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Linux for Chromebooks: Secure Development (Google I/O ’19)

We are here to talk to you about Linux, for Chrome box, also known as crostini. We will start by introducing ourselves. My name is Sudan. I am a designer on crostini for Chromebooks hi, I’m Dylan, I’m the Chrome, OS virtualization lead and I’m Tom product manager for Linux on Chromebooks. Now it’s the end of day two at i/o and you’ve probably already been to a bunch of different sessions that have talked about all the new frameworks that you need to be using or the platforms that you need to be building for and everyone’s right.

You absolutely should be, but we’re not really here to talk about that. Instead, what we want to talk about is you as developers and how you can get more peace of mind by using Linux on Chromebooks, we give you that peace of mind by balancing simplicity and security. On that note, let’s do a quick user study. How many of you are developers in the audience? Wow, that’s full room as expected. Keep your hands raised, how many of you use your computers for anything else, other than development like doing your taxes, checking email, again, 100 % of you, okay, one last question: how many of you are worried about security? Good, that’s pretty! I mean you all should be so I’m glad to see many hands up anyway, so I don’t know about you, but when I start a new project I I get stuck a lot right.

I hit a lot of walls and I hit a lot of barriers and go to look for a problem, go to look for a solution and I turn to Google. Luckily, Google’s almost always got a great answer for me. Unluckily. Sometimes the answer looks like this, and I know I shouldn’t run this script from evil site, comm and pipe it to soo do. But you know that deadlines coming up. This may be, the site, looks kind of legit, so in this case I’ll make an exception and I’ll do this and then it happens again and again and eventually I end up with a system that I don’t trust as much as I should, because I don’t really Know what code I’ve run on it anymore? I don’t have time to read all these scripts.

My solution to this has been to carry two laptops, one for my developer world and one for my everything else world that I want to be secure in, but recently I switched to using one laptop and Tom’s going to talk about how I do that. So our goal with Chrome OS has been to give you a simple and secure experience from the start, but if you tried it previously, you might have seen that it wasn’t quite ready for developers in order to be simple and secure.

We couldn’t run all of the tools that developers need to get their job done, but that all changed at i/o. Last year, when we announced that we were going to start supporting Linux on Chromebooks Linux on Chromebooks lets, you run all of your favorite editors, IDs and tools, and it’s now supported on over 50 % of Chromebooks, including great devices with eighth generation. Intel CPUs like the Lenovo yoga book C 630 and the Acer Chromebook spin 13.

If you haven’t seen it we’re going to run through a few examples. First off, how do you get it? It’s really easy! It’s just a couple clicks now. In the background, this is downloading a virtual machine setting up containers configuring, it all Dylan’s, going to go more into that in a few minutes. But for you as a developer, it’s just a couple clicks to get started and this adds a terminal to your launcher.

Now, if you open that terminal, you’ll see that you have a pretty standard, debian environment and we’ve already loaded in a bunch of the tools that developers expect like git and vim. And if you need anything else, you have the apt package manager and you can install whatever packages you need and if you want to, instead install files or install programs via dev files, you can do that too. This gives you access to thousands of world-class developer tools.

Now, once you’ve installed, any graphical you’ll find that they all show up in your launcher, just like the rest of your Chrome, OS apps, and if you open them, they show up in your window manager again just like the rest of your Chrome, OS apps. This is the simple experience that people have come to expect from Chrome OS and we didn’t want to change that with Linux. But importantly, this is also secure.

You don’t have to worry about malware, accessing your files, snooping on your traffic or infecting your peripherals. I’d ask you to trust us on that, but this is way too important for you to take on Trust alone. So over the course of this talk, Dylan and Sudha are going to walk you through the principles behind the architecture and design of crostini. We’re then, going to dissect some common developer flows to show you how these principles apply and, finally, we’re going to share some tips and tricks for advanced usage for the power users out there.

So now I’m going to hand it over to Dylan to talk about the architecture. Okay, so Chrome OS has always had a layered approach to security, and our big layer has always been the browser and the renderer and running all untrusted code in a nice, isolated renderer, and that keeps the attack surface of your core system to an absolute minimum. They’re not allowed to make a lot of system calls, they can’t poke at random bits of your kernel and that worked really well for webpages web apps.

However, for developer tools, I need to install a lot of different programs. They need a lot of different privileges. They can do anything any app on Linux can do and that wasn’t acceptable for us on the core of Chrome OS. So we need that a layer, so we added a virtualization layer and that lives in the main, Chrome OS layer and that spins up a VM. And now this VM has a much more limited interface, while still exposing a full Linux kernel to the programs that run inside the VM.

