diff --git a/fs/namespace.c b/fs/namespace.c
index 68789f896f08..df137ba19d37 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4238,6 +4238,13 @@ static int build_mount_idmapped(const struct mount_attr *attr, size_t usize,
 		err = -EPERM;
 		goto out_fput;
 	}
+
+	/* We're not controlling the target namespace. */
+	if (!ns_capable(mnt_userns, CAP_SYS_ADMIN)) {
+		err = -EPERM;
+		goto out_fput;
+	}
+
 	kattr->mnt_userns = get_user_ns(mnt_userns);
 
 out_fput: