vfs: Carefully propogate mounts across user namespaces

As a matter of policy MNT_READONLY should not be changable if the
original mounter had more privileges than creator of the mount
namespace.

Add the flag CL_UNPRIVILEGED to note when we are copying a mount from
a mount namespace that requires more privileges to a mount namespace
that requires fewer privileges.

When the CL_UNPRIVILEGED flag is set cause clone_mnt to set MNT_NO_REMOUNT
if any of the mnt flags that should never be changed are set.

This protects both mount propagation and the initial creation of a less
privileged mount namespace.

Cc: stable@vger.kernel.org
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Reported-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
{
	struct vfsmount *mnt;
	mnt = vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
	if (!IS_ERR(mnt)) {
		/*
		 * it is a longterm mount, don't release mnt until
		 * we unmount before file sys is unregistered
		*/
		real_mount(mnt)->mnt_ns = MNT_NS_INTERNAL;
	}
	return mnt;
}

struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
{
	struct vfsmount *mnt;
	mnt = vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
	if (!IS_ERR(mnt)) {
		/*
		 * it is a longterm mount, don't release mnt until
		 * we unmount before file sys is unregistered
		*/
		real_mount(mnt)->mnt_ns = MNT_NS_INTERNAL;
	}
	return mnt;
}
