A Firmware Update Architecture for Internet of Things Devices
draft-ietf-suit-architecture-06
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| Authors | Brendan Moran , Milosch Meriac , Hannes Tschofenig , David Brown | ||
| Last updated | 2019-09-13 (Latest revision 2019-04-09) | ||
| Replaces | draft-moran-suit-architecture | ||
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draft-ietf-suit-architecture-06
SUIT B. Moran
Internet-Draft Arm Limited
Intended status: Informational M. Meriac
Expires: March 16, 2020 Consultant
H. Tschofenig
Arm Limited
D. Brown
Linaro
September 13, 2019
A Firmware Update Architecture for Internet of Things Devices
draft-ietf-suit-architecture-06
Abstract
Vulnerabilities with Internet of Things (IoT) devices have raised the
need for a solid and secure firmware update mechanism that is also
suitable for constrained devices. Incorporating such update
mechanism to fix vulnerabilities, to update configuration settings as
well as adding new functionality is recommended by security experts.
This document lists requirements and describes an architecture for a
firmware update mechanism suitable for IoT devices. The architecture
is agnostic to the transport of the firmware images and associated
meta-data.
This version of the document assumes asymmetric cryptography and a
public key infrastructure. Future versions may also describe a
symmetric key approach for very constrained devices.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 16, 2020.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Agnostic to how firmware images are distributed . . . . . 7
3.2. Friendly to broadcast delivery . . . . . . . . . . . . . 8
3.3. Use state-of-the-art security mechanisms . . . . . . . . 8
3.4. Rollback attacks must be prevented . . . . . . . . . . . 8
3.5. High reliability . . . . . . . . . . . . . . . . . . . . 9
3.6. Operate with a small bootloader . . . . . . . . . . . . . 9
3.7. Small Parsers . . . . . . . . . . . . . . . . . . . . . . 10
3.8. Minimal impact on existing firmware formats . . . . . . . 10
3.9. Robust permissions . . . . . . . . . . . . . . . . . . . 10
3.10. Operating modes . . . . . . . . . . . . . . . . . . . . . 10
3.11. Suitability to software and personalization data . . . . 12
4. Claims . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5. Communication Architecture . . . . . . . . . . . . . . . . . 13
6. Manifest . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Device Firmware Update Examples . . . . . . . . . . . . . . . 18
7.1. Single CPU SoC . . . . . . . . . . . . . . . . . . . . . 18
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7.2. Single CPU with Secure - Normal Mode Partitioning . . . . 18
7.3. Dual CPU, shared memory . . . . . . . . . . . . . . . . . 18
7.4. Dual CPU, other bus . . . . . . . . . . . . . . . . . . . 18
8. Bootloader . . . . . . . . . . . . . . . . . . . . . . . . . 19
9. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
11. Security Considerations . . . . . . . . . . . . . . . . . . . 24
12. Mailing List Information . . . . . . . . . . . . . . . . . . 25
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 27
14.1. Normative References . . . . . . . . . . . . . . . . . . 27
14.2. Informative References . . . . . . . . . . . . . . . . . 27
14.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
When developing IoT devices, one of the most difficult problems to
solve is how to update the firmware on the device. Once the device
is deployed, firmware updates play a critical part in its lifetime,
particularly when devices have a long lifetime, are deployed in
remote or inaccessible areas where manual intervention is cost
prohibitive or otherwise difficult. Updates to the firmware of an
IoT device are done to fix bugs in software, to add new
functionality, and to re-configure the device to work in new
environments or to behave differently in an already deployed context.
The firmware update process, among other goals, has to ensure that
- The firmware image is authenticated and integrity protected.
Attempts to flash a modified firmware image or an image from an
unknown source are prevented.
- The firmware image can be confidentiality protected so that
attempts by an adversary to recover the plaintext binary can be
prevented. Obtaining the firmware is often one of the first steps
to mount an attack since it gives the adversary valuable insights
into used software libraries, configuration settings and generic
functionality (even though reverse engineering the binary can be a
tedious process).
