On Thu, Sep 07, 2017 at 03:30:47PM +0200, Dominick Grift wrote: > On Thu, Sep 07, 2017 at 03:22:42PM +0200, Dominick Grift wrote: > > 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?
Actually, It probably the absence of polkit that is causing this
> >
> > $ 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
>
> So to answer your question as to whether i would be able to block this access
> without polkit:
>
> Yes i would be able to do this by just not allowing the systemd --user
> instances to execute a shell
>
> >
> > >
> > > 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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> Dominick Grift
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Dominick Grift
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