[PATCH 1/3] qemu_passt: Don't make passt transition to svirt_t/virt_domain on start

[Daniel P. Berrangé]

On Tue, Feb 21, 2023 at 10:49:46PM +0100, Stefano Brivio wrote:
> On Tue, 21 Feb 2023 19:43:33 +0000
> Daniel P. Berrangé <berrange at redhat.com> wrote:
> 
> > On Tue, Feb 21, 2023 at 08:19:05PM +0100, Stefano Brivio wrote:
> > > qemuSecurityCommandRun() causes an explicit domain transition of the
> > > new process, but passt ships with its own SELinux policy, with
> > > external interfaces for libvirtd, so we simply need to transition
> > > from virtd_t to passt_t as passt is executed. The qemu type
> > > enforcement rules have little to do with it.  
> > 
> > Can you clarify the difference here ?
> 
> Between...?
> 
> I mean, virCommandRun() will just keep things running under virtd_t, so
> that passt later can transition to passt_t.
> 
> With qemuSecurityCommandRun(), there would be a transition from virtd_t
> to svirt_t (it's the function that's called to start qemu, but
> shouldn't be called here), and not to passt_t.
> 
> But I'm not really sure that's what you were asking for.

Yes, it is.

> 
> > Runing passt under 'svirt_t' is not desirable as that allows
> > many actions that are only relevant to QEMU.
> 
> Right, that's what this patch avoids. There are also actions, such as
> starting passt or killing it, that we don't want to allow QEMU to do.
> 
> > Running passt under the MCS label that is associated with the
> > VM is highly desirable though. Two passt instances belonging
> > to separate VMs are isolated from each other if they each use
> > the VM specific MCS label, than if they use the global default
> > MCS label.
> > 
> > To use the VM specific MCS label would require libvirt to
> > explicitly set the desired selinux label on exec, it can't
> > happen automatically via an SELinux transition rule.
> > 
> > We do stil want to use the passt_t type though.
> > 
> > IOW, if we have a VM running
> > 
> >   svirt_t:s0:c710,c716
> > 
> > IMHO we would its corresponding passt instance to be
> > running
> > 
> >   passt_t:s0:c710,c716
> > 
> > 
> > not
> > 
> >   passt_t:s0:c0.c1023
> 
> Practically speaking, it doesn't make a huge difference for passt
> because it unshares any relevant namespace right after it starts --
> that's *in theory* a strictly stronger isolation compared to what
> SELinux provides (at least once we reach the main loop).

Even docker/podman will apply SELinux isolation per container,
rather than only relying on namespaces. 

> But it makes sense, and I guess we should relabel to a specific MCS
> with still 'virtd_t' as a type, then later the domain would transition
> to passt_t. This could probably be done as an extension of
> qemuSecurityCommandRun(), I haven't looked into it yet. I will.
> 
> Anyway, right now, I think this provides better security than
> 'setenforce 0', which is the only way to run passt from libvirt at the
> moment on some distributions.

If running with SELinux permissive, this patch has no effect, as it
is unconfined regardless. That's not a situation we care about. If
people want to run without protection they get to keep the pieces
when it all goes wrong.

> I'm not sure how you handle these cases on libvirt, but generally
> speaking, this patch is a vast improvement on the current situation,
> and I can follow up with an extension or a different version of
> qemuSecurityCommandRun() later.

No, I don't think it is a vast improvement.

The goal of sVirt is to guarantee isolation between VMs.

Running passt under svirt_t:MCS is not ideal because the svirt_t
policy allows things that passt should not get. It does still
guarantee isolation between VMs, because the MCS label is present.

Switching to running passt_t:c0.c1023 will be more correct in terms
of what permissions passt should be allowed, but it has disabled
isolation of passt between VMs. This is a clear degradation of
capabilities from the POV of sVirt's goals.

With regards,
Daniel
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