More details about the security goals are discussed in Section 5 and
requirements are described in Section 3.
2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
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"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This document uses the following terms:
- Manifest: The manifest contains meta-data about the firmware
image. The manifest is protected against modification and
provides information about the author.
- Firmware Image: The firmware image is a binary that may contain
the complete software of a device or a subset of it. The firmware
image may consist of multiple images, if the device contains more
than one microcontroller. The image may consist of a differential
update for performance reasons. Firmware is the more universal
term. Both terms are used in this document and are
interchangeable.
- Bootloader: A bootloader is a piece of software that is executed
once a microcontroller has been reset. It is responsible for
deciding whether to boot a firmware image that is present or
whether to obtain and verify a new firmware image. Since the
bootloader is a security critical component its functionality may
be split into separate stages. Such a multi-stage bootloader may
offer very basic functionality in the first stage and resides in
ROM whereas the second stage may implement more complex
functionality and resides in flash memory so that it can be
updated in the future (in case bugs have been found). The exact
split of components into the different stages, the number of
firmware images stored by an IoT device, and the detailed
functionality varies throughout different implementations. A more
detailed discussion is provided in Section 8.
- Microcontroller (MCU for microcontroller unit): An MCU is a
compact integrated circuit designed for use in embedded systems.
A typical microcontroller includes a processor, memory (RAM and
flash), input/output (I/O) ports and other features connected via
some bus on a single chip. The term 'system on chip (SoC)' is
often used for these types of devices.
- System on Chip (SoC): An SoC is an integrated circuit that
integrates all components of a computer, such as CPU, memory,
input/output ports, secondary storage, etc.
- Homogeneous Storage Architecture (HoSA): A device that stores all
firmware components in the same way, for example in a file system
or in flash memory.
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- Heterogeneous Storage Architecture (HeSA): A device that stores at
least one firmware component differently from the rest, for
example a device with an external, updatable radio, or a device
with internal and external flash memory.
- Trusted Execution Environments (TEEs): An execution environment
that runs alongside of, but is isolated from, an REE.
- Rich Execution Environment (REE): An environment that is provided
and governed by a typical OS (e.g., Linux, Windows, Android, iOS),
potentially in conjunction with other supporting operating systems
and hypervisors; it is outside of the TEE. This environment and
applications running on it are considered un-trusted.
- Trusted applications (TAs): An application component that runs in
a TEE.
For more information about TEEs see [I-D.ietf-teep-architecture].
The following entities are used:
- Author: The author is the entity that creates the firmware image.
There may be multiple authors in a system either when a device
consists of multiple micro-controllers or when the the final
firmware image consists of software components from multiple
companies.
- Firmware Consumer: The firmware consumer is the recipient of the
firmware image and the manifest. It is responsible for parsing
and verifying the received manifest and for storing the obtained
firmware image. The firmware consumer plays the role of the
update component on the IoT device typically running in the
application firmware. It interacts with the firmware server and
with the status tracker, if present.
- (IoT) Device: A device refers to the entire IoT product, which
consists of one or many MCUs, sensors and/or actuators. Many IoT
devices sold today contain multiple MCUs and therefore a single
device may need to obtain more than one firmware image and
manifest to succesfully perform an update. The terms device and
firmware consumer are used interchangably since the firmware
consumer is one software component running on an MCU on the
device.
- Status Tracker: The status tracker offers device management
functionality to retrieve information about the installed firmware
on a device and other device characteristics (including free
memory and hardware components), to obtain the state of the
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firmware update cycle the device is currently in, and to trigger
the update process. The deployment of status trackers is flexible
and they may be used as cloud-based servers, on-premise servers,
embedded in edge computing device (such as Internet access
gateways or protocol translation gateways), or even in smart
phones and tablets. 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. How much
functionality a status tracker includes depends on the selected
configuration of the device management functionality and the
communication environment it is used in. In a generic networking
environment the protocol used between the client and the server-
side of the status tracker need to deal with Internet
communication challenges involving firewall and NAT traversal. In
other cases, the communication interaction may be rather simple.
