Would a regular libvirt installation benefit from having libvirtd run untrusted too (I think so)? especially if its on the network? so instead of making it plugable, maybe the architecture should be updated and libvirtd never does trusted operations and maybe the solution can be shared between libvirt without kubevirt and libvirt+kubevirt? Thanks, Kevin ________________________________________ From: kubevirt-dev(a)googlegroups.com <kubevirt-dev(a)googlegroups.com&gt; on behalf of Andrea Bolognani <abologna(a)redhat.com&gt; Sent: Tuesday, November 29, 2022 9:05 AM To: libvirt-list(a)redhat.com Cc: kubevirt-dev(a)googlegroups.com Subject: [kubevirt-dev] RFC: New APIs for delegation of privileged operations Check twice before you click! This email originated from outside PNNL. Hi, this is a proposal for introducing a new family of APIs in libvirt, with the goal of improving integration with management applications. KubeVirt is intended to be the primary consumer of these APIs. Background ---------- KubeVirt makes it possible to run VMs on a Kubernetes cluster, side by side with containers. It does so by running QEMU and libvirtd themselves inside a container. The architecture is explained in more detail at https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fkubevir... but for the purpose of this discussion we only need to keep in mind two components: * virt-launcher - runs in the same container as QEMU and libvirtd - one instance per VM * virt-handler - runs in a separate container - one instance per node Conceptually, these two components roughly map to QEMU processes and libvirtd respectively. run workloads under unprivileged user accounts and without additional capabilities. Again, this is similar to how libvirtd itself runs as root but the QEMU processes it starts are under the unprivileged "qemu" account. KubeVirt has been working towards the goal of running VMs as completely unprivileged workloads and made excellent progress so far. Some of the operations needed for running a VM, however, inherently require elevated privilege. In KubeVirt, the conundrum is solved by having virt-handler (a privileged component) take care of those operations, making it possible for virt-launcher (as well as QEMU and libvirtd) to run in an unprivileged context. Examples -------- Here are a few examples of how KubeVirt has been able to reduce the privilege required by virt-launcher by selectively handing over responsibilities to virt-handler: * Remove SYS_RESOURCE capability from launcher pod https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.... * Drop SYS_RESOURCE capability https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.... * Housekeeping cgroup https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.... * Real time VMs fail to change vCPU scheduler and priority in non-root deployments https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.... * virt-launcher: Drop SYS_PTRACE capability https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.... The pattern we can see is that, initially, libvirt just assumes that it can perform a certain privileged operation. This fails in the context of KubeVirt, where libvirtd runs with significantly reduced privileges. As a consequence, libvirt is patched to be more resilient to such lack of privilege: for example, instead of attempting to create a file and erroring out due to lack of permissions, it will instead first check whether the file already exists and, if it does, assume that it has been prepared ahead of time by an external entity. Limitations ----------- This approach works fine, but only for the privileged operations that would be performed by libvirt before the VM starts running. Looking at the "housekeeping cgroup" PR in particular, we notice that the VM is initially created in paused state: this is necessary in order to create a point in time in which all the VM threads already exist but, crucially, none of the vCPUs have stated running yet. This is the only opportunity to move threads across cgroups without invalidating the expectations of a real time workload. When it comes to live migration, however, there is no way to create similar conditions, since the VM is running on the destination host right out of the gate. As a consequence, live migration has to be blocked when the housekeeping cgroup is in use, which is an unfortunate limitation. Moreover, there's an overall sense of fragility surrounding these interactions: both KubeVirt and, to some extent, libvirt need to be acutely aware of what the other component is going to do, but there is never an explicit handover and the whole thing only works if you just so happen to do everything with the exact right order and timing. Proposal -------- In order to address the issues outlined above, I propose that we introduce a new set of APIs in libvirt. These APIs would expose some of the inner workings of libvirt, and as such would come with *massively reduced* stability guarantees compared to the rest of our public API. The idea is that applications such as KubeVirt, which track libvirt fairly closely and stay pinned to specific versions, would be able to adapt to changes in these APIs relatively painlessly. More traditional management applications such as virt-manager would simply not opt into using the new APIs and maintain the status quo. Using memlock as an example, the new API could look like typedef int (*virInternalSetMaxMemLockHandler)(pid_t pid, unsigned long long bytes); int virInternalSetProcessSetMaxMemLockHandler(virConnectPtr conn, virInternalSetMaxMemLockHandler handler); The application-provided handler would be responsible for performing the privileged operation (in this case raising the memlock limit for a process). For KubeVirt, virt-launcher would have to pass the baton to virt-handler. If such an handler is installed, libvirt would invoke it (and likely go through some sanity checks afterwards); if not, it would attempt to perform the privileged operation itself, as it does today. This would make the interaction between libvirt and the management application explicit rather than implicit. Not having to stick to our usual API stability guarantees would make it possible to be more liberal in exposing the internals of libvirt as interaction points. Scope ----- I think we should initially limit the new APIs to the scenarios that have already been identified, then gradually expand the scope as needed. In other words, we shouldn't comb through the codebase looking for potential adopters. Since the intended consumers of these APIs are those that can adopt a new libvirt release fairly quickly, this shouldn't be a problem. Once the pattern has been established, we might consider introducing support for it at the same time as a new feature that might benefit from it is added. Caveats ------- libvirt is all about stable API, so introducing an API that is unstable *by design* is completely uncharted territory. To ensure that the new APIs are 100% opt-in, we could define them in a separate <libvirt/libvirt-internal.h> header. Furthermore, we could have a separate libvirt-internal.so shared library for the symbols and a corresponding libvirt-internal.pc pkg-config file. We could even go as far as requiring a preprocessor symbol such as VIR_INTERNAL_UNSTABLE_API_OPT_IN to be defined before the entry points are visible to the compiler. Whatever the mechanism, we would need to make sure that it's usable from language bindings as well. Internal APIs are amendable to not only come and go, but also change semantics between versions. We should make sure that such changes are clearly exposed to the user, for example by requiring them to pass a version number to the function and erroring out immediately if the value doesn't match our expectations. KubeVirt has a massive suite of functional tests, so this kind of change would immediately be spotted when a new version of libvirt is imported, with no risk of an incompatibility lingering in the codebase until it affects users. Disclaimer ---------- This proposal is intentionally vague on several of the details. Before attempting to nail those down, I want to gather feedback on the high-level idea, both from the libvirt and KubeVirt side. Credits ------- Thanks to Michal and Martin for helping shape and polish the idea from its initial rough state. -- Andrea Bolognani / Red Hat / Virtualization -- You received this message because you are subscribed to the Google Groups "kubevirt-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to kubevirt-dev+unsubscribe(a)googlegroups.com. To view this discussion on the web visit https://gcc02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgroups.....

