[RFC] initoverlayfs - a scalable initial filesystem

[Luca Boccassi]

On Mon, 11 Dec 2023 at 20:43, Demi Marie Obenour
<demi at invisiblethingslab.com> wrote:
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> On Mon, Dec 11, 2023 at 08:15:27PM +0000, Luca Boccassi wrote:
> > On Mon, 11 Dec 2023 at 17:30, Demi Marie Obenour
> > <demi at invisiblethingslab.com> wrote:
> > >
> > > On Mon, Dec 11, 2023 at 10:57:58AM +0100, Lennart Poettering wrote:
> > > > On Fr, 08.12.23 17:59, Eric Curtin (ecurtin at redhat.com) wrote:
> > > >
> > > > > Here is the boot sequence with initoverlayfs integrated, the
> > > > > mini-initramfs contains just enough to get storage drivers loaded and
> > > > > storage devices initialized. storage-init is a process that is not
> > > > > designed to replace init, it does just enough to initialize storage
> > > > > (performs a targeted udev trigger on storage), switches to
> > > > > initoverlayfs as root and then executes init.
> > > > >
> > > > > ```
> > > > > fw -> bootloader -> kernel -> mini-initramfs -> initoverlayfs -> rootfs
> > > > >
> > > > > fw -> bootloader -> kernel -> storage-init   -> init ----------------->
> > > > > ```
> > > >
> > > > I am not sure I follow what these chains are supposed to mean? Why are
> > > > there two lines?
> > > >
> > > > So, I generally would agree that the current initrd scheme is not
> > > > ideal, and we have been discussing better approaches. But I am not
> > > > sure your approach really is useful on generic systems for two
> > > > reasons:
> > > >
> > > > 1. no security model? you need to authenticate your initrd in
> > > >    2023. There's no execuse to not doing that anymore these days. Not
> > > >    in automotive, and not anywhere else really.
> > > >
> > > > 2. no way to deal with complex storage? i.e. people use FDE, want to
> > > >    unlock their root disks with TPM2 and similar things. People use
> > > >    RAID, LVM, and all that mess.
> > > >
> > > > Actually the above are kinda the same problem in a way: you need
> > > > complex storage, but if you need that you kinda need udev, and
> > > > services, and then also systemd and all that other stuff, and that's
> > > > why the system works like the system works right now.
> > > >
> > > > Whenever you devise a system like yours by cutting corners, and
> > > > declaring that you don't want TPM, you don't want signed initrds, you
> > > > don't want to support weird storage, you just solve your problem in a
> > > > very specific way, ignoring the big picture. Which is OK, *if* you can
> > > > actually really work without all that and are willing to maintain the
> > > > solution for your specific problem only.
> > > >
> > > > As I understand you are trying to solve multiple problems at once
> > > > here, and I think one should start with figuring out clearly what
> > > > those are before trying to address them, maybe without compromising on
> > > > security. So my guess is you want to address the following:
> > > >
> > > > 1. You don't want the whole big initrd to be read off disk on every
> > > >    boot, but only the parts of it that are actually needed.
> > > >
> > > > 2. You don't want the whole big initrd to be fully decompressed on every
> > > >    boot, but only the parts of it that are actually needed.
> > > >
> > > > 3. You want to share data between root fs and initrd
> > > >
> > > > 4. You want to save some boot time by not bringing up an init system
> > > >    in the initrd once, then tearing it down again, and starting it
> > > >    again from the root fs.
> > > >
> > > > For the items listed above I think you can find different solutions
> > > > which do not necessarily compromise security as much.
> > > >
> > > > So, in the list above you could address the latter three like this:
> > > >
> > > > 2. Use an erofs rather than a packed cpio as initrd. Make the boot
> > > >    loader load the erofs into contigous memory, then use memmap=X!Y on
> > > >    the kernel cmdline to synthesize a block device from that, which
> > > >    you then mount directly (without any initrd) via
> > > >    root=/dev/pmem0. This means yout boot loader will still load the
> > > >    whole image into memory, but only decompress the bits actually
> > > >    neeed. (It also has some other nice benefits I like, such as an
> > > >    immutable rootfs, which tmpfs-based initrds don't have.)
> > > >
> > > > 3. Simply never transition to the root fs, don't marke the initrds in
> > > >    systemd's eyes as an initrd (specifically: don't add an
> > > >    /etc/initrd-release file to it). Instead, just merge resources of
> > > >    the root fs into your initrd fs via overlayfs. systemd has
> > > >    infrastructure for this: "systemd-sysext". It takes immutable,
> > > >    authenticated erofs images (with verity, we call them "DDIs",
> > > >    i.e. "discoverable disk images") that it overlays into /usr/. [You
> > > >    could also very nicely combine this approach with systemd's
> > > >    portable services, and npsawn containers, which operate on the same
