- Spectrum Discuss - spectrum-os.org

GPU virtualization in Spectrum
by Alyssa Ross 03 Nov '21

03 Nov '21

Table of Contents ───────────────── 1. Display controllers and renderers 2. Overview of GPU virtualization technologies .. 1. Direct passthrough .. 2. Hardware virtualization and paravirtualization ..... 1. Intel ..... 2. NVIDIA ..... 3. AMD ..... 4. ARM .. 3. Virgil 3D 3. Seamless windowing .. 1. virtio-gpu 2d .. 2. virtio-wl .. 3. virtio-gpu context types 4. Putting it together — GPUs in Spectrum VMs This report documents the current state of the GPU virtualization ecosystem as it relates to Spectrum. GPU virtualization is a very controversial topic in the world of compartmentalized operating systems. Qubes doesn't support GPU acceleration of anything but the GUI VM (the equivalent of Spectrum's compositor VM), and (I think as a result of this) a way to provide GPU accelerate to applications is *the* most requested Spectrum feature by a large margin. But there are serious security concerns with GPU virtualization. GPUs have a frankly awful track record when it comes to isolation. This might improve going forward, especially with SR-IOV-based solutions, because business demand for secure GPU virtualization is increasing and GPU vendors are trying to meet that demand. For example, AMD says of their GPU virtualization implementation[1]: The hardware-enforced memory isolation logic provides strong data security among the VFs, which helps prevent one VM from being able to access another VM’s data. With security being a bare minimum requirement for any virtualization solution, AMD’s hardware-based virtualized GPU solution offers a strong deterrent to unauthorized users who traverse the software or application layers seeking means to extract or corrupt GPU user data from the virtual machines. Although a VF can access full GPU capabilities at its own GPU partition, it does not have access to the dedicated local memory of its sibling VFs. But it remains to be seen whether these claims will stand up in practice, especially because SR-IOV is not yet widely available. The alternatives (software multiplexing and GPU emulation) have both seen exploitable security issues. Because of this, my current plan is that access to the GPU will be highly restricted in a default Spectrum system. I do think, though, that there needs to be a way for users to opt in to using whatever virtualization features their hardware provides, because I've heard so many accounts from people who find themselves unable to use compartmentalized systems /at all/ because of certain tasks where they need graphics acceleration. My primary goal with Spectrum is to bring compartmentalized computing to people who are not currently using it, and to people who're currently using mainstream systems where any application they run might be stealing their private SSH keys or ransomwaring all their files, being potentially vulnerable to zero-days in GPU drivers or hardware is the least of their problems. 1 Display controllers and renderers ═══════════════════════════════════ Standalone GPUs, and Intel integrated GPUs, tend to be display controllers and renderers all in a single package. The distinction between them will be important, though, so briefly: A *display controller* is the hardware that takes care of making a pixel grid show up on a screen. A *renderer* does computation (3D renderering, etc.) to create an image that can be given to a display controller to display. (Naturally in can also be used for other computations as well.) 2 Overview of GPU virtualization technologies ═════════════════════════════════════════════ 2.1 Direct passthrough ────────────────────── Direct passthrough is the simplest and least interesting option for accelerated graphics in a VM. The host simply gives the VM control over a whole GPU. Since we're aiming for one VM per application instance in Spectrum, direct passthrough might be useful if you have a dual GPU system, and want to accelerate one particular application, like a game. Of course, direct passthrough is also the most secure sort of GPU virtualization — data leaks are far less likely to happen when each GPU user has their own dedicated hardware! We can definitely support this. 2.2 Hardware virtualization and paravirtualization ────────────────────────────────────────────────── By *hardware virtualization*, I mean that the virtualization is implemented by the GPU itself, and by *paravirtualization* I mean that the host kernel implements virtual GPUs by multiplexing. The standard way of doing hardware virtualization is called SR-IOV, and it's widely used today with server network cards, and to a lesser extent NVMe drives. It's not widely available on GPUs, especially not consumer ones, but it seems like that's about to change. When using this sort of virtualization, all that's being virtualized is the renderer. The display controller stays attached to the host. I'm not aware of any implementation for hardware that is both a display controller and a renderer that supports separately passing through a display controller to a VM[2]. 