Trustzone usually runs code from eMMC. These chips are programed in factory with a secret key in the RPMB partiton. It's a one-time operation - the user can't replace it. Without that key you can't update the code Trustzone executes. Only the manufacturer can update it.
Also, any ring -2 code can be used for secure boot locking the device to manufacturer approved OS, enforce DRM, lock hardware upgrades and repairs, spy, call home, install trojans by remote commands, you name it. And you can't audit what it does.
> Trustzone usually runs code from eMMC.
This might be true in so far as the largest number of systems using Trustzone may be using eMMC, but there's nothing magical about eMMC here (my phone, which absolutely uses Trustzone, has no eMMC). But when you then go on to say:
> Without that key you can't update the code Trustzone executes. Only the manufacturer can update it.
you're describing the same sort of limitation that you decried with SMM. As commonly deployed, Trustzone is strictly worse for user freedom than SMM is. This isn't an advantage for Arm.
> Also, any ring -2 code can be used for secure boot locking the device to manufacturer approved OS
No, the secure boot code that implements cryptographic validation of the OS is typically running in an entirely normal CPU mode.
> enforce DRM
This is more typical, but only on Arm - on x86 it's typically running on the GPU in a more convoluted way.
> lock hardware upgrades and repairs
Typically no, because there's no need at all to do any sort of hardware binding at that level - you can implement it more easily in normal code, why make it harder?
> spy
When you're saying "can be used", what do you mean here? Code running in any execution environment is able to spy.
> call home
Code in SMM or Trustzone? That isn't literally impossible but it would be far from trivial, and I don't think we've seen examples of it that don't also involve OS-level components.
> install trojans by remote commands
Again, without OS support, I'm calling absolute bullshit on this. You're going to have an SMM trap on every network packet to check whether it's a remote command? You're going to understand a journaling filesystem and modify it in a way that remains consistent with whatever's in cache? This would be an absolute nightmare to implement in a reliable way.
> And you can't audit what it does.
Trustzone blobs do have a nasty habit of being encrypted, but SMM is just… sitting there. You can pull it out of your firmware. It's plain x86, an extremely well understood architecture with amazing reverse engineering tools. You can absolutely audit it, and in many ways it's easier to notice backdoors in binary than it is in source.
Trustzone is mostly deployed on Devicetree-based platform. What saves you here isn't the choice of firmware interface, it's whether the platform depends on hostile code. If you don't care about secure boot (or if you do but don't care about updating the validation keys at runtime), you can implement a functional UEFI/ACPI platform on x86 with zero SMM.
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.
