On Mon, Dec 11, 2023 at 08:58:58PM +0000, Luca Boccassi wrote: > On Mon, 11 Dec 2023 at 20:43, Demi Marie Obenour > <demi@xxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > -----BEGIN PGP SIGNED MESSAGE----- > > Hash: SHA512 > > > > 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@xxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > On Mon, Dec 11, 2023 at 10:57:58AM +0100, Lennart Poettering wrote: > > > > > On Fr, 08.12.23 17:59, Eric Curtin (ecurtin@xxxxxxxxxx) 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. What about non-optional packages? The goal is to _require_ the on-disk image to boot, so that full-featured UI toolkits can be used to e.g. prompt for LUKS passphrases. Ideally, the initramfs would be as minimal as possible. > > > 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. Neither Chrome OS devices nor Macs with Apple silicon use UEFI, and both have better platform security than any UEFI-based device on the market I am aware of. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab
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