The only way the VM can talk to Chrome OS proper is through a small API that that cross VM program on the left up there exposes to the guest. This was pretty good. Now we’ve got a lot greatly reduced attack surface. We were pretty happy with this. We wanted to go a little further, so we made sure that the guest VM was also signed by Google and somewhat trusted. This lets us trust some of the actions the guest VM takes, and it’s also read-only.

So users can only break things so much and that no matter what you do, you’re going to be able to boot a VM. However, with all that security solved, we’re back in a situation where you don’t have enough flexibility, your apps can’t do anything. It’s a read-only thing: you can’t install anything in it, so we had another layer and for this we stole used lxd from canonical. That teams been very helpful in getting this spun up with us.

It’s a pretty standard container run time. It’s built for running system containers and in our case we started a system container of Debian and exposed to that to the user so that cross VM layer. I was talking about that’s kind of the most important part of the security story. Here it’s the last line of defense before something gets into Chrome OS. So we went. We focused on this for a long time and made sure we got that as secure as possible.

We wrote it in a memory safe programming language. We chose rust. This eliminates buffer overflows and integer overflows a lot of common bugs related to memory safety that are exploited by attackers. We were pretty happy with that, but we again added another layer of security here in that we broke up the virtualization program into pillars and made sure that each pillar that interfaces with the guest only has access to small parts of your hosts Chrome OS system.

So your host Chrome, OS system, you’ve got your bank’s web page open. You’ve got your online tax filing thing. Open you’ve got all kinds of personal identifiable information everywhere. We really wanted to protect that, but we needed to give the guest access to things like a random number, a display, a USB device, so each of those got their own jail and they can only see the thing they need, so our random number generator can generate Random numbers: it can’t access any files, it’s in an empty file system.

From its perspective, it doesn’t have any network access the display driver, it can access the display again, it can’t touch the network, it can’t go, grab your files and upload them, even if somebody gets into it and tries to make it do things we didn’t intend it To this is all a little complicated, but we’ve added a great amount of system UI to make this easy for you to use. So when you’re just doing your job as a developer, you don’t have to worry about these.

Pretty pictures I’ve drawn for you and I’ll show you of what we did. Thank You. Dylan security is absolutely top of mind for us. While crafting the Linux experience on Chromebooks, we came up with three high-level design goals. The first goal was to keep your experience intuitive. Everyone here in this room has been using computers for a long time and you have just established your workflows and habits.

So, basically, what we wanted to do is to match to those those expectations. We wanted to provide an experience, that’s natural to you. We want developers everywhere to be using Chromebooks and feel right at home doing it. The second goal was to make your experience native. We could have taken the easy path by giving you a full Linux desktop in a VM, but that wasn’t good enough. Our goal was to bring the Linux apps.

You depend on for development into your native Chrome, OS experience. The third goal was to make your experience simple, and I think this is very important. There’s a lot of complexity, that’s going on under the hood, and we want to leave it there. Our guiding principle is that complexity shouldn’t interfere with the user experience. There’s a couple of things. We are trying to balance here. The security concerns that come with installing Linux apps on Chromebooks and the simplicity that comes with sticking to design patterns established by Chrome, OS and our mission was to find that sweet spot all right.

So now we’re going to talk about three common developer flows and see how they work with crusting. The first of these is accessing files as developers. We have to do this all the time our editors need to access files, as do our compilers, our source control and a whole lot more, but the problem is that our file systems have a lot more than just code. They have our personal photos, our tax returns.

Maybe that novel that you’ve been working on a lot can go wrong. Ransomware can hold all of that data hostage. Malware can upload your files to some rain server, or maybe you just get something that goes and deletes everything for the fun of it. We built crostini with those threats in mind to limit what can go wrong and Dylan will tell you how so our goal, sharing files with your VM and with your container, was to make it easy for you to get the files you needed for your development tasks.

Where you need them, but not expose things, you don’t want exposed to untrusted code, because ultimately we don’t trust the code. That’s running inside this VM. To do this, we took a layered approach, your files all live in Chrome OS at the very bottom, and we share them out to the VM with a 9p server. We named it 9s again. We wrote it in rust, so it’s memory safe. We fuzzed it to make sure unexpected inputs, don’t cause unexpected behavior and we put it in a in a tight jail.