On Tue, Nov 29, 2022 at 09:05:33 -0800, Andrea Bolognani wrote: [...] As proposed, with 'pid'/'bytes' as arguments the callbacks don't really feel to provide any form of privilege restriction to libvirt. Libvirt can choose to invoke the callback on any PID it likes and the application registering the callback doesn't have any way to check that the pid actually belongs to the started VM. I'm afraid that with many of those you'll have the same issue. At the very least the API/callback should carry additional information outlining to which object the callback invocation is tied to. (I understand that kubevirt most likely can assume that it's the single VM it's handling inside that specific container, though) By carefuly scoping this though the management layer can at least in some situations trust this the data (e.g. if no untrusted code ran yet). In fact libvirt does the same e.g. when probing vCPU thread pids at startup (while cpus) are paused. I simply want to point out that by this mechanism the code inside the "unprivileged" container is not as unprivileged as it has backdoors to higher layers to do privileged operations and the abstraction in the callbacks needs to be carefully chosen so that it is not simply a security charade. I think we also need to carefully consider how to do the callbacks in our RPC. Currently events don't wait for the client to respond and these would need to do so. Also use of these should e.g. require keepalive to be used. Another possibility could be to only allow them with the embedded mode of the qemu driver where RPC would not be used. This would also ensure that the lifetime of the callbacks is the same as lifetime of the process in which the libvirt hypervisor driver runs, preventing situations such as that the management app disconnects and libvirt wants to perform an async operation (e.g. device unplug finishing). The API contract should also require the management application registering a callback to few usage restrictions/requirements: - not allowing multiple callback registrations - guarantee that the callback returns to prevent stuck driver

