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

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); newLabel = context_str(s); virCommandSetSELinuxLabel(cmd, newLabel); virCommandRun(); }