How to disallow the Docker Daemon to mount host's root file system into the container

Solution 1:

SELinux will prevent anything not correctly labelled to be mounted as a volume inside a docker container, as proof, here using a single file, same policy applies to directories:

$ sestatus
SELinux status:                 enabled
SELinuxfs mount:                /sys/fs/selinux
SELinux root directory:         /etc/selinux
Loaded policy name:             targeted
Current mode:                   enforcing
Mode from config file:          enforcing
Policy MLS status:              enabled
Policy deny_unknown status:     allowed
Max kernel policy version:      30

Using the sample file:

$ cat sample_script.sh 
echo 'Hello, world'

Its default security context is:

$ ls -lrtZ sample_script.sh 
-rw-------. 1 david david unconfined_u:object_r:user_home_t:s0 20 Oct  3 17:18 sample_script.sh

Trying to use this file inside a container fails as expected:

$ docker run -v /home/david/sample_script.sh:/sample_script.sh --rm ubuntu bash sample_script.sh
bash: sample_script.sh: Permission denied

And an AVC denial will be logged:

$ sudo ausearch -m avc -ts recent
time->Mon Oct  3 17:39:28 2016
type=AVC msg=audit(1475512768.444:784): avc:  denied  { read } for  pid=28720 comm="bash" name="sample_script.sh" dev="dm-13" ino=101062112 scontext=system_u:system_r:svirt_lxc_net_t:s0:c457,c992 tcontext=unconfined_u:object_r:user_home_t:s0 tclass=file permissive=0

After changing the security context to one Docker can use:

$ sudo chcon -Rt svirt_sandbox_file_t sample_script.sh
$ ls -lrtZ sample_script.sh 
-rw-------. 1 david david unconfined_u:object_r:svirt_sandbox_file_t:s0 20 Oct  3 17:18 sample_script.sh 

The container now has access to the file:

$ docker run -v /home/david/sample_script.sh:/sample_script.sh --rm ubuntu bash sample_script.sh
Hello, world

More information about Docker and SELinux in the official Red Hat documentation and this article by Dan Walsh.