Re: [Tsv-art] Tsvart last call review of draft-ietf-suit-architecture-11

Bob Briscoe <> Tue, 11 August 2020 12:50 UTC

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Subject: Re: [Tsv-art] Tsvart last call review of draft-ietf-suit-architecture-11
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I notice that the review upload process has munged some of the line 
wrap. I've re-instated it below.

On 10/08/2020 00:33, Bob Briscoe via Datatracker wrote:
> Reviewer: Bob Briscoe
> Review result: Ready with Issues
> This document has been reviewed as part of the transport area review team's
> ongoing effort to review key IETF documents. These comments were written
> primarily for the transport area directors, but are copied to the document's
> authors and WG to allow them to address any issues raised and also to the IETF
> discussion list for information.
> When done at the time of IETF Last Call, the authors should consider this
> review as part of the last-call comments they receive. Please always CC
> if you reply to or forward this review.
> This review is long. For the benefit of busy readers, it is structured with 7
> important issues listed first (and tagged either as technical or editorial),
> followed by minor editorial comments for the authors.
> Altho' it is ostensibly from the Transport Area Review Team, this review
> identifies only one transport-related issue (see item #6a). Most of the major
> discussion points are offered with a security hat on.
> First I want to say that there's a lot of useful stuff in the draft. So I'd
> like to apologize that the review comments raise issues, and do not dwell on
> praising all the good stuff.
> == Important Issues ==
> 1. Motivation for publication by the IETF [Editorial]
> Until I reached the summary of the recent IoT IAB workshop in the first para of
> the Security Considerations section, I was wondering why the IETF needed to
> publish this. It seemed to be a description of what is already done in the
> industry, but framed as an architecture. Most of this first para of the
> Security Considerations section motivates this work, and ought to be moved to
> the Introduction.
> Even then, a document that describes what the industry already does isn't a
> sufficient response to a security problem. Given (I believe) the intention is
> to encourage the industry to systematically cater for firmware updates, perhaps
> the draft needs to be a little more hard-hitting (without being patronizing of
> course). Rather than giving the impression (except in the abstract) that it is
> just describing current industry practice. For instance, see item #2 below
> about saying what not to do. I would also suggest that it should highlight the
> simplest architecture, only giving optional more complex extras later (see item
> #4 below).
> 2. Is Anything Not Allowed by this Architecture? [Technical+Editorial]
> a) A good architecture precludes as well as includes. Would it be useful to
> list some common practices that are insecure, and perhaps some common
> misconceptions about secure firmware update?
> b) I could hardly find anything in this draft that did not equally apply to
> firmware update of "Non-Things". It would indeed be useful to define a 'Thing'
> (at least what this document means by it). I suggest:
> * unattended operation
> * not within the operator's physical security control
> c) On the subject of ruling things out, I felt the list of items ruled out of
> scope in the Security Considerations include some items that are so central to
> IoT that they should not have been ruled out of scope, and in the first two
> cases quoted below, they didn't need to be ruled out of scope, because the
> document addresses them:
> "
>    - installing firmware updates in a robust fashion so that the update
>      does not break the device functionality of the environment this
>      device operates in.
>    - the distribution of the actual firmware update, potentially in an
>      efficient manner to a large number of devices without human
>      involvement
>    - energy efficiency and battery lifetime considerations.
> "
> And, wouldn't it be better to move scoping statements to just after the Intro,
> rather than in Security Considerations? (And, yes, I know that not all Things
> are energy-challenged, but the size of the subset that are is significant.)
> 3. Relying on Software with Security Vulnerabilities to Patch Security
> Vulnerabilities [Technical]
> The Intro only mentions 'software updates' generally, and doesn't explicitly
> mention patching security vulnerabilities (altho the abstract does). Only
> having read the Security Considerations section, do I discover that the draft
> is primarily meant to be about patching firmware vulnerabilities.
