8.2 KiB
Docker --privileged
What Affects
When you run a container as privileged these are the protections you are disabling:
Mount /dev
In a privileged container, all the devices can be accessed in /dev/. Therefore you can escape by mounting the disk of the host.
{% tabs %} {% tab title="Inside default container" %}
# docker run --rm -it alpine sh
ls /dev
console fd mqueue ptmx random stderr stdout urandom
core full null pts shm stdin tty zero
{% endtab %}
{% tab title="Inside Privileged Container" %}
# docker run --rm --privileged -it alpine sh
ls /dev
cachefiles mapper port shm tty24 tty44 tty7
console mem psaux stderr tty25 tty45 tty8
core mqueue ptmx stdin tty26 tty46 tty9
cpu nbd0 pts stdout tty27 tty47 ttyS0cker run --rm --privileged -it alpine sh
[...]
{% endtab %} {% endtabs %}
Read-only kernel file systems
Kernel file systems provide a mechanism for a process to alter the way the kernel runs. By default, we don't want container processes to modify the kernel, so we mount kernel file systems as read-only within the container.
{% tabs %} {% tab title="Inside default container" %}
# docker run --rm -it alpine sh
mount | grep '(ro'
sysfs on /sys type sysfs (ro,nosuid,nodev,noexec,relatime)
cpuset on /sys/fs/cgroup/cpuset type cgroup (ro,nosuid,nodev,noexec,relatime,cpuset)
cpu on /sys/fs/cgroup/cpu type cgroup (ro,nosuid,nodev,noexec,relatime,cpu)
cpuacct on /sys/fs/cgroup/cpuacct type cgroup (ro,nosuid,nodev,noexec,relatime,cpuacct)
{% endtab %}
{% tab title="Inside Privileged Container" %}
# docker run --rm --privileged -it alpine sh
mount | grep '(ro'
{% endtab %} {% endtabs %}
Masking over kernel file systems
The /proc file system is namespace-aware, and certain writes can be allowed, so we don't mount it read-only. However, specific directories in the /proc file system need to be protected from writing, and in some instances, from reading. In these cases, the container engines mount tmpfs file systems over potentially dangerous directories, preventing processes inside of the container from using them.
{% hint style="info" %} tmpfs is a file system that stores all the files in virtual memory. tmpfs doesn't create any files on your hard drive. So if you unmount a tmpfs file system, all the files residing in it are lost for ever. {% endhint %}
{% tabs %} {% tab title="Inside default container" %}
# docker run --rm -it alpine sh
mount | grep /proc.*tmpfs
tmpfs on /proc/acpi type tmpfs (ro,relatime)
tmpfs on /proc/kcore type tmpfs (rw,nosuid,size=65536k,mode=755)
tmpfs on /proc/keys type tmpfs (rw,nosuid,size=65536k,mode=755)
{% endtab %}
{% tab title="Inside Privileged Container" %}
# docker run --rm --privileged -it alpine sh
mount | grep /proc.*tmpfs
{% endtab %} {% endtabs %}
Linux capabilities
Container engines launch the containers with a limited number of capabilities to control what goes on inside of the container by default. Privileged ones have all the capabilities accesible. To learn about capabilities read:
{% content-ref url="../linux-capabilities.md" %} linux-capabilities.md {% endcontent-ref %}
{% tabs %} {% tab title="Inside default container" %}
# docker run --rm -it alpine sh
apk add -U libcap; capsh --print
[...]
Current: cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap=eip
Bounding set =cap_chown,cap_dac_override,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_net_bind_service,cap_net_raw,cap_sys_chroot,cap_mknod,cap_audit_write,cap_setfcap
[...]
{% endtab %}
{% tab title="Inside Privileged Container" %}
# docker run --rm --privileged -it alpine sh
apk add -U libcap; capsh --print
[...]
Current: =eip cap_perfmon,cap_bpf,cap_checkpoint_restore-eip
Bounding set =cap_chown,cap_dac_override,cap_dac_read_search,cap_fowner,cap_fsetid,cap_kill,cap_setgid,cap_setuid,cap_setpcap,cap_linux_immutable,cap_net_bind_service,cap_net_broadcast,cap_net_admin,cap_net_raw,cap_ipc_lock,cap_ipc_owner,cap_sys_module,cap_sys_rawio,cap_sys_chroot,cap_sys_ptrace,cap_sys_pacct,cap_sys_admin,cap_sys_boot,cap_sys_nice,cap_sys_resource,cap_sys_time,cap_sys_tty_config,cap_mknod,cap_lease,cap_audit_write,cap_audit_control,cap_setfcap,cap_mac_override,cap_mac_admin,cap_syslog,cap_wake_alarm,cap_block_suspend,cap_audit_read
[...]
{% endtab %} {% endtabs %}
You can manipulate the capabilities available to a container without running in --privileged mode by using the --cap-add and --cap-drop flags.
Seccomp
Seccomp is useful to limit the syscalls a container can call. A default seccomp profile is enabled by default when running docker containers, but in privileged mode it is disabled. Learn more about Seccomp here:
{% content-ref url="seccomp.md" %} seccomp.md {% endcontent-ref %}
{% tabs %} {% tab title="Inside default container" %}
# docker run --rm -it alpine sh
grep Seccomp /proc/1/status
Seccomp: 2
Seccomp_filters: 1
{% endtab %}
{% tab title="Inside Privileged Container" %}
# docker run --rm --privileged -it alpine sh
grep Seccomp /proc/1/status
Seccomp: 0
Seccomp_filters: 0
{% endtab %} {% endtabs %}
# You can manually disable seccomp in docker with
--security-opt seccomp=unconfined
Also, note that when Docker (or other CRIs) are used in a Kubernetes cluster, the seccomp filter is disabled by default
AppArmor
AppArmor is a kernel enhancement to confine containers to a limited set of resources with per-program profiles. When you run with the --privileged flag, this protection is disabled.
{% content-ref url="apparmor.md" %} apparmor.md {% endcontent-ref %}
# You can manually disable seccomp in docker with
--security-opt apparmor=unconfined
SELinux
When you run with the --privileged flag, SELinux labels are disabled, and the container runs with the label that the container engine was executed with. This label is usually unconfined and has full access to the labels that the container engine does. In rootless mode, the container runs with container_runtime_t. In root mode, it runs with spc_t.
{% content-ref url="../selinux.md" %} selinux.md {% endcontent-ref %}
# You can manually disable selinux in docker with
--security-opt label:disable
What Doesn't Affect
Namespaces
Namespaces are NOT affected by the --privileged flag. Even though they don't have the security constraints enabled, they do not see all of the processes on the system or the host network, for example. Users can disable individual namespaces by using the --pid=host, --net=host, --ipc=host, --uts=host container engines flags.
{% tabs %} {% tab title="Inside default privileged container" %}
# docker run --rm --privileged -it alpine sh
ps -ef
PID USER TIME COMMAND
1 root 0:00 sh
18 root 0:00 ps -ef
{% endtab %}
{% tab title="Inside --pid=host Container" %}
# docker run --rm --privileged --pid=host -it alpine sh
ps -ef
PID USER TIME COMMAND
1 root 0:03 /sbin/init
2 root 0:00 [kthreadd]
3 root 0:00 [rcu_gp]ount | grep /proc.*tmpfs
[...]
{% endtab %} {% endtabs %}
User namespace
Container engines do NOT use user namespace by default. However, rootless containers always use it to mount file systems and use more than a single UID. In the rootless case, user namespace can not be disabled; it is required to run rootless containers. User namespaces prevent certain privileges and add considerable security.