The virt-login-shell binary is a setuid program that takes
no arguments. When invoked it looks at the invoking uid,
resolves it to a username, and finds an LXC guest with the
same name. It then starts the guest and runs the shell in
side the namespaces of the container.

Given this set of tasks the virt-login-shell binary needs
to connect to libvirtd, make various other libvirt API calls.
This is a problem for setuid binaries as various libraries
that libvirt.so links to are not safe. For example, they have
constructor functions which execute an unknown amount of code
that can be influenced by env variables.

For this reason virt-login-shell doesn't use libvirt.so,
but instead links to a custom, cut down, set of source files
sufficient to be a local client only.

This introduces a problem for integrating glib2 into libvirt
though, as once integrated, there would be no way to build
virt-login-shell without an external dependancy on glib2 and
this is definitely not setuid safe.

To resolve this problem, we split the virt-login-shell binary
into two parts. The first part is setuid and does almost
nothing. It simply records the original uid+gid, and then
invokes the virt-login-shell-helper binary. Crucially when
it does this it completes scrubs all environment variables.
It is thus safe for virt-login-shell-helper to link to the
normal libvirt.so. Any things that constructor functions
do cannot be influenced by user control env vars or cli
args.
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>