> That raises the question of how secure it is to download new firmware from a
> device booted from firmware that is potentially already compromised. As a
> minimum, surely the draft needs to mention this point. And preferably:
> * whether anything can be trusted once firmware is compromised, and if so what.
> * whether it is still worth updating firmware, even once a vulnerability in the
> firmware update process has been identified, given:
>    o identification of a vulnerability does not necessarily imply it has been
>    exploited, or not prevalently exploited
>    o a vulnerability might not make the firmware update process itself
>    vulnerable (with an explanation of how to tell)
> * describe which aspects of the firmware update process need to be run within a
> TEE (and which not if any)
> * should the TEE lock the device against booting if a firmware authentication
> or integrity check fails
>    o how to prevent tampering with firmware integrity from itself being used as
>    an attack, e.g.
>      - by ensuring that, once a device is locked against booting, firmware
>      re-update is never completely disabled
>      - by ensuring firmware updates are not immediately retried without an
>      exponentially increasing timer back-off, otherwise retries could lead to
>      the devices flooding their own network with fruitless update traffic.
> 4. Please Focus More on the Simplest Architecture [Technical]
> All the following increase system complexity, but are not /essential/ for
> strong security:
> a) Status Tracking Per Device
> b) Confidentiality of the firmware binary
> c) Robustness against rendering the device unbootable
> d) Supporting both Message Authentication and Object Authentication (see item
>     #5)
> e) Broadcast Friendly (see item #6)
> This draft is meant to be persuading the 'industry of Things' to provide
> built-in secure firmware update. It tends to fall into the common trap of
> setting the security bar so high that practitioners might give up in despair.
> a) Per-device status tracking certainly might be preferred by many operators,
> but the alternative of the operator not knowing the status of each individual
> device might be acceptable (as in the example in Figure 5). Per-device status
> tracking introduces the following complexity:
> * a need to separately identify each device, both on each device, and in the
> status tracker.
> * a need to securely identify each separate device (to prevent compromised
> devices masquerading as all the other devices to give a false sense of
> security), requiring management of separate public or shared keys
> b) Confidentiality certainly might provide defence in depth against reverse
> engineering the binaries, but it is ultimately security by obscurity, and so
> ultimately optional. By definition (see item #2b) 'Things' are not in a
> physically secure environment. So, unless all devices decrypt all downloaded
> binaries within a TEE and store them in tamper-proof memory, once the binaries
> are stored on each device, they will be accessible to external inspection
> anyway. So the document should be less dogmatic about confidentiality
> protection (3rd para of Intro), and at least explain that, with IoT,
> confidentiality on the wire is moot unless there is also confidential device
> storage as well.
> c) Robustness against rendering the device unbootable
> Often, when I initiate an (attended) firmware update, the OS warns me that this
> is a sensitive process that could render the device useless if the power fails
> part-way through. So clearly, this is a cost-tradeoff that device designers are
> willing to compromise on. Therefore, I don't think the IETF is entitled to
> pronounce a requirement against this practice. I would rather see this text
> moved from Requirements to somewhere else in the doc, as a commentary on the
> implementation issues, rather than stating it as a requirement. Climbing down a
> bit at the end by saying it is only an implementation requirement doesn't help.
> 5. Both Message Authentication and Whole Object Authentication? [Technical]
> Message authentication codes aren't specifically mentioned, until sections 7 &
> 8, where they are mentioned as if they might be used, without saying why or
> how. The document needs to discuss the merits of MACs vs. authentication of the
> whole manifest and/or the whole firmware binary.
> Ultimately, if an object's authenticity and integrity will be verified once it
> is fully delivered, there is no need for MACs as well. However, using message
> authentication reduces the risk that the device is talking with an imposter at
> an early stage in the transmission, rather than having to wait until it is
> complete. And it is easy to arrange message authentication to cumulatively
> authenticate the whole object, without additional infrastructure for
> whole-object verification. Therefore using MACs could avoid the need to provide
> enough storage for a complete update of the firmware as well as the current
> version - after verifying the manifest and the first message, the device could
> even start to overwrite the firmware it is currently booted from.