2.2.1 Intel ╌╌╌╌╌╌╌╌╌╌╌ Intel's GPU paravirtualization technology is called GVT-g. Unlike solutions from other GPU companies, GVT-g is very widely available — it's supported for integrated graphics starting from Broadwell (launched 2014). GVT-g is definitely something we can support in Spectrum, but one limitation I found is that on my laptop (a Google Pixelbook), I can create at most two vGPUs, so don't think you're going to be able to have every application using accelerated graphics using GVT-g even if you're okay with the security implications. The most recent Intel GPUs no longer support GVT-g — they have hardware SR-IOV implementations instead. I don't believe this is supported in Linux yet, but I imagine it will be at some point. 2.2.2 NVIDIA ╌╌╌╌╌╌╌╌╌╌╌╌ NVIDIA officially supports GPU virtualization for most of their datacentre and professional GPUs, through their proprietary driver. Because of NVIDIA's general allergy to publishing code or documentation, I don't know how this GPU virtualization is implemented, but my guess is that it's mostly in hardware, because their product briefs claim their cards support SR-IOV. There also exists third-party software that can modify NVIDIA's software to support some consumer cards. The GPUs that support it tend to support between 16 and 32 virtual GPUs. Obviously, running a proprietary driver in the Spectrum host kernel would have /horrible/ security implications, and I will always strongly advise against it. But, we probably could make it work if we wanted to. 2.2.3 AMD ╌╌╌╌╌╌╌╌╌ There are five AMD GPUs that implement hardware (SR-IOV) virtualization. The Radeon Pro V520 is "only available as a public cloud offering" and so isn't a product you can actually buy. The Radeon Pro V340 is only supported by VMware, not AMD's open source Linux driver. The AMD S7100X, S7150, and S7150 x2 are all supported with KVM, but none of them have been produced since 2016. It would certainly be possible to support those last three in Spectrum, but due to just how obscure AMD GPU virtualization is, I don't see myself prioritizing it any time soon. 2.2.4 ARM ╌╌╌╌╌╌╌╌╌ There's single ARM GPU that supports hardware virtualization, the Mali-G78AE. It's intended for "automotive and industrial" applications, so I doubt it's going to show up in any hardware anybody wants to run Spectrum on. 2.3 Virgil 3D ───────────── Virgil 3D takes a different approach, by doing entirely software-based GPU virtualization. It implements OpenGL in software, in a way that's still backed by the host system's hardware GPU. This is naturally less performant than hardware virtualization or paravirtualization due to the extra overhead, but the advantages are that you can get a lot of virtual GPUs out of it, since every VM is just another application using the GPU on the host, and that this approach works even when there isn't special support for virtualizing a particular GPU. 3 Seamless windowing ════════════════════ One of the earliest features that Spectrum committed to was a seamless windowing experience, where applications running in different VMs would appear as native windows in the same Wayland compositor. 3.1 virtio-gpu 2d ───────────────── The simplest way to do seamless windowing is to run a Wayland compositor in each VM, and have it render windows to a virtual 2D GPU, where they can be displayed by a host compositor. To the host compositor, windows are basically just opaque rectangles. This is effectively how WSLg, the Windows Subsystem for Linux GUI, works. This works okay, but you lose the ability to have any integration between the application windows and the window manager they're being displayed on. It's not actually seamless. That's why, in screenshots of WSLg, you see Weston window chrome, instead of Windows ones. The WSLg developers can either choose to draw Windows window decorations around every application (which will look bad for GNOME programs that draw their own decorations), or none of them (which means they have to use Weston's decorations, or there'd be windows with no decorations), because they can't directly speak the client-side decoration negotiation protocol to the applications. This particular use case doesn't matter for Spectrum, because CSD is incompatible with unspoofable window decorations, but the point is that if you're not letting applications and window managers speak Wayland to each other, you need to come up with ad-hoc protocols every time you /do/ want them to be able to cooperate. Fortunately, there's another approach to seamless windowing, that preserves Wayland as the method of communication between applications and window manager. 