So it can access only the files you share with it and it takes those files and exports them to the VM. The VM mounts the 9p thing, that’s built into Linux and then lxd takes that mount and exposes it into your container, where your development tools are running. The important thing here is that your container can only see files. You say I want to share with my development environment. Your VM can only see those same files and even the server that we wrote running on Chrome OS can only see those files.

It doesn’t get to see everything. So somebody exploits this stack all the way back into Chrome OS. They still don’t have access to the files you haven’t shared with the container. That’s a lot of stuff to set up setting up 9 P mounts bind mounting things into containers. We had to do this manually for a while. We were developing it. It was painful, so let’s do to show you how easy we made it for you.

There are a lot of layers going on, but let’s see how simple this is in the UI right out of the box, you have a directory called Linux files, which is your home directory within Linux. Anything in this directory is automatically shared with Linux. Outside of this directory anywhere else on the device, Linux doesn’t have access to anything until you grant permissions I’ll walk you through a couple of examples here, let’s say you’re working on a project, and you see yourself needing files from this.

One folder called illustrations to share this: all you have to do is access the right-click menu and click on share with Linux. In as simple as two steps, you now share this folder with Linux. If you notice, this is in Google Drive and that’s a cool thing when you don’t want to share this anymore, you can do that by going to settings and unshare here’s another example where we made quick edits, really simple for you.

You have a data file in your downloads folder and when you double-click it automatically opens in vs code when this happens in the background it’s implicitly shared and the sharing last until you restart. This is the balance of security and simplicity. We wanted to bring you. Thank you so, for our second developer flow that we’re going to talk about we’re going to look at running a web server. Now being Chrome OS.

We care a lot about people making great web apps and we want to make sure that they can create those on a Chromebook and being able to run a web server is pretty central to being able to build any web app. Unfortunately, web servers also need a pretty low level of access, and that can cause some problems. The code that can run a web server is also capable of snooping on your internet traffic. It can know what sites you’re accessing and, in some cases, even see the contents of those pages.

This means that a malicious web server could potentially track everything that you’re doing now again, we thought of this as we design crostini, and we made sure that we prevented this kind of attack. Linux Dylan will tell you how I can be called Linux. It’s my job. All right so starting a web server from crostini, simple we’ve got a good demo over in the web, dev sandbox already type of command. You fire up your web server, just like you would on any Linux distribution out there what’s actually happening under.

The hood, though, is you’re in a container, and you open up a port that ports in a network namespace inside a VM running under our special hypervisor, which puts its network stack in another namespace on the host and then finally out to Chrome, so Chrome can’t get Back in which is great for security, you’ve got wonderful isolation, but if I want to test this new PWA or webpage, I’m running in my VM, how do I get chrome to talk to it? This was not simple.

So for that we had to add some demons along the way. Actually, every layer gets a daemon for this there’s the first one is running in the in the VM and it’s sitting there waiting to check if any container that’s running happens to open a port, and then it’s got to figure out which container open that port and Bundles that information up sends it to Chrome OS, so hey this port in this container is listening.

The user might want to use that port and on the Chrome, OS side we say – ok, the other daemon responds says I will set up a route to do some forwarding I’m going to forward all of this over Vee sock, which is a protocol used to talk To local VMS, on on machines, that’s kept under the hood, so either end talks HTTP in in to the demons and the daemons dog Vee sock to each other. So the key here is that the web server gets to talk.

Http Chrome gets to talk, HTTP everything’s, normal everything works. Just like you would well under the hood. We’ve got all this extra daemons and V sock forwarding going, but we’ve hidden that one other important thing we’ve made it trusted. So you can get all your PWA features. You can install it to your desktop, even though it’s not technically the same machine. We know it is because we’ve got the information we set up the VM, so we allow that to be trusted domain and all this complexity, I think, makes one of our best demos.

Today of how complicated we made it under the hood and how simple you’re going to see it is to actually use. I totally agree that this is very complicated under the hood, but in the UI it’s exactly like you would expect it to be. Let’s say: you’re experimenting with building this cool PWA here in terminal you’re in your folder pwe, a starter kit, you’re running a commands to start your web server and if you see at the bottom of this screen, it’s listening at port 8080.

At this point, you can launch your browser, go to localhost 8080 and test your web app on the screen here on the left. You have your web app in Chrome and on the right if you’re noticing it it’s in Chrome. Yes, you can test your web app on a Chromebook in Firefox too, if you noticed, we did not prompt you to give any permissions while we were in this flow. This is because the host is accessing the VM and not the other way around again.

