Re: [Suit] Wording for integrated payload size

David Brown <david.brown@linaro.org> Wed, 11 December 2019 17:54 UTC

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Date: Wed, 11 Dec 2019 10:54:51 -0700
From: David Brown <david.brown@linaro.org>
To: Russ Housley <housley@vigilsec.com>
Cc: Brendan Moran <Brendan.Moran@arm.com>, suit <suit@ietf.org>
Message-ID: <20191211175451.GB832567@davidb.org>
References: <734509A8-7562-4B47-AAE5-54F840C4A298@arm.com> <355773AB-A971-4690-9958-D32B24CAE23A@vigilsec.com>
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Subject: Re: [Suit] Wording for integrated payload size
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On Fri, Dec 06, 2019 at 03:32:40PM -0500, Russ Housley wrote:

> I agree with the approach that you are taking here, but I am not
> sure that every implementation will perform the checks in that
> order.  They all need to be checked, but as long as they are all
> checked before the firmware is installed, then the implementation
> should be considered conformant.  I think the following would be
> better:
> 
> .....   If the manifest is too large to fit in RAM, then it must be
> processed modularly, which includes evaluating delegation chains,
> validating the security container, processing the actual manifest,
> and verifying the integrated payload. ...

I have some similar concerns about this assumption that the manifest
must be in RAM.  With the existing MCUboot implementation we always
process the manifest directly in-place in flash.  There are steps of
this processing that are done at every boot (we need to verify the
signature of the image before booting).

What is unclear to me is whether this wording is talking about the raw
manifest itself, or some kind of in-memory decoded CBOR.  I would
expect smaller targets to always interpret the manifest directly, and
not decode it into some kind of intermediate representation.

David