3.2 virtio-wl ───────────── The original plan for seamless windowing in Spectrum was to use virtio-wl, a technology from Chromium OS. virtio-wl (plus Sommelier, a component that runs in guest userspace) presents a socket-like interface, over which Wayland clients running in VMs can send and receive arbitrary data, as well as file descriptors, to and from a Wayland compositor running on the host. Because implementing support for sending arbitrary file descriptors out of a guest would be impossible, only certain types needed by the Wayland protocol are supported — pipes and host-allocated shared memory. This worked fine for Wayland, but because of how Wayland-specific it is, virtio-wl was never considered suitable for upstreaming. This was a big problem for Spectrum specifically, because in virtio-wl there is no way for guests to share memory with the host. In virtio-wl's intended use case, the compositor runs on the host, clients run in guests, and the compositor handles all memory allocation, so memory only needs to ever be shared in one direction, host → guest. But in Spectrum, we'd like to have the compositor in its own VM, so we'd need some extra mechanism to let the compositor VM allocate host memory[3]. 3.3 virtio-gpu context types ──────────────────────────── Google's second attempt at seamless VM Wayland windowing is a new mechanism called virtio-gpu context types. Despite the "gpu" in the name, there's not (necessarily) any GPU involved — it's just convenient to use virtio-gpu as a transport since it already has primitives for sharing memory between guest and host. Context types allow new protocols, like Wayland or Vulkan, to be sent over virtio-gpu, in addition to the 2D and Virgil 3D protocols it was designed for. Context types should be available in Linux from v5.15 (the next release at the time of writing). Support for Wayland over virtio-gpu has already been implemented in crosvm and Sommelier. Even more excitingly, virtio-gpu has a mechanism by which guest userspace can do a special memory allocation that can be shared with the host! So we won't need to add any special memory allocation mechanism just for Spectrum — we'll just need to make sure the compositor VM userspace knows how to allocate memory and send it to the host in this way. 4 Putting it together — GPUs in Spectrum VMs ═════════════════════════════════════════════ After extensively researching all this, my current plan for graphics in Spectrum is as follows: each VM that runs a graphical application will get a virtio-gpu device, over which it will be expected to speak Wayland to the compositor. VMs with accelerated graphics (which will be opt-in per-VM) will additionally get another GPU device to use for rendering — this could be, for example, a GVT-g vGPU, or it could be a Virgil 3D virtio-gpu. They can use that accelerator (or software rendering if they don't have one) to render window contents into a buffer that can then be shared with the host (and by extension the compositor VM) by way of Wayland over virtio-gpu. Rendering on a GPU while using Wayland over virtio-gpu isn't currently supported by Sommelier, but Google has it planned[4]. A further problem we need to solve is how the compositor renders to the display controller. If we can pass the display controller through to the compositor VM (this might be a possibility on some ARM systems), all's good. But otherwise, we'd need to render to a virtual 2D GPU, and then have the host display that on the actual display controller. This is something that could probably be implemented fairly easily in crosvm's virtio-gpu stack — it already supports showing output as Wayland or X11 windows. And in the meantime, we could cheat by running the world's simplest Wayland compositor on the host, which would just show the crosvm window and take care of talking to the display controller. Graphics and windowing is definitely something that we'll be iterating on for a very long time. We can get started with the happy path Google already has working, with the compositor on the host. From there, we can move the compositor into a guest, with direct GPU passthrough. And then we can start looking at GPU virtualization. This field is evolving /extremely/ quickly, so by the time we get there things will undoubtedly have moved on. That means it's very important not to work too much on it too early, because that work has a very high chance of being obsoleted shortly afterwards. The purpose of this report is to demonstrate to the community that GPU accelerated applications in Spectrum will be possible by presenting how they would be implemented if I were to do so today. But in reality, it makes much more sense to focus on almost everything else first, because no other area of virtualization is moving as fast as this one. Footnotes ───────── [1] <https://www.amd.com/system/files/documents/amd-mxgpu-white-paper.pdf> [2] (This would be direct passthrough, because if you have a graphics card with one DisplayPort output, there's physically not much you can do to share that output between multiple VMs.) [3] I actually implemented this, before Wayland-over-virtio-gpu emerged as a viable alternative that didn't need an extra mechanism for this: <https://spectrum-os.org/git/crosvm/tree/servers/src/memfd.rs?h=interguest&i…> [4] <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9592#note_867838>