> The above strategy would not be without risk, but my point is not just to
> suggest this particular strategy. The document ought to at least discuss the
> trade-offs between MACs and whole-objection authentication, and whether both
> are really necessary.
> 6. Friendly to Broadcast Delivery? [Technical]
> Section 3. states this as one of the "Requirements", although the text softens
> it to "may be desirable for some networks". However, broadcast delivery
> introduces the three significant problems below, wrt a) reliable transport; b)
> device energy efficiency; and c) broadcast message authentication.
> a) Reliable Broadcast Transport
> Delivery of binary objects needs to recover lost or corrupt packets. Reliable
> broadcast delivery at scale is extremely challenging. It needs either fountain
> coding [1] or reliable multicast.
> * Fountain coding delivers an object in a continually repeating stream and
> ensures that the data in any missing packet can be reconstructed from data in
> a subsequent different packet. But this would increase device complexity.
> * For broadcast delivery, per-packet acknowledgements (ACKs) from each device
> do not scale. Negative ACKs (NACKs) can be used but they also do not scale. If
> a loss is experienced close to the root of the broadcast/multicast, it still
> causes an implosion of negative ACKs (NACKs) on the sender. Reliable multicast
> (e.g. PGM [RFC3208]) arranges a spreading tree of delivery nodes each of which
> handles NACKs solely from its next-degree downstream neighbours. Clearly this
> increases network or CDN complexity.
> b) Broadcast Energy Efficiency
> If the IoT device is wireless and needs to take care with its energy
> consumption, it will need to initiate all communications, rather than have to
> sit with its radio powered up listening for an incoming message. However, of
> course, it is not possible for each device to independently initiate an
> incoming broadcast. It would be possible for a broadcast to be scheduled, and
> for each device to poll for the schedule. But this would add complexity,
> particularly because all the device clocks would have to be fairly closely
> synchronized.
> c) Broadcast Message Authentication
> Message authentication has potential advantages over whole-object
> authentication (see #5). When MACs are used over unicast, typically the cost of
> asymmetric crypto for each message is avoided by using asymmetric crypto just
> once to transmit a shared key, which is then used to verify each MAC. However,
> that process is only secure for unicast. For broadcast or multicast delivery,
> the sender only sends each message once, using one key for the MAC that would
> therefore have to be shared with every receiver. Then any receiver could
> masquerade as the genuine sender. TESLA is a solution to this [RFC4082], but it
> would again increase the complexity of each device and the servers, not least
> because it requires loose clock synch (nonetheless, uTESLA has been implemented
> for challenged devices [2]).
> Aside regarding broadcast encryption:
> In section 3.3. "Use state-of-the-art security mechanisms", it says:
>    "The information that is encrypted individually for each device must
>    maintain friendliness to Content Distribution Networks, bulk storage,
>    and broadcast protocols."
> That implies a magic encyption scheme that is beyond any state-of-the-art that
> I am aware of! If information is encrypted individually for each device, surely
> by definition it will not be friendly to broadcast protocols. Actually, I
> suspect the authors did not mean to say "encrypted individually for each
> device", because a shared group key is adequate for confidentiality - a shared
> group key is only problematic for message or source authentication (see above).
> 7. Missing Security Concerns [Technical]
> a) Avoiding Reliance on the Device's System Clock
> I suggest that the document makes the point that it is preferable for the
> firmware update process not to rely on the device's system clock.
> Reasoning: Even if the TEE maintains the system clock, protection against
> attacks on this clock rely on voting between multiple time sources. No amount
> of authentication provides any proof of message timing. So, it is hard for a
> TEE to protect against tampering with the timing of its messages, given they
> pass via the untrusted execution environment of the rest of the device, similar
> to the problem of a secure time source for virtualized functions [3].
> I think IoT developers can be reassured that none of the requirements for
> firmware update need to rely on the system clock. For instance roll-back attack
> prevention (section 3.4) only requires comparison between version numbers, not
> comparison between a release time and the clock.
