On Thu, Sep 07, 2017 at 08:55:23AM -0400, Stephen Smalley wrote:
> On Thu, 2017-09-07 at 11:05 +0200, Dominick Grift wrote:
> > pam_selinux requirements are generally pretty simple: its used to
> > associate a context with a login shell.
> >
> > With systemd things have becomes a bit more complicated.
> >
> > systemd uses pam_selinux to associate a context with both a login
> > shell (via container-shell@.service) as well as with a systemd --user
> > instance.
> >
> > Ideally one would not associate a login shell context with a systemd
> > --user instance because a systemd --user instance needs permissions
> > that do not make sense for a login shell to have.
> >
> > I am not aware of any way to make pam_selinux associate a context
> > based on variable like for example: if its a login shell then
> > associate this context , and if its a systemd --user instance then
> > associate that context
> >
> > This is an issue for me currently because if users are allowed to
> > "host-shell" (machinectl shell .host) then they are able to open a
> > shell with the context of the systemd --user instance and escape
> > their shell restrictions
> >
> > For now I can just block host-shell access with polkit but I am
> > trying to figure out what it would take to address this challenge
> > with pam_selinux
>
> The logic for determining a security context from pam_selinux (using
> libselinux get_ordered_context_with_level) can take the source/from
> context (i.e. the context in which the caller of pam_selinux is running
> ) into account as a factor in selecting which context to use, since it
> checks reachability (transition permission) as part of
> security_compute_user(). So that's a possible approach if you can
> distinguish what contexts are reachable for systemd --user vs login
> daemons.
The challenge here is that the source/from context is the same whether systemd --system spawns a login shell on behalf of a login user or whether systemd --system spawns a systemd --user instance on behalf of a login user
>
> What SELinux permissions are required to host-shell? Can you block
> that via SELinux policy rather than just polkit?
Might be possible but probably tricky. Not sure what it would take.
machinectl shell can also be used to spawn a shell in a container and this doesnt use pam_selinux, only machinectl shell .host makes systemd --system use pam_selinux to determine the pam characteristics to associate with the login shell on the host for the user
So "machinectl shell $container" makes machinectl itself a shell in $container (no pam), and "machinectl shell .host" makes machinectl tell systemd --system to spawn a login shell on the host (where systemd --system is using pam_selinux)
>
> Running machinectl shell .host locally doesn't seem to work:
> $ machinectl shell .host
> Failed to get shell PTY: Access denied
Probably SELinux blocking then?
$ id -Z
wheel.id:wheel.role:wheel.subj:s0
$ machinectl shell .host
==== AUTHENTICATING FOR org.freedesktop.machine1.host-shell ===
Authentication is required to acquire a shell on the local host.
Authenticating as: kcinimod
Password:
==== AUTHENTICATION COMPLETE ===
Connected to the local host. Press ^] three times within 1s to exit session
# journalctl -rb
sh: journalctl: command not found
# bash
# journalctl -rb | grep denied | head -n 1
Sep 07 15:14:29 julius audit[1983]: AVC avc: denied { getattr } for pid=1983 comm="sh" path="/usr/bin/journalctl" dev="dm-1" ino=2756183 scontext=sysadm.id:sysadm.role:sysadm_systemd.subj:s0 tcontext=sys.id:sys.role:journalctl.cmd_file:s0 tclass=file permissive=0
# ps auxZ | grep sysadm_systemd.subj
sysadm.id:sysadm.role:sysadm_systemd.subj:s0 root 1983 0.2 0.0 14452 4500 pts/4 Ss 15:14 0:00 -bin/sh
sysadm.id:sysadm.role:sysadm_systemd.subj:s0 root 1988 0.2 0.0 82036 7956 ? Ss 15:14 0:00 /usr/lib/systemd/systemd --user
So systemd --system runs -bin/sh using its pam_selinux which tells it to associate "sysadm_systemd.subj" with the login process, but that type is for the systemd --user instance of this user
then there is an automatic transition from sysadm_systemd.subj to sysadm.subj on cmd.shell.cmd_file that eventually makes the transition work when manually running bash
I put in that auto type transition (sysadm_systemd.subj > (cmd.shell.cmd_file) -> sysadm.subj) not realizing that the shell initially ends up with sysadm_systemd.subj via pam_selinux
So i should remove that auto type transition as well since i shouldnt end up in sysadm_systemd.subj in the first place
>
> Ideally, I'd like to get rid of security_compute_user() entirely, take
> the logic for determining user context entirely to userspace, and
> simplify it significantly, both in libselinux and in pam_selinux.
> That's been discussed previously on the list.
>
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Dominick Grift
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