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Quick update about further updates
by Alyssa Ross 07 Oct '21

07 Oct '21

Hi everyone, especially people who're new to the project after seeing it on Hacker News recently. Those of you who've been around for a while will remember that the end of last year was a busy time for Spectrum because of funding cycles, and that's going to be the case again this year. As a result of this, I'm going to have my head down until the end of the year, and am not planning on doing more This Week in Spectrum this year. I love TWiS, but it takes hours to write every week I manage to get it done, and I need to spend those hours elsewhere at the moment. It'll be back next year. If you do still want to keep up with development (and I hope you do!), I'm trying to talk in more detail about what I'm working on in the #spectrum IRC channel. There's been a good reaction to this so far, and the conversation has been extremely lively for the last few days. There are consistently over 120 people in the channel now, which is amazing! You can find information about how to join the channel through Matrix or IRC on the website[1]. Hope to see you there! [1]: https://spectrum-os.org/participating.html#irc

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This Week in Spectrum, 2021-W31
by Alyssa Ross 14 Sep '21

14 Sep '21

What a week. Progress has felt a bit slow, but the work has been consistently interesting, and there have been some exciting developments in the ecosystem. Nixpkgs ------- Last week I'd posted the patches required to get all of Spectrum working with a more up-to-date Nixpkgs. After last week's email, Cole reviewed the patch set, so I applied it, and merged the nixpkgs-update branch into master, so now Spectrum is using a reasonably up-to-date Nixpkgs for the first time in a long time. I also said > Once that's done, it's time for another chromiumOSPackages upgrade, but > that should be pretty easy this time because we're only one version behind. Alas, it's never that simple. Firstly, the chromiumOSPackages update script broke, because the information about the currently released Chromium OS build seems to have gone missing. Google is apparently[1] serving build number 13982, but their published build metadata[2] includes builds 13981 and 13983, but not 13982. This means it's not possible for me to know what Git revisions are used in the currently released Chromium OS. Assuming this is just a one time thing, I hacked the update script to just look at the previous build, but we should keep an eye on this. If it ever happens again we should probably implement some sort of mitigation in the update script. Once I had new versions of the Chromium OS packages it was time to get them to build, which was straightforward enough for everything except crosvm. For Spectrum, we have a patch[3] for crosvm to make it support VIRTIO_NET_F_MAC[4], which is a mechanism by which the host system can indicate to the guest kernel what it should set as the MAC address of a virtual network device. After the update, this patch no longer applied, because all of a sudden crosvm has two different virtual network device implementations. This turns out to be because crosvm has implemented vhost-user[5], a protocol to allow virtual devices to be implemented outside the VMM program! This is great news, but it's also surprising. virtiofs[6] was designed to be implemented with vhost-user, but when crosvm implemented it a while ago, they became the only implementer to do so in-VMM. It's great to see them moving in the vhost-user direction, because it makes it much easier to mix and match virtual device implementation and VMMs. Most excitingly, I saw a reference to vhost-user-wl, meaning a standalone implementation of Virtio Wayland. This would allow us to use Virtio Wayland with other, non-crosvm VMMs, which is great because I think cloud-hypervisor[7] is probably going to end up being a better fit for Spectrum, but Virtio Wayland was crosvm's killer app. I'd even thought about trying to port crosvm's virtio wayland implementation to vhost-user myself, so it's great to know that when the time comes, that'll already have been done for me. While I could have fixed my crosvm patch for all this new virtual network device code, the introduction of vhost-user support means we should be able to drop the patch altogether. cloud-hypervisor provides a vhost-user-net implementation[8] that already supports VIRTIO_NET_F_MAC, so if we can just get crosvm to talk to cloud-hypervisor's vhost-user-net implementation we shouldn't have to carry any patch for this any more. [1]: https://cros-updates-serving.appspot.com/ [2]: https://chromium.googlesource.com/chromiumos/manifest-versions [3]: https://spectrum-os.org/git/nixpkgs/tree/pkgs/os-specific/linux/chromium-os… [4]: https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.html [5]: https://qemu.readthedocs.io/en/latest/interop/vhost-user.html [6]: https://virtio-fs.gitlab.io/index.html#overview [7]: https://github.com/cloud-hypervisor/cloud-hypervisor [8]: https://github.com/cloud-hypervisor/cloud-hypervisor/tree/db2159b5638eb4ccf… cloud-hypervisor / rust-vmm --------------------------- So I started looking at cloud-hypervisor to try to hook this all up, but it looks like cloud-hypervisor still has some issues where it doesn't quite follow the vhost-user specification. As I was trying to debug this, I noticed some UB in rust-vmm[9] (the shared utility code project for crosvm and its derivatives like cloud-hypervisor and Firecracker[10]). Being a good citizen, I started working on a fix for this, and then I encountered some more issues with functions that should have been marked as unsafe but weren't. So that's going to need to be fixed too. But the rust-vmm code is otherwise very high quality and easy to work with, and they're very responsive, so it shouldn't take long to get these issues fixed. The affected code is fortunately all to do with communication between vhost-user backends and the VMM, so it's very very unlikely that it's anything that could be exploited by a guest. These sorts of problems are exactly the sort of thing that Rust is supposed to prevent, so it was disappointing to discover these issues. But on the other hand, Rust made them stand out like sore thumbs to me, so even though these issues managed to sneak in, there's definitely still a huge benefit to using Rust for these sorts of programs. [9]: https://github.com/rust-vmm [10]: https://firecracker-microvm.github.io/ In the next week, I'm hoping to get all the rust-vmm issues I've discovered fixed, and maybe get cloud-hypervisor's backends to speak proper vhost-user. I'm getting my second vaccination next week as well though, so we'll see. I might just end up being sick.