> However, I think not relying on the clock is worth mentioning, because key
> expiry and key revocation have to be designed carefully to avoid relying on
> secure time, and this is a subtle point that might not be appreciated by IoT
> device designers.
> b) Key revocation
> When keys are in tamper-resistant storage but otherwise not within a physically
> secure site, the question of revocation surely has to be addressed. In
> particular, there should be a discussion about the advisability or otherwise of
> pre-loading the same keys into multiple devices.
> == Minor Editorial Issues ==
> 1. Intro
>    "Updates to the firmware of an IoT device are done to fix bugs in software..."
> This would be a good place to highlight the focus on patching security
> vulnerabilities.
> "This version of the document assumes... Future versions may also describe..."
> I assume this aspiration needs to be deleted now?
> 2. Terminology
> There are ~22 occurrences of lower case 'must' in this document, and one
> 'should' (excluding multiple uses in rhetorical questions). I'm not sure
> whether it is intentional to make it seem like this is an RFC that is mandating
> behaviour, perhaps for readers who don't understand the subtleties of the IETF
> informational track. I would prefer it to be clear that this document is not
> mandating anything, by using alternatives to 'must' like 'ought to' or 'has
> to'. Otherwise it could be considered disingenuous.
>    "The term ’system on chip (SoC)’ is often used for these types of devices."
> Perhaps more useful:
>    "The term ’system on chip (SoC)’ is often used interchangeably with MCU, but
>    MCU tends to imply more limited peripheral functions."
>    "The following entities are used:"
> The list is a mix of stakeholders and functions, which tends to show that the
> authors themselves might not be clear about the distinction. It would be useful
> to split into two lists.
>    "The terms device and
>    firmware consumer are used interchangeably since the firmware
>    consumer is one software component running on an MCU on the
>    device."
> I didn't notice them being used interchangeably. If they are anywhere, why not
> just edit to use whichever term is more appropriate and delete this sentence?
> Status Tracker
>    "While the IoT device itself runs the client-
>    side of the status tracker it will most likely not run a status
>    tracker itself unless it acts as a proxy for other IoT devices in
>    a protocol translation or edge computing device node."
> The client-side of a status tracker surely does run a status tracker itself
> (the clue is in the name). I know what is intended, but the writer was clearly
> in two minds as to whether a status tracker is the combination of client and
> server or just the server.
> 3. Requirements
> 3.5 "High reliability" -> 'Robust against becoming unbootable'.
> The title for this requirement otherwise implies a much more general
> requirement than the description under it.
> 3.6 Small bootloader
> "...again using firmware updates over serial,
> USB or even wireless connectivity like a limited version of
> Bluetooth Smart."
> Don't see why it has to be "...a limited version of...". Suggest these words
> are deleted.
> s/poses a risk in reliability/
>   /poses a reliability risk/
> s/must fit in the available RAM/
>   /must fit in the available memory/
> (not necessarily RAM)
> s|there are not other task/processing running|
>   |there are not other tasks/processes running|
> s/unlike it may be the case/
>   /unlike that which may be the case/
> s/Note: This is an implementation requirement./
>   /Note: This last paragraph is an implementation requirement./
> (Otherwise, 'this' could ambiguously refer to the whole requirement)
> 3.7 Small Parsers
> "Since parsers are known sources of bugs they must be minimal."
> To be honest, I suspect the target audience will find this sentence and others
> like it rather pious. Given the purpose of this document is meant to be to
> encourage implementers to provide secure firmware update, I think these
> peripheral "requirements" will just serve to make any implementers reading this
> feel they are being patronized.
> As with the earlier requirement about 'robustness against becoming unbootable',
> I think many of these 'requirements' would be easier to stomach within a
> discussion of tradeoffs, rather than as a list of pronouncements that demand
> perfection.
> 3.8
> s/Minimal impact on existing firmware formats/
>   /No impact on existing firmware formats/
> Reason: This is what the text underneath says.
