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

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(a)redhat.com&gt; 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(a)redhat.com&gt; 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.