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This Week in Spectrum, 2021-W33
by Alyssa Ross 22 Aug '21

22 Aug '21

Hi! I've had a busy day today and am pretty tired, so I'm not sure how coherent my writing is at the moment. But I'd rather get this out on time, especially since tomorrow is also busy today. As I said last week[1], I took some time off this week as a preventative measure against burnout. [1]: https://spectrum-os.org/lists/archives/spectrum-devel/87lf51ybwk.fsf@alyssa… rust-vmm -------- One of my patches[2] was accepted, but another[3] is still waiting. As I said last week, I have more fixes for rust-vmm planned, but want to let them catch up with the changes I've already sent upstream first so I know what base I'm using for the next stuff. I expect the one that's still waiting to be accepted next week. [2]: https://github.com/rust-vmm/vmm-sys-util/pull/135 [3]: https://github.com/rust-vmm/vhost/pull/69 spectrum-live ------------- Last week, I'd just integrated dm-verity into the Spectrum live image I've been working on. When it came time to work on the actual root filesystem, instead of the initramfs, I hit a bit of a brick wall. I realised that trying to generate a whole operating system image using Nix was giving me real writers' block. There was too much inbetween me and how the files ended up on disk, and that meant there was too much overhead to keep in mind when I was thinking about how things should be designed and laid out. It might feel like making a Linux root filesystem should be a solved problem, but Spectrum has a bunch of special requirements. You might want to do something like start a VM for each hardware device of a specific type, and that's something that isn't really addressed by most standard stuff. All this stuff is definitely solveable, but it requires some experimentation to get right, and Nix was getting a bit in the way of that. So I created a new directory, and I wrote a Makefile that builds an ext4 image, and I just started putting files in an etc/ directory. This made reasoning about the system way easier, and I was immediately making progress. Nix is great for building known targets, and for making customisable systems (there's no way my Makefile-based system would allow the amount of customisation I'd easily be able to provide with Nix), but for experimentation, it's a lot nicer to be closer to the end product. So once I know how all this should look I'll make it Nix-aware. Currently, I have a root filesystem with a service manager that can respond to hardware appearing and disappearing. The next (more exciting) step will be to have it start some VMs, and assign hardware to them appropriately. I'm looking forward to getting to that next week. An interesting challenge I'll have to solve will be figuring out simple categories (e.g. "ethernet device") from the huge amount of very specific information the kernel provides. I think I might be able abuse the modules.alias file from the kernel, that defines the mappings from PCI etc. information to default drivers. Then all I'll have to do will be to write mappings from default drivers to whatever categories I come up with, or what VM I want to assign them to, or whatever. One neat thing I'm using for the first time here is tar2ext4[4], a utility program that's part of a larger Microsoft open source project I don't entirely understand the purpose of. It's really useful for me it builds ext4 images entirely in userspace, which will be great for using in Nix derivations where it's not possible to just mount an ext4 image and write directly to it. For previous Spectrum experiments, I'd always used SquashFS[5], entirely because I already knew of a tar2sqfs program[6] that made creating filesystem images really easy. I've added tar2ext4 to Nixpkgs[7], which will hopefully help other people who have similar problems discover it. [4]: https://github.com/microsoft/hcsshim/blob/master/cmd/tar2ext4/tar2ext4.go [5]: https://en.wikipedia.org/wiki/SquashFS [6]: https://github.com/AgentD/squashfs-tools-ng [7]: https://github.com/NixOS/nixpkgs/pull/134650 musl ---- While I was working on the root filesystem, I noticed that mount -a, which mounts all filesystems described in fstab(5), wasn't working. This turned out to be because of a bug in Musl's implementation of getmntent(3), a libc function for parsing fstab files. So I wrote some tests and a fix and sent them to Musl[8]. [8]: https://inbox.vuxu.org/musl/20210821085420.474615-1-hi@alyssa.is/ Hope that all made sense. Next week, I'll continue working on Spectrum live, and maybe fix the next rust-vmm issue I have on my todo list if my final outstanding PR is merged.