> 3.9 Robust permissions
>    "...the authorization policy is separated from the
>    underlying communication architecture. This is accomplished by
>    separating the entities from their permissions."
> I'm not sure whether either of these sentences makes much sense (at least not
> to me). Perhaps the first sentence means to say that
>    "...the authorization policy is separated from the
>    firmware it applies to"
> And then the second sentence could be deleted. I'm not sure the second sentence
> would ever be necessary, because entities are always separate from their
> permissions (otherwise you would have to access an entity to find out you
> weren't allowed to access it). To be honest, I don't really see the point of
> the whole requirement. So if it is important, maybe its meaning needs to be
> clarified for people like me. Otherwise, if it's just stating the obvious,
> maybe it's not necessary at all.
> 3.10. Operating modes
> Later, in S.5. the term 'delivery modes' is used. If these are meant to mean
> the same thing, then the same term should be used consistently. In my
> experience, the term 'interaction model' is used to describe things like polled
> request-reply, push, publish-subscribe, etc.
> "The pre-authorisation step involves verifying..."
> When describing a distributed system, pls avoid passive sentences like this,
> which don't specify which entity is performing the action. It is followed up
> later by "...the firmware consumer must also...", which implies the subject is
> the firmware consumer, but it's best not to rely on implication, especially not
> if it requires two passes to understand.
>    "Pushing a manifest and firmware image to the transfer to
>    the Package resource of the LwM2M Firmware Update object"
> Garbled?
>    " may need to wait for a trigger from the
>    status tracker to initiate the installation, may trigger the update
>    automatically, or may go through a more complex decision making
>    process to determine the appropriate timing for an update"
> I had to read this a few times before realizing it was a list.
> How about:
>    "... to initiate the installation, it may either need to wait for a trigger
>    from the status tracker; or trigger the update automatically; or go through a
>    more complex decision making process to determine the appropriate timing for
>    an update"
> 3.11.
> s/Suitability to software and personalization data/
>   /Suitability for software and personalization data/
> The document suddenly jumps into a different style at the start of 3.11, more
> like an log of WG activity than a requirement. Pls consider making the style
> consistent, especially given it switches back after the first sentence of the
> 2nd para.
> 4. Claims
> s/Only install firmware with a matching vendor/
>   /Only install firmware with a matching author/ ?
> 5. Communication Architecture
> The document often repeats that it's agnostic to the communication
> architecture, then this section starts with the phrase:
>    "Figure 1 shows the communication architecture..."
> Perhaps it means 'firmware update architecture'?
> Or, possibly this implies that the authors are in two minds as to what
> 'communications architecture' means. Or the heading was intended to be
> 'Communications Architectures' (plural) and the first phrase was meant to say
>    "Figure 1 shows an example communication architecture..."
> The text needs to make it clear that a status tracker is optional in the client
> pull case but not in the server push case (see item #4a earlier).
> It would be useful for the doc to say what it means for an operator circle to
> enclose a function. For instance the 'Device Operator' in Fig 1 encloses the
> status tracker, which to me implies it controls the status tracker. However,
> the network operator encloses the device, which probably doesn't imply it
> operates the device. Perhaps an enclosing circle means 'within the physical
> security control of'? The network operator isn't mentioned in the text - why is
> it in the diagram, given it has no role in the firmware update, other than as a
> common carrier of opaque bits?
>    "The following assumptions are made to allow the firmware consumer to
>    verify the received firmware image and manifest before updating
>    software:"
> The following three bullets aren't really assumptions. Perhaps 'statements
> about the verification process' would be a better phrase. Would another
> reference to suit-information-model here be useful, to explain why the details
> are not given here?
> See item #4b) above about highlighting that confidentiality is optional, not
> just 'deployment specific'.
>    "There are different types of delivery modes, which are illustrated
>    based on examples below."
> Shouldn't this sentence start section 5? (Also see my earlier point about
> 'operating modes' / 'interaction modes' terminology).
> Fig 3 is inconsistent with Fig 1, in that it omits the firmware consumer
> function.
