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From:   Jann Horn <jannh@google.com>
Date:   Wed, 8 Apr 2020 18:24:16 +0200
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Subject: Re: [PATCH 0/8] loopfs
To:     Christian Brauner <christian.brauner@ubuntu.com>
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On Wed, Apr 8, 2020 at 5:23 PM Christian Brauner
<christian.brauner@ubuntu.com> wrote:
> One of the use-cases for loopfs is to allow to dynamically allocate loop
> devices in sandboxed workloads without exposing /dev or
> /dev/loop-control to the workload in question and without having to
> implement a complex and also racy protocol to send around file
> descriptors for loop devices. With loopfs each mount is a new instance,
> i.e. loop devices created in one loopfs instance are independent of any
> loop devices created in another loopfs instance. This allows
> sufficiently privileged tools to have their own private stash of loop
> device instances. Dmitry has expressed his desire to use this for
> syzkaller in a private discussion. And various parties that want to use
> it are Cced here too.
>
> In addition, the loopfs filesystem can be mounted by user namespace root
> and is thus suitable for use in containers. Combined with syscall
> interception this makes it possible to securely delegate mounting of
> images on loop devices, i.e. when a user calls mount -o loop <image>
> <mountpoint> it will be possible to completely setup the loop device.
> The final mount syscall to actually perform the mount will be handled
> through syscall interception and be performed by a sufficiently
> privileged process. Syscall interception is already supported through a
> new seccomp feature we implemented in [1] and extended in [2] and is
> actively used in production workloads. The additional loopfs work will
> be used there and in various other workloads too. You'll find a short
> illustration how this works with syscall interception below in [4].

Would that privileged process then allow you to mount your filesystem
images with things like ext4? As far as I know, the filesystem
maintainers don't generally consider "untrusted filesystem image" to
be a strongly enforced security boundary; and worse, if an attacker
has access to a loop device from which something like ext4 is mounted,
things like "struct ext4_dir_entry_2" will effectively be in shared
memory, and an attacker can trivially bypass e.g.
ext4_check_dir_entry(). At the moment, that's not a huge problem (for
anything other than kernel lockdown) because only root normally has
access to loop devices.

Ubuntu carries an out-of-tree patch that afaik blocks the shared
memory thing: <https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/eoan/commit?id=4bc428fdf5500b7366313f166b7c9c50ee43f2c4>

But even with that patch, I'm not super excited about exposing
filesystem image parsing attack surface to containers unless you run
the filesystem in a sandboxed environment (at which point you don't
need a loop device anymore either).