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This (Last) Week in Spectrum, 2021-W32
by Alyssa Ross 16 Aug '21

16 Aug '21

It's a short update this week, because most of what I did was a continuation of stuff from last week. rust-vmm -------- Last week, I mentioned I'd identified some Rust safety issues in rust-vmm. Most of the patches for these are now up[1][2][3]. The first has been accepted already, and I expect another to be accepted later today. There's still a UB issue I'm aware of and haven't sent a fix for yet, because there are a number of ways to fix it and I wanted to get my other patches in first before I decided how to fix that one. I deliberately haven't made any progress on using cloud-hypervisor's vhost-user-net backend with crosvm, which is what got me looking at this code in the first place. I want to make sure I can work on rust-vmm-adjacent things at a pace where I don't get overwhelmed with having to keep track of loads of patches and whether I've got them upstream yet. So I'll be putting that work on hold until the current round of patches are upstreamed. [1]: https://github.com/rust-vmm/vhost/pull/68 [2]: https://github.com/rust-vmm/vmm-sys-util/pull/135 [3]: https://github.com/rust-vmm/vhost/pull/69 spectrum-live ------------- For the past little while, in the time when I wasn't writing regular updates, I've been working on a live system for testing Spectrum. This will be especially useful for testing things like GPU support, because I can just build a live image with everything I might need, plug it into all the computers I want to test, and have everything be automatic from there. It will also probably evolve directly into what becomes the Spectrum base system that we'll hopefully all be running as the host system on our machines at some point. I shifted my focus back to this this week because of wanting to not get ahead of myself with rust-vmm. (I have a funding milestone for GPU support, so getting that checked off soon would be good.) The main thing I did this week was integrate dm-verity[4], which I did mostly for fun and to satisfy my curiosity. dm-verity is a Linux mechanism to efficiently ensure that a read-only filesystem hasn't been tampered with, by constructing a Merkle tree out of filesystem block hashes, and providing the root hash to the kernel when the filesystem is mounted. dm-verity is a _great_ fit for Nix, because we can generate the hashes at the same time as creating the filesystem image, and then embed the hash into the initramfs we're also building. Getting this all working took less than a day. The idea is that (long) in the future, we'll also implement Secure Boot, which will make sure the kernel and initramfs haven't been tampered with, and dm-verity will extend that integrity guarantee to the host system's root filesystem. I recommend reading "Producing a trustworthy x86-based Linux appliance"[5] by Matthew Garrett for an overview of how this all comes together. dm-verity is something that's particularly exciting to me, because it's very useful to us, but it's something that's generally used to frustrate end user attempts to control computers they own. In Spectrum, it's instead a tool that protects the end user against malicious filesystem changes, while being almost completely transparent to the user if they do want to modify their own system. Protecting against root filesystem tampering (which would require a VM escape or physical device access) is hardly the biggest priority for Spectrum, but integrating dm-verity was fun, interesting, and provided good motivation for working on the live image, which is one of the highest priority bits of the system. (Because I'm tired of having to say "you can't" when people ask me how they can try out Spectrum.) [4]: https://lwn.net/Articles/459420/ [5]: https://mjg59.dreamwidth.org/57199.html This week, I'm going to take a bit of time off as an anti-burnout defense, but probably not the whole week. I'll still keep an eye on the rust-vmm patches throughout this time as well, to make sure they're not delayed in getting accepted upstream.

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This (Last) Week in Spectrum, 2021-W30
by Alyssa Ross 02 Aug '21