On Thu, Dec 01, 2022 at 10:17:49AM +0000, Daniel P. Berrangé wrote: Exactly what I was afraid of with another approach I discussed with someone else a while ago. If you start providing ways to do arbitrary privileged operations, then you are effectively giving away privileged access. In this case I think it was an unfortunate choice of an API. If the API is *designed* to provide the proper identifying information, then the management application can then choose the action properly. My idea how to avoid callback failures was to split e.g. virDomainCreate() similarly to how virDomainMigrate...() is, i.e. into phases which would be invisible from the caller unless they want to do "something special" with the domain. Of course this does not work for actions initiated by guest/libvirt, but to start small this was one of the ideas. One counterargument to this was that it would require mgmt app developers to write an .so, or at least a C-compatible plugin. While I don't see that as an issue (apart from it possibly hindering the adoption) I can offer an idea which came before we even discussed this particular topic. It actually started from the other end, but let me give you some idea how it might be more usable approach for mgmt apps like kubevirt. If we provide a way to initialise this "plugin" at runtime, then there is no need to mangle with reloading the daemon. If we provide a way to do this over the already existing connection, then it might be even easier to use. However that would require a way to transfer a coded algorithm and being able to run it (ideally with reduced access to the system for security purposes). A format that could be pretty usable for this is WASM. And before anyone rolls their eyes give me a minute to mention few points about it: - there is no limit to the source code language used - it can be built particularly for the libvirt daemon dynamically (if someone wants that) - it can be ran safely by giving it access only to a limited set of predefined APIs - libvirt can run it itself, the runtime can be built in I'm not saying this is the solution to the points in question, just an idea I had few months ago which lead nowhere because I did not have enough time to make a simple bytecode run in a C program. Of course most of the above points can be solved without utilising WASM, e.g. by allowing plugins to be handled by admin APIs, running them in a sandbox, etc. Martin

On Wed, Dec 07, 2022 at 12:07:11PM +0000, Daniel P. Berrangé wrote: What I meant was something more along the lines of "place_vm_in_cgroup" where we would offload the cgroup placement to kubevirt. It already has to trust us with the information we provide now (which threads are placed in which cgroups). The benefit of having the callback per-connection and calling it on the connection that is starting the VM would kind of make the argument easier. Fair point. Me neither, I suggested that if someone wants to mend the behaviour of libvirt in a specific way (e.g. requests that the daemon does not call some function) can already be solved (or at least tried and tested) by LD_PRELOAD-ing a library. The first time it was said in a bit of an anger, but thinking about it the point is still valid. Of course that's not fitting for any production usage for which we'd want something at least resembling a plug-in system. Once there is an interface defined (for example the plugin) then it can do anything, of course. The advantage was mainly with code transfer from the client to the server, once that is out of the question (well it was never in) the other advantages are fall apart too. Static plug-ins are the safest (and probably easiest) choice. The argument about writing them, and mostly other arguments as well, are mostly constructed until people from mgmt apps weigh in with their opinions.

On Tue, Nov 29, 2022 at 09:05:33AM -0800, Andrea Bolognani wrote: I don't think this is a workable approach. A client connection to libvirt is a transient thing. We expect disconects+reconnects, and the VM needs to continue functioning throughout. We also expect there to be multiple clients sometimes. Even if most callbacks are only needed at VM startup, for KubeVirt, if we're going to design a privilege offload mechanism, we need to consider the broader picture. We need a mechanism that works for all VMs at all phases of their lifecycle, regardless of client connections that may or may not be present. IOW this is not something that can be wired into a public facing client API, it needs to be in-process for virtqemud. So what we're looking at here is better thought off as a platform backend "plugin". IOW, something would provide a plugin .so file for virtqemud, which can expose callbacks that can handle various low level operations. The plugin to be loaded would be configured at host deploy time. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|

On Wed, Nov 30, 2022 at 11:48 AM Andrea Bolognani <abologna(a)redhat.com&gt; wrote: +Andrew Burden <aburden(a)redhat.com&gt; This sounds like a reasonable request. Who would be in charge of manual moderation if we took this approach?

On Thu, Dec 01, 2022 at 11:17:02AM +0100, Andrew Burden wrote: The bounces we're getting look like this: Date: Thu, 01 Dec 2022 02:18:04 -0800 (PST) From: Mail Delivery Subsystem <mailer-daemon(a)googlemail.com&gt; To: berrange(a)redhat.com Subject: Delivery Status Notification (Failure) Hello berrange(a)redhat.com, We're writing to let you know that the group you tried to contact (kubevirt-dev) may not exist, or you may not have permission to post messages to the group. A few more details +on why you weren't able to post: * You might have spelled or formatted the group name incorrectly. * The owner of the group may have removed this group. * You may need to join the group before receiving permission to post. * This group may not be open to posting. If you have questions related to this or any other Google Group, visit the Help Center at https://groups.google.com/support/. Thanks, Google Groups Obviously the group exists, so it must be a permissions setting on the group. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|

This should now be working as intended. Sorry for the inconvenience, folks. On Thu, Dec 1, 2022 at 12:36 PM Daniel P. Berrangé <berrange(a)redhat.com&gt; wrote: