Landlock looks promising (a first for LSMs) but largely unneeded IMO. Namespaces + seccomp pretty much fully suffice. Seccomp to block all newfangled syscalls and trace/debug type stuff, and ns's for virtualizing resources.
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For the most part namespaces don't restrict what processes do. They gain capabilities via the kernel and other processes via file descriptors.
Mount namespaces give them their own path hierarchy but don't sandbox their filesystem access. Similar for most other than userns.
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Gain capabilities? That's not a thing unless you botched the basics (nosuid).
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In a user namespace, you can't gain any new capabilities outside of your user namespace. User namespaces are where all the most important stuff happens.
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And mount namespaces do sandbox filesystem access but you have to know how to use them. It involves bind mounting over things that should not be accessible (or that you want to interpose different content over) and then making a new nested namespace so they can't be undone.
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That's changing what's accessible via paths, not sandboxing filesystem access. The program isn't restricted from accessing files passed to it from outside the sandbox in any way. If the user opens a file with an application via system UI, something is being passed to the app, etc
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I'm aware of how mount namespaces work. Changing what can be accessed via paths is a much different thing than restricting which files can actually be accessed. Mount namespaces are doing the former rather than the latter. Same applies to other non-user namespaces.
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I'm assuming if you pass a file descriptor into the sandbox from outside it, you're okay with it being accessed. If you don't want that, don't do it.
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Look at this small fraction of what they're building:
flatpak.github.io/xdg-desktop-po
This doesn't include a whole lot of the baseline IPC.
It's going to get 100x larger and far more complicated as they fill out the capabilities to actually be able to sandbox more complex real apps.
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So how does an application do something like using OpenGL, opening a file with user consent, taking a picture with user consent, obtaining access to take multiple pictures within the current session (or persistently, for a camera app), and so on and so on?
I have never once wanted a desktop app to "take a picture". If I want to take a picture with my webcam I'll do it with a native utility I trust then open the file. This is a general principle: vetted native apps for privileged things, 3p junk in a HARD sandbox.
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But of course this is a replay of the FDO folks trying to recreate all the mistakes of Windows on the Linux Desktop, only this time it's recreating all the mistakes of Android... 🤦🤦🤦
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