02 Aug '21

Hi, it's been a long time since I've done one of these, but so much interesting stuff happened this week, and there's been a bit of renewed interest in the project, that it _really_ needed an update. So here it is. As always when I'm not keeping up with TWiS, this update is limited to only things that happened in the last week, because otherwise it would just be far too long. Wayland ------- A problem I've been thinking about for a long time is Wayland access control. The Linux desktop ecosystem is moving towards access controls for most functionality through xdg-desktop-portal[1] (which sadly doesn't seem to have a website). But it doesn't cover stuff that's part of the core Wayland protocol, like access to the clipboard. And some compositors (wlroots-based ones) provide extra Wayland protocols for things like screenshots. And up to now, Wayland compositors haven't really done any authorization for these potentially dangerous APIs. (All of this is only really meaningful for sandboxed applications -- running in Flatpak or a Spectrum VM or something. Linux isn't really set up to do separation between processes running as the same user without namespaces.) So earlier this week, I posted[2] to the Wayland mailing list with an idea I'd been thinking about for a while[3], which was to place a proxy program in front of the Wayland compositor, that would intercept client->compositor requests and handle access control. I was quickly convinced that a proxy wasn't a good idea, but there was a lot of discussion, and it was really helpful to me figuring out what the right way to do it might be. There are really two problems to be solved here, one of which I hadn't even thought much about. The first is securely identifying a Wayland client. A compositor needs to be able to form the question "Should client X be able to do this?", and to do that it needs to be able to identify a client as client X, and know if it tries to interact with client X later, that it'll be talking to the same client. In a non-virtualized system, the obvious way to do this would be getting the pid of the client from the connection socket and then looking it up in proc(5) to find out executable path, but this approach is fundamentally insecure[4]. The way forward here (and one that would work for Spectrum) appears to be the proposed Security Contexts protocol[5], which would allow a sandbox implementation to provide a security context identifier for a client before handing off the Wayland connection to the client. Once the security context had been set up, it wouldn't be allowed to be modified, so once the sandboxed client was given the connection, it wouldn't be able to change the security context identifier. In Spectrum, the security context identifier here would likely be a unique, user-provided name for the VM, and the security context setup would be done by the virtio wayland implementation in the VMM. The second part of this puzzle is how the compositor should decide whether a client should be allowed to perform a particular operation, like a paste or going fullscreen (which is risky because it might spoof a desktop). There was actually a previous attempt at this a long time ago. libwsm[7] (short for Wayland Security Module) was a library that compositors would have integrated to make authorization decisions. But it wasn't adopted by compositors, and it some things we now know[8] to be bad ideas, like setting policy based on the executable path. It also made compositors responsible for any sort of authorization UI. In my opinion, it's better to have that done by the external piece, so that compositors have as little work to do as possible and therefore authorization is implemented as widely as possible. The compositor could implement an authorization system entirely on its own, but this would be a lot of code for each compositor to write, and it would limit the user to whatever permissions system the compositor came up with, which might not be able to accommodate their needs. (An example of this would be a Spectrum user that wanted to allow pasting between two applications, but not allow pasting between either of those and any other application.) It could also be implemented by having the compositors integrate with something like polkit[6], but compositor authors are reluctant to integrate directly with a single system like that, and even polkit might not support everything that would be desirable in an authorization system. (For example, it might be nice to implement libwsm's concept of a soft allow, where an action is permitted, but a notification is shown so the user is aware it's happened.) So a third solution, suggested by ifreund, a Wayland compositor author, in the Spectrum IRC channel, would be to have a privileged Wayland client that the compositor could ask authorization questions to, with a new protocol. The compositor would know that this particular client should be privileged because it either wouldn't be in a security context, or would be given a special security context identifier known to the compositor ahead of time. Then, implementations of this protocol could do authorization however they wanted, with the only limitation being the questions the compositor was programmed to ask them. I think this is a good way forward, but it'd be important to discuss with more compositor authors before getting to excited about it. Next steps from here with Wayland are: * Figure out what needs to happen to move the security contexts proposal forward. If it needs an experimental implementation, maybe we could help with that? * Inquire about the authorization protocol idea, and see how other compositor offers would feel about it. If there's a generally positive reaction, figure out how to move forward with it. [1]: https://github.com/flatpak/xdg-desktop-portal/ [2]: https://lists.freedesktop.org/archives/wayland-devel/2021-July/041896.html [3]: https://spectrum-os.org/lists/archives/spectrum-discuss/8735ueudel.fsf@alys… [4]: https://gitlab.freedesktop.org/wayland/weston/-/issues/206#note_176699 [5]: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/68 [6]: https://www.freedesktop.org/software/polkit/docs/latest/polkit.8.html [7]: https://github.com/mupuf/libwsm [8]: https://lore.kernel.org/lkml/87r1xplsku.fsf@x220.int.ebiederm.org/ Nixpkgs ------- I created a branch in Spectrum's nixpkgs repository[9] for the long-overdue merge with upstream, did the merge, and posted some patches[10] to spectrum-devel that fix all the builds that broke as a result. I've been letting them sit for a few days hoping for a review. Once that's done, it's time for another chromiumOSPackages upgrade, but that should be pretty easy this time because we're only one version behind. [9]: https://spectrum-os.org/git/nixpkgs [10]: https://spectrum-os.org/lists/archives/spectrum-devel/20210729100928.196534… Thanks for keeping up with Spectrum. :)

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#spectrum IRC channel has moved to Libera.Chat
by Alyssa Ross 31 May '21

31 May '21

The Spectrum IRC channel is now #spectrum on irc.libera.chat. It can also be joined through the Matrix bridge as #spectrum:libera.chat. I understand there are some teething problems with the Matrix bridge, but hopefully they should be resolved soon. Apologies for the inconvenience.

