Re: [RFC PATCH 1/5] rpmb: add Replay Protected Memory Block (RPMB) subsystem

["Alex Bennée" <alex.bennee@linaro.org>]

Hector Martin <marcan@marcan.st> writes:

> On 11/03/2021 23.31, Linus Walleij wrote:
>> I understand your argument, is your position such that the nature
>> of the hardware is such that community should leave this hardware
>> alone and not try to make use of RPMB  for say ordinary (self-installed)
>> Linux distributions?
>
> It's not really that the community should leave this hardware alone, so 
> much that I think there is a very small subset of users who will be able 
> to benefit from it, and that subset will be happy with a usable 
> kernel/userspace interface and some userspace tooling for this purpose, 
> including provisioning and such.
>
> Consider the prerequisites for using RPMB usefully here:
>
> * You need (user-controlled) secureboot
> * You need secret key storage - so either some kind of CPU-fused key, or 
> one protected by a TPM paired with the secureboot (key sealed to PCR 
> values and such)
> * But if you have a TPM, that can handle secure counters for you already 
> AIUI, so you don't need RPMB
> * So this means you must be running a non-TPM secureboot system
>
> And so we're back to embedded platforms like Android phones and other 
> SoC stuff... user-controlled secureboot is already somewhat rare here, 
> and even rarer are the cases where the user controls the whole chain 
> including the TEE if any (otherwise it'll be using RPMB already); this 
> pretty much excludes all production Android phones except for a few 
> designed as completely open systems; we're left with those and a subset 
> of dev boards (e.g. the Jetson TX1 I did fuse experiments on). In the 
> end, those systems will probably end up with fairly bespoke set-ups for 
> any given device or SoC family, for using RPMB.
>
> But then again, if you have a full secureboot system where you control 
> the TEE level, wouldn't you want to put the RPMB shenanigans there and 
> get some semblance of secure TPM/keystore/attempt throttling 
> functionality that is robust against Linux exploits and has a smaller 
> attack surface? Systems without EL3 are rare (Apple M1 :-)) so it makes 
> more sense to do this on those that do have it. If you're paranoid 
> enough to be getting into building your own secure system with 
> anti-rollback for retry counters, you should be heading in that directly 
> anyway.
>
> And now Linux's RPMB code is useless because you're running the stack in 
> the secure monitor instead :-)

Well quiet - the principle use-case of virtio-rpmb is to provide a RPMB
like device emulation for things like OPTEE when running under QEMU's
full-system emulation. However when it came to testing it out I went for
Thomas' patches because that was the only virtio FE implementation
available.

When I finished the implementation and Ilias started on the uBoot
front-end for virtio-rpmb. uBoot being firmware could very well be
running in the secure world so would have a valid use case accessing an
RPMB device. We ran into API dissonance because uboot's driver model is
roughly modelled on the kernel so expects to be talking to eMMC devices
which lead to requirements to fake something up to keep the driver
stacks happy.

I guess what we are saying is that real secure monitors should come up
with their own common API for interfacing with RPMB devices without
looking to the Linux kernel for inspiration?

-- 
Alex Bennée