the topmost mount for a mount namespace file

attach_recursive_mnt() preallocates a mountpoint for the root of the
topmost mount of the source so that a mount that already sits at the
destination can be put on top of the source or on top of one of its
propagated copies.

The copies are made without CL_COPY_MNT_NS_FILE so their chain of
overmounts is shorter than the source's. It ends below the first bind
mount of a mount namespace file. So while the loop walks up the chain of
the source it remembers the mountpoint of that mount in "shorter" and
the existing mount is put there for the copies.

But the loop ends at the topmost mount without ever looking at it. If
the topmost mount is the only mount namespace file in the chain
"shorter" stays NULL and mnt_change_mountpoint() attaches the existing
mount of the copy below the root of the mount namespace file. That's a
dentry of nsfs. No path walk in the copy's mount namespace ever gets
there. The mount is gone until its parent goes and the copy that took
its place can't be unmounted synchronously because it has a child:

S bind mount of a file
N bind mount of a mount namespace file on top of S
A shared mount, B a slave of A, Q mounted on B/file

move_mount(S, A/file)
cat B/file -> the content of S instead of Q
umount B/file -> EBUSY

Look at every mount of the chain including the topmost one.