[PATCH 1/3] qemu_passt: Don't make passt transition to svirt_t/virt_domain on start
Daniel P. Berrangé
berrange at redhat.com
Wed Feb 22 11:35:16 UTC 2023
On Wed, Feb 22, 2023 at 12:21:09PM +0100, Michal Prívozník wrote:
> On 2/22/23 11:05, Stefano Brivio wrote:
> > On Wed, 22 Feb 2023 09:46:42 +0000
> > Daniel P. Berrangé <berrange at redhat.com> wrote:
> >
> >> 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.
> >
> > Sure, I'm not saying it's not desirable -- but still, many (most?)
> > host-facing services they rely on are not isolated in this sense. Same
> > for the current implementation of libvirt with dnsmasq, for example.
> >
> >>> 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.
> >
> > The current implementation simply does not and cannot work with SELinux
> > in enforcing mode, so users have no other choice.
> >
> >>> 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.
> >
> > It's a bit more than that: it's not ideal because libvirt simply won't
> > start passt. There's no isolation and no VMs.
> >
> >> 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.
> >
> > It's not a degradation because VMs can't start passt with SELinux in
> > enforcing mode, without this patch. No service, no degradation.
> >
> > I looked into options to rework
> > virSecurityManagerSetChildProcessLabel() and friends with a more
> > flexible modeling/building of labels -- just doing some trick all the
> > way down from qemuSecurityCommandRun() implies a number of layering
> > violations that I would like to avoid.
> >
> > It all looks doable, but implementing the type of functionality/API
> > that's currently missing there isn't a small rework -- some refactoring
> > of interfaces is definitely needed. I started, but it's not quick.
> >
> > So for the moment being I would suggest that, if passt can't be
> > relabeled as passt_t (i.e. if this patch or equivalent can't be
> > applied), the whole passt back-end should be dropped.
>
>
> I don't think we need such drastic measure. I think you can use:
>
> qemuPasstStart()
> {
>
>
> seclabel = virDomainDefGetSecurityLabelDef(vm->def, "selinux");
> s = context_new(seclabel->label);
> context_type_set(s, "virt_t);
I presume you meant "passt_t" here.
> newLabel = context_str(s);
Probably don't want code directly exposed to SELinux APIs
though.
We also likely shouldn't hardcode 'passt_t' either, as that's
not an inherant property of passt, but rather of the policy.
This problem is not unique to passt either - we'll want to
deal with this for almost any helper binary we spawn off
per-VM.
We can avoid hardcoding labels by calling getfilecon() on the
binary we're going to run, to query its assigned type label
that is on disk. Then concatenate the MCS label.
So I'm inclined to say we need a
virSecurityManagerGetHelperLabel(virSecurityManager *mgr,
virDomainDef *def,
const char *helper_path);
And we would call
virSecurityManagerGetHelperLabel(mgr, vm->def, "/usr/bin/passt")
> virCommandSetSELinuxLabel(cmd, newLabel);
>
> virCommandRun();
> }
With regards,
Daniel
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