> Fig 4 is inconsistent with Figs 1 & 3, in that there is also an arrow from the
> status tracker to the author. What does this imply?
>    "This architecture does not mandate a specific delivery mode but a
>    solution must support both types.
> Whatever for? This requirement surely over-plays the IETF's hand, which is not
> in a position to make such a demand? Is the intention really that being
> agnostic to the delivery mode means every solution must support all delivery
> modes?
> 6. Manifest
> Given each of the items in the second bullet list addresses one of the
> questions in the first bullet list, it would be useful to tabulate them
> side-by-side and to put them in a more meaningful order, e.g. in the order they
> occur during firmware update. Also, the the first question bullet (author
> trust) is not specifically addressed in the second list - implied within the
> last bullet, but not explicitly stated.
> 7.1
> s/Combined with the non-relocatable nature of the code/
>   /Due to the non-relocatable nature of the code/
> 7.3
>    "This configuration has two or more CPUs in a single SoC that share
>    memory (flash and RAM). Generally, they will be a protection
>    mechanism to prevent one CPU from accessing the other’s memory."
> I know what is intended, but it reads as if line 1 contradicts line 3. Perhaps:
>   "...
>    mechanism to prevent one CPU from unintentionally accessing memory currently
>    allocated to the other."
> 9. Example
> In at least one example figure, it would be useful to show the initial
> pre-loading of keys, policy logic and trust anchor into the firmware consumer /
> bootloader.
> s/starting with an author uploading the new firmware to firmware server/
>   /starting with an author uploading the new firmware to the firmware server/
>    "This setup does
>    not use a status tracker and the firmware consumer component is
>    therefore responsible for periodically checking whether a new
>    firmware image is available for download."
> It needs to be much clearer that the status tracker has both a monitoring
> function and an update triggering function. So, altho it is essential in the
> server push model - to trigger updates, it's monitoring function means it is
> not ruled out for the client pull model.
> Fig 5 & 6 are inconsistent, in that the former omits the IoT device box around
> the Firmware consumer and bootloader.
> s/Figure 6 shows an example follow with the device using a status tracker./
>   /Figure 6 shows an example with the device using a status tracker./
>    "For editorial reasons the author publishing the manifest at
>    the status tracker and the firmware image at the firmware server is
>    not shown."
> How about:
>    "Depiction of the author publishing the manifest at
>    the status tracker and the firmware image at the firmware server would
>    be the same as in Figure 5. So for brevity they are not shown."
> 11. Security Considerations
> Between
>    "A report about this workshop can be found at [RFC8240]."
> and
>    "A standardized firmware manifest format..."
> there either needs to be some glue text to explain that the initial manifest
> format was an output of the workshop (if it was), or a new para if the second
> sentence really doesn't follow from the first.
> Note also that I suggest (item #1) that the motivating text about the workshop
> should be moved to the introduction. I also say (in item 2c) that the scoping
> bullets would be better at the end of the Intro too. However, I can also see a
> case for them remaining under Security Considerations; to admit that the
> document does not fully address all possible security concerns.
> Given this could leave nothing in the Security Considerations section, it would
> be appropriate to merely point to all the sections of the document that already
> cover security matters.
> == References ==
> [1] Byers, J.; Luby, M.; Mitzenmacher, M. & Rege, A. A Digital Fountain
> Approach to Reliable Distribution of Bulk Data Proc. ACM SIGCOMM'98, Computer
> Communication Review, 1998, 28
> [2] Perrig, A.; Szewczyk, R.; Wen, V.; Culler, D. E. & Tygar, J. D. SPINS:
> Security Protocols for Sensor Networks Proc. ACM International Conference on
> Mobile Computing and Networks (Mobicom'01), 2001, 189-199
> [3] Briscoe (Ed.), B. & others Network Functions Virtualisation; Security;
> Problem Statement ETSI NFV Industry Specification Group (ISG), ETSI NFV
> Industry Specification Group (ISG), 2014
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Bob Briscoe