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[libvirt] LXC: making the private root filesystem more secure



When I wrote the private root filesystem stuff for LXC (which I just 
committed) I noted that we couldn't actually make this secure, because
someone inside the chroot can just 'mknod' and access the host devices.

What I completely forgot was that cgroups as of 2.6.26 has device ACLs
If we place every container in a cgroup (which was planned anyway), then
we can trivially prevent containers accessing host devices

One time setup

    mount -t cgroups  /dev/cgroups
    mkdir /dev/cgroups/libvirt
    mkdir /dev/cgroups/libvirt/lxc

For each new container 'NAME'

    mkdir /dev/cgroups/libvirt/lxc/{NAME}
    echo "a" > /dev/cgroups/libvirt/lxc/{NAME}/devices.deny
    echo "c 1:3 rwm" > /dev/cgroups/libvirt/lxc/{NAME}/devices.allow
    echo "c 1:5 rwm" > /dev/cgroups/libvirt/lxc/{NAME}/devices.allow
    echo "c 1:7 rwm" > /dev/cgroups/libvirt/lxc/{NAME}/devices.allow
    echo "c 5:1 rwm" > /dev/cgroups/libvirt/lxc/{NAME}/devices.allow
    echo "c 1:8 rwm" > /dev/cgroups/libvirt/lxc/{NAME}/devices.allow
    echo "c 1:9 rwm" > /dev/cgroups/libvirt/lxc/{NAME}/devices.allow

This denies all devices, and then allows null, zero, full, console, random
and urandom. Allowing use of 'random' is debatable.

The 'devpts' namespace stuff is also needed to provide private PTYs. 
The 'user' namespace stuff is needed to prevent an unprivileged user
in the host OS from killing off processes with same UID inside the
container. There looks to be active patchsets for both of these being
discussed, so we're getting close to having a genuinely useful
container based virt driver with LXC

Daniel
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