mount: /tmp/cgrp permission denied - docker

I am trying to follow this article to reproduce a container escape vulnerability that exists in docker. As mentioned in the article (Refining the proof of concept section), I first run,

docker run --rm -it --cap-add=SYS_ADMIN --security-opt apparmor=unconfined ubuntu bash

to create a docker container with needed privileges. After, when I run

mkdir /tmp/cgrp && mount -t cgroup -o rdma cgroup /tmp/cgrp && mkdir /tmp/cgrp/x

which is the next step but I get an error saying mount: /tmp/cgrp: permission denied. Not sure why that happens. I tried the solutions mentioned in this (adding --privileged flag etc) but nothing seemed to work.

Here is the output of the docker version command.

Client:
 Version:           20.10.5+dfsg1
 API version:       1.41
 Go version:        go1.15.9
 Git commit:        55c4c88
 Built:             Wed Aug  4 19:55:57 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server:
 Engine:
  Version:          20.10.5+dfsg1
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.15.9
  Git commit:       363e9a8
  Built:            Wed Aug  4 19:55:57 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.12~ds1
  GitCommit:        1.4.12~ds1-1~deb11u1
 runc:
  Version:          1.0.0~rc93+ds1
  GitCommit:        1.0.0~rc93+ds1-5+b2
 docker-init:
  Version:          0.19.0
  GitCommit:

How do I go about this?