by M95D | 2026-03-12 08:52:13+00:00
I appreciate your detailed reply. I think we are looking from different perspectives. You are correct in an item-by-item way, but you need to put them all together and see the bigger picture. In my comment, it may have made a mess confusing technologies and their capabilities, but I was looking at the forest, not the trees.

There are only two viable firmware alternatives in the world right now: ring 0 U-boot* or the ones that use ring -2: UEFI* and various bootloaders +TrustZone in Android world (read the footnotes!). Manufacturers usually focus on only one of the two: either ring -2 (locked bootloaders, UEFI +ACPI +SMM +whatever crapware they may want to add) protected by secure boot or ring 0 U-boot +a device tree +their GPL source code. The ones interested in locked-down platforms choose the ring -2 option and they are not going to make it open source, nor provide the signing keys to allow it to be replaced by FOSS alternatives.

I appreciate freedom. Linux kernel is free (ring 0). U-boot and coreboot are free (ring -2 if they include ACPI / SMM, else still ring 0). When I run a Linux kernel, I don't want it preempted and sabotaged by a ring -2 component. If that ring -2 includes proprietary blobs, then it's a hard "no" from me. You may argue that SMM (and ACPI) brings useful features such as overheating shutdown when the kernel froze/crashed or the system is stuck at bootloader, but let's face it: practically there's no free alternative to manufacturer's blobs when it comes to ring -2. The FOSS community barely keeps u-boot and the device tree working. Barely! An open source UEFI + all that complexity for every single board out there is a no-go from the start. If you ported Coreboot, i'm sure you know how difficult it is.

I recently learned that ACPI can be decompiled to source code, so that's an improvement, but not by much. Unlike a device tree, which is only a hardware description, ACPI is executable code. I see that as a risk and I'm not the only one. Even Linus had something to say about it - the quote is on wikipedia article. Some of that code executes in ring -2. It can also install components in the OS - spyware components - you can also read about that in the wikipedia article. U-boot has the capability of creating files on some filesystems and you can argue that a proprietary fork could maliciously install OS components by dropping something in init.d, but I've never heard of it being misused that way, and a manufacturer must publish the GPL source code, so it would be difficult to hide. A device tree can't to that at all. If you use UEFI, then every single blob published by the manufacturer must be decompiled and be inspected. U-boot + ACPI is probably simpler than porting Coreboot, but it still won't happen. There are simply too many systems to support.

So, as a conclusion. I see ring -2 as a dangerous capability (even if the malware itself doesn't execute in ring -2) because there are no viable open source alternatives. For this reason I encourage you to not support or promote UEFI and ring -2.

> Trustzone is strictly worse for user freedom than SMM is. This isn't an advantage for Arm.

> Trustzone is mostly deployed on Devicetree-based platform.

True, but ARM world still has unlocked CPUs that can boot unsigned firmware. There are none left in x86 world. (Or at least none that I know about.)

> No, the secure boot code that implements cryptographic validation of the OS is typically running in an entirely normal CPU mode.

OK, valid observation, I may have used "ring -2" to describe features that are not typically running in ring -2. I tried to avoid these technologies as much as possible and I don't have much hands-on experience about what runs where.

> you can implement a functional UEFI/ACPI platform on x86 with zero SMM.

One dev could probably implement and maintain one or maybe 5-10 systems if they are related (same CPU, mostly same hardware). How many systems are there and how many devs? Not possible, but for very very few exceptions, as long as some random dev got one of these systems for himself and does it as a pet project.

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* When I say U-boot, I mean mainline U-boot plus a device tree, or forks with pubished GPL source code. I know U-boot can include ACPI and secure boot, but that's not what I mean in the context of this comment. Sure, you can set up secure boot with open source U-boot if you want to. There's nothing wrong with that.

* When I say UEFI, I mean all related technologies: ACPI, SMM, secure boot, signed firmware, etc. The whole forest.