one should *not* be allowed to mount one of those, new API or not.

Reported-by: Denis Arefev <arefev@swemel.ru>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
[[ I still want to see the rest of the reproducer - report smells like a missing
d_can_lookup() somewhere, on top of fsmount(2) bug]]
diff --git a/fs/namespace.c b/fs/namespace.c
index fe919abd2f01..17777c837683 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4499,6 +4499,10 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
 	new_mnt = vfs_create_mount(fc);
 	if (IS_ERR(new_mnt))
 		return PTR_ERR(new_mnt);
+	if (new_mnt->mnt_sb->s_flags & SB_NOUSER) {
+		mntput(new_mnt);
+		return -EINVAL;
+	}
 	new_mnt->mnt_flags = mnt_flags;
 
 	new_path.dentry = dget(fc->root);