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I'm on a panel at the NGI Forum tomorrow
by Alyssa Ross 29 May '21

29 May '21

Tommorow (2021-05-19) at 7:30 UTC, I'm going to be part of a panel discussion at the NGI Forum. NGI is the EU initiative that funds my work on Spectrum. The topic of the panel is "Internet of Trust", and it'll be streamed here: https://2021.ngiforum.eu/ There's a bit more information about the panel in the agenda here: https://prod5.assets-cdn.io/event/6431/assets/8379007230-30f8d8b6fb.pdf

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Proxying Wayland for untrusted clients
by Alyssa Ross 25 May '21

25 May '21

I've been thinking a lot about this for a while, thanks to conversations with Thomas and Puck, CCed here. I think it's time to put it into words properly and start working towards making it happen. One of the benefits that Wayland is supposed to have over X11 is security. A Wayland application isn't supposed to be able to record the screen without user permission, for example. But in most compositors, it can, with no restrictions. Existing Wayland compositors are monolithic, and each one would have to implement its own access controls. (Mutter already does this to some extent, at least for screen sharing, I believe.) The popular Wayland compositors are largely focused on being feature-complete reimplementations of their X11 equivalents, and so taking advantage of the security features and access controls the Wayland protocol makes possible hasn't been a priority for them. Additionally, every popular Wayland compositor is written in a memory-unsafe language, and this combined with the complexity of the Wayland protocol, with all the extensions involved, presents a serious concern to applications of Wayland that involve untrusted clients. To solve these problems, I propose a proxy program that sits between Wayland clients and the compositor, in the same privelege domain as the compositor. The proxy would decode and re-encode every Wayland request (client->compositor message), and would discard any request it didn't understand. This would mitigate the problem of a large, privileged program written in a memory-unsafe language being exposed to untrusted inputs. Additionally, the proxy would support a plugin interface, through which the user of the proxy (or their distributor) could configure custom behaviour. This could be used to prompt the user for confirmation before allowing a screen capture request, or even to implement a similar thing for e.g. clipboard access, for which there is no support in the Wayland protocol. It could even be used to modify surfaces, to implement things like Qubes-style unspoofable coloured window borders. This approach would allow permissions systems and other custom Wayland behaviour to be implemented in a compositor-independent manner. Distributions which suppor tseveral compositors could implement customisations in a single place, and users of compositors which lack security features and the assurances memory-safety can provide against untrusted input would gain access to those things. I'd like to hear feedback here, but I think early in the life of this idea we should also reach out to the broader Wayland community. I think there's a lot of potential for this idea beyond Spectrum, and it would be great if it could be something developed with input from a big breadth of Wayland users. If we can do that, it might be sensible for it to live at freedesktop.org? I'm not sure how that works. Let me know what you all think. :)

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Re: Proxying Wayland for untrusted clients
by Josh DuBois 22 May '21

22 May '21

On May 22, 2021, at 8:05 AM, Alyssa Ross <hi(a)alyssa.is> wrote: > > One of the benefits that Wayland is supposed to have over X11 is > security. A Wayland application isn't supposed to be able to record the > screen without user permission, for example. But in most compositors, > it can, with no restrictions. <snip> > > To solve these problems, I propose a proxy program that sits between > Wayland clients and the compositor, in the same privelege domain as the > compositor. <snip> > If we can do that, it might be sensible for > it to live at freedesktop.org? I'm not sure how that works. I am curious, if you have time, to hear more on why the approach of a proxy vs picking a compositor and implementing security there. If the problem is that the Wayland community so far has not considered security a priority, it seems that a security proxy may suffer from those same forces. Basically, will it be easier to attract developers or gain widespread adoption of a proxy as opposed to getting buy-in to do security directly in a compositor? You mention writing in a memory safe language and having a compositor neutral solution as technical advantages. Do you think a proxy is a good choice primarily because it can achieve a better technical result, or is the choice of a new component more a matter of difficulty getting community buy-in from a popular compositor and doing security there? How would you weigh the upsides of a new project against the difficulties of getting a new thing off the ground and adopted? (This is really just curiosity on my part and my $0.02 from the outside. You may have already had a lot of discussions about that, or even already tried talking to compositor folk and not gotten traction. Seems worth some explicit consideration.)

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