fs/mount_setattr: tighten permission checks

We currently don't have any filesystems that support idmapped mounts
which are mountable inside a user namespace. That was a deliberate
decision for now as a userns root can just mount the filesystem
themselves. So enforce this restriction explicitly until there's a real
use-case for this. This way we can notice it and will have a chance to
adapt and audit our translation helpers and fstests appropriately if we
need to support such filesystems.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
CC: linux-fsdevel@vger.kernel.org
Suggested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

fs/namespace.c

@@ -3855,8 +3855,12 @@ static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
 	if (!(m->mnt_sb->s_type->fs_flags & FS_ALLOW_IDMAP))
 		return -EINVAL;
 
+	/* Don't yet support filesystem mountable in user namespaces. */
+	if (m->mnt_sb->s_user_ns != &init_user_ns)
+		return -EINVAL;
+
 	/* We're not controlling the superblock. */
-	if (!ns_capable(m->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
+	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
 	/* Mount has already been visible in the filesystem hierarchy. */