This is another way we kind of balanced the security and simplicity factor. We were talking about all right for finally for our third demo, we’re going to talk about testing an Android app now this is really exciting, because just yesterday we announced that Android studio is officially supported on Chromebooks and we even created an installer just for Chrome OS To make it really easy to get started with now, of course, Android studio isn’t the only thing that you need in order to build a great Android app.

You also need something to test that app on usually a phone and well. You could do that over Wi-Fi with ADB remote all this sort of stuff. We wanted to make it easy, just the experience that you’d expect on any other vice. I can plug my phone in over USB and test my app that way. Now, if I’m an Android developer, sure I’ll plug my phone in to test my app, but I’m also going to plug in a lot of other devices over USB over the course of my day, I’m a plug in a USB Drive that has a lot of family Photos on it, I’m a plug in a wearable that has some health information I may even plug in my security key for work.

That gives me all of my access. Malware can take advantage of these devices to uniquely identify you as you move between machines to spread itself or even to make changes to them again. We thought of these threats at when designing crostini and made sure that we were preventing them. Implementing USB was a lot of fun, for us might have been our most painful stack same principles. Apply, we’ve got our layers, we protect the host there’s a lot of attack surface in a hosts USB stack to very complicated kind of loosely spec to protocol.

It’s an exact spec, that’s loosely implemented by a lot of people, so we’ve hidden that kept that on the host side wrote a device that we live in cross VM jail again, we’ve got a USB driver, it’s pretty complicated. It’s got a lot of code in it. I’r sure there’s a bug or two, so we made sure it was very well isolated. It can’t get to your files, they can’t get to the network, it also can’t get to any USB device.

You have to explicitly say hey. I want to give this USB device to my development environment. We’ve tried to make that as easy as possible and what actually happens under the hood. We’ve always got an emulated USB bus running so that the guest always sees hey. I’ve got a USB bus. There’s nothing plugged in and once you indicate that I want to give this to my VM, it says: ok, I’m going to add this device to this bus and then we show it to the guest and then the guest again in turn, has to forward that into The container and the container can see it there’s two things we were really focused on here.

One was security: again we dressed that with the jail, and we made sure the attack surface was as minimal as possible. It’s also written in rust and it’s nice and memory safe and it’s fuzz. But the other issue here is its privacy, because people somehow use lists of USB devices attached to machines to fingerprint and track users, and we wanted to make sure the untrusted code running inside the container couldn’t be another way to do that again.

This is a lot of steps. We have to create a device, we have to export it to a VM. We have to export it to a container. We have to decide which device to export and not and again we’ll have a demo that shows how easy it is. Okay, what this is the last demo, let’s say: I’m on my Linux and Abel Chromebook and you’re plugging in your phone you’ll see a notification that prompts you to connect to Linux.

At this point, only Chrome OS has knowledge of your phone. Linux doesn’t even know that your phone exists and that’s a good thing. If you see here, your phone is not listed in the USB list, but when you rerun the command once you connect on the notification, your phone shows up in the list. At this point, you established access to Linux to your phone. Let’s say: you’re working on a project, you’re developing a cool app again in Android studio and you’re, ready to test it out.

You hit, run and select the phone and boom just like that. You’re able to test your phone test, your app on your phone at this point you can debug and test out your app. Finally, you can go to settings and manage what Linux access to at any point of time. So you can see how security is at the core of your Linux experience on Chromebooks you, the user, are in full control at all times of what linux has access to.

We take advantage of a variety of UX patterns to make it simple to use and also native to Chrome OS. The combination of principles of Chrome, OS and crostini make this experience pretty unique thanks. My turn all right good. We got plenty of time, so we’ve been talking about a lot of details and I’ve been talking a lot about layers and jails and all that’s important and it’s a good reason for you to trust our normal flows and at when I’m using my Chromebook.

I almost always stay within these common workflows that we’ve polished and made sure work. However, a lot of that technical detail I was talking about is still usable and we’ve left hooks in for you to play with it. So I’m glad I’ve got time left. So I can go through a few of these examples and kind of just wait. Your appetite for what else you can do. We don’t test this stuff. We don’t support this stuff.

We really want the standard flow to be enough for everybody, but every once in a while there might be a reason you want to do something a little more advanced or you know you might just want to go, have fun and play with things under the hood. We’re tinkerers right supposed to be so we’ll go through and show how some of this stuff works. All this is going to be from the Chrome, OS shell. This has been in Chrome OS since longer than I have and so ctrl alt T gets you a shell.