> > > >    authenticated images]. At MSFT we have a major product that works
> > > >    exactly like this: the OS runs off a rootfs that is loaded as an
> > > >    initrd, and everything that runs on top of this are just these
> > > >    verity disk images, using overlayfs and portable services.
> > > >
> > > > 4. The proposal in 3 also addresses goal 4.
> > > >
> > > > Which leaves item 1, which is a bit harder to address. We have been
> > > > discussing this off an on internally too. A generic solution to this
> > > > is hard. My current thinking for this could be something like this,
> > > > covering the UEFI world: support sticking a DDI for the main initrd in
> > > > the ESP. The ESP is per definition unencrypted and unauthenticated,
> > > > but otherwise relatively well defined, i.e. known to be vfat and
> > > > discoverable via UUID on a GPT disk. So: build a minimal
> > > > single-process initrd into the kernel (i.e. UKI) that has exactly the
> > > > storage to find a DDI on the ESP, and set it up. i.e. vfat+erofs fs
> > > > drivers, and dm-verity. Then have a PID 1 that does exactly enough to
> > > > jump into the rootfs stored in the ESP. That latter then has proper
> > > > file system drivers, storage drivers, crypto stack, and can unlock the
> > > > real root. This would still be a pretty specific solution to one set
> > > > of devices though, as it could not cover network boots (i.e. where
> > > > there is just no ESP to boot from), but I think this could be kept
> > > > relatively close, as the logic in that case could just fall back into
> > > > loading the DDI that normally would still in the ESP fully into
> > > > memory.
> > >
> > > I don't think this is "a pretty specific solution to one set of devices"
> > > _at all_.  To the contrary, it is _exactly_ what I want to see desktop
> > > systems moving to in the future.
> > >
> > > It solves the problem of large firmware images.  It solves the problem
> > > of device-specific configuration, because one can use a file on the EFI
> > > system partition that is read by userspace and either treated as
> > > untrusted or TPM-signed.
> >
> > All those problems are already solved, without inventing a new shell
> > scripting solution - we have DDIs and credentials. This is the exact
> > opposite of the direction we are pursuing: we want to _kill_ all these
> > initrd-specific infrastructure, tools, build systems, dependency
> > management and so on, because they are difficult to maintain, they
> > create a completely different environment that what is "normally" ran,
> > and they end up reinventing everything the 'normal' image does. We
> > want to build initrds from packages - as in normal distribution
> > packages, not special sauce initrd-only packages, so that the same
> > code and the same configuration is used everywhere, in different
> > runtime modes. Because that's what distributions are good to do:
> > creating package-based ecosystems, with good tooling, infrastructure
> > and so on.
> >
> > The end goal is to build images without initramfs-tools/dracut and
> > just using packages, not to stick yet another glue script in front of
> > them, that needs yet more special initrd-only arcane magic to put
> > together, in order to save a handful of KBs.
>
> The initramfs being a RAM filesystem is exactly why keeping it small is
> so critical.  Lennart's suggestion solves this problem by eagerly
> loading an image from disk, which is much less size-constrained.  One
> would use distribution packages to build this on-disk image.

This is already solved by using extension DDIs for optional packages.

> > And for ancient, legacy platforms that do not support modern APIs, the
> > old ways will still be there, and can be used. Nobody is going to take
> > away grub and dracut from the internet, if you got some special corner
> > case where you want to use it it will still be there, but the fact
> > that such corner cases exist cannot stop the rest of the ecosystem
> > that is targeted to modern hardware from evolving into something
> > better, more maintainable and more straightforward.
>
> The problem is not that UEFI is not usable in automotive systems.  The
> problem is that U-Boot (or any other UEFI implementation) is an extra
> stage in the boot process, slows things down, and has more attack
> surface.

Whatever firmware you use will have an attack surface, the interface
it provides - whether legacy bios or uefi-based - is irrelevant for
that. Skipping or reimplementing all the verity, tpm, etc logic also
increases the attack surface, as does adding initrd-only code that is
never tested and exercised outside of that limited context. If you are
running with legacy bios on ancient hardware you also will likely lack
tpm, secure boot, and so on, so it's all moot, any security argument
goes out of the window. If anybody cares about platform security, then
a tpm-capable and secureboot-capable firmware with a modern, usable
interface like uefi, running the same code in initrd and full system,
using dm-verity everywhere, is pretty much the best one can do.