There’s a set of debug commands. You can run we’re going to focus on one command, which is the the VMC command that we added to control virtual machines and containers the basic command. You can do a VM C list. It’ll show you what VMs you have installed on your system. I, the default VM, is called termina, so hopefully the fonts big enough and you can see what size it is. The terminal VM is the one that all the demos were done for the slides before.

So it’s up and running, we’ve made a shortcut to enter a container inside of yem. So if you want to go into the default container, the containers name penguin again, that’s that’s where we were doing all these demos from so the there’s, a BMC container command to get you into there will pop out of there and then we’ll pop back into just The VM so VMC start enters your virtual machine without entering your container.

So if you go back to my layers, it’s the one in the middle. The thing that LXDE runs in – and the reason you want to be in here is if you want to manipulate or change containers, so I mentioned we used lxd, there’s going to be a lot of LXE commands, that’s the lxd control program. This is well documented online and most of it will work inside Chrome OS, just like it does on a default. It want to install the first one you can do is a list you can see, we’ve got penguin running, we have one container it’s up and running.

It’s got an IP address, so we’ve got our one container. We might want to play with it a little bit and before we do, maybe I want to make sure I can get back to a state where I know it’s good right, because I’ve broken them before. It’s nice to be able to just go back to where I was and play around without worrying, so standard LXE command, it’s called snapshot and you give it your container name and you can give it the name of your snapshot and now you’ve got an image saved.

That you can go back to if you break things, there’s a copy on right. We use butter FS in the VM, so you’re not eating up a ton of disk space. We can get info on our container. This gives a bunch of information. Again, you can go poke around with this on a Chromebook. If you want to the important bit here, is that we’ve got one snapshot at the bottom, the i/o one snapshot we just created, you can have multiple snapshots.

It’s got a date on it to help. You remember if you didn’t use a very creative name and then, when you want to restore it back, Alex see restore these are well-named commands. They did a better job with this than I did. If you really want to go and play with different things, sometimes you want more than one container, so I’ve got my penguin container and I’m going to go say, install some different libraries in this one.

Maybe I want to have a container. That’s got Python two seven and a different one. That’s got Python three or maybe I want a different container for writing. Go then the container I have for writing rust. So we let you do that you can create as many containers as you want disk space limited. These do do cost this space. The most basic way to start off a new container is to copy an existing one. There’s an LXE copy command.

The example up here copies the the default penguin container over to a new container named Kingfisher. You can list the containers. We’ve got two by default: containers are stopped, so we have to start them now. We can list two there it’s running and you can jump in you say hey. I want to run bash in Kingfisher and now I’ve got a shell in my new container and I can go off and install whatever random toolchain.

I didn’t want in my default container, taking that one step further. We chose Debian because it was kind of the easiest thing for us to do. We didn’t want to tie you down to that, though. We support the Debian workflow. We support some guest packages that are installed in Debian by default, but some people want to use their favorite distro and there is a huge amount of distres available from the image server.

That canonical runs will install an arch one here, I’m not I’m not an arch guy. I don’t really know much about arch, but some of my co-workers talked me into doing this and playing with it. So now you can see, we’ve got three containers and I’ve got two Debian containers, my penguin and my Kingfisher, and now I’ve got something called arch test and again I can enter it by telling it to run bash, and if I want to install packages in this One I’ll use pac-man instead of app it’s actually it’s actually arch.

I promise that’s just a taste of what you can do from here. If you go and look at the LXE and lxd documentation online, you can get some more ideas, there’s even some help online about installing other ones and getting them to integrate better with the GUI. If you want more than just a command line, all right, so Dillon just showed you a bunch of the really cool tricks you can do with crostini. When you go under the hood and if you’re interested in this kind of thing, we really recommend checking out the crostini subreddit.

The folks they’re buying features as soon as we release them, sometimes even sooner and they’re, also really welcoming to new users of Linux on Chromebooks. So if you have any questions, please check it out and a big thanks to the folks there. So that’s Linux. On Chromebooks, as you can see, we already support a lot of web and Android developer flows and there’s a lot more to come, both in supporting other developers and in expanding what we can do with new capabilities like multiple containers and backup and restore we’re going to Keep applying these principles of simplicity and security to give you the best developer experience possible whenever you’re ready, we hope you’ll join us.

Thank you. You


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