This architecture document does not impose requirements on the
status tracker.
- Firmware Server: The firmware server stores firmware images and
manifests and distributes them to IoT devices. Some deployments
may require a store-and-forward concept, which requires storing
the firmware images/manifests on more than one entity before
they reach the device. There is typically some interaction
between the firmware server and the status tracker but those
entities are often physically separated on different devices for
scalability reasons.
- Device Operator: The actor responsible for the day-to-day
operation of a fleet of IoT devices.
- Network Operator: The actor responsible for the operation of a
network to which IoT devices connect.
In addition to the entities in the list above there is an orthogonal
infrastructure with a Trust Provisioning Authority (TPA) distributing
trust anchors and authorization permissions to various entities in
the system. The TPA may also delegate rights to install, update,
enhance, or delete trust anchors and authorization permissions to
other parties in the system. This infrastructure overlaps the
communication architecture and different deployments may empower
certain entities while other deployments may not. For example, in
some cases, the Original Design Manufacturer (ODM), which is a
company that designs and manufactures a product, may act as a TPA and
may decide to remain in full control over the firmware update process
of their products.
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The terms 'trust anchor' and 'trust anchor store' are defined in
[RFC6024]:
- "A trust anchor represents an authoritative entity via a public
key and associated data. The public key is used to verify digital
signatures, and the associated data is used to constrain the types
of information for which the trust anchor is authoritative."
- "A trust anchor store is a set of one or more trust anchors stored
in a device. A device may have more than one trust anchor store,
each of which may be used by one or more applications." A trust
anchor store must resist modification against unauthorized
insertion, deletion, and modification.
3. Requirements
The firmware update mechanism described in this specification was
designed with the following requirements in mind:
- Agnostic to how firmware images are distributed
- Friendly to broadcast delivery
- Use state-of-the-art security mechanisms
- Rollback attacks must be prevented
- High reliability
- Operate with a small bootloader
- Small Parsers
- Minimal impact on existing firmware formats
- Robust permissions
- Diverse modes of operation
- Suitability to software and personalization data
3.1. Agnostic to how firmware images are distributed
Firmware images can be conveyed to devices in a variety of ways,
including USB, UART, WiFi, BLE, low-power WAN technologies, etc. and
use different protocols (e.g., CoAP, HTTP). The specified mechanism
needs to be agnostic to the distribution of the firmware images and
manifests.
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3.2. Friendly to broadcast delivery
This architecture does not specify any specific broadcast protocol.
However, given that broadcast may be desirable for some networks,
updates must cause the least disruption possible both in metadata and
payload transmission.
For an update to be broadcast friendly, it cannot rely on link layer,
network layer, or transport layer security. In addition, the same
message must be deliverable to many devices, both those to which it
applies and those to which it does not, without a chance that the
wrong device will accept the update. Considerations that apply to
network broadcasts apply equally to the use of third-party content
distribution networks for payload distribution.
3.3. Use state-of-the-art security mechanisms
End-to-end security between the author and the device, as shown in
Section 5, is used to ensure that the device can verify firmware
images and manifests produced by authorized authors.
The use of post-quantum secure signature mechanisms, such as hash-
based signatures, should be explored. A migration to post-quantum
secure signatures would require significant effort, therefore,
mandatory-to-implement support for post-quantum secure signatures is
a goal.
A mandatory-to-implement set of algorithms has to be defined offering
a key length of 112-bit symmetric key or security or more, as
outlined in Section 20 of RFC 7925 [RFC7925]. This corresponds to a
233 bit ECC key or a 2048 bit RSA key.
If the firmware image is to be encrypted, it must be done in such a
way that every intended recipient can decrypt it. The information
that is encrypted individually for each device must be an absolute
minimum, for example AES Key Wrap [RFC5649], in order to maintain
friendliness to Content Distribution Networks, bulk storage, and
broadcast protocols.
3.4. Rollback attacks must be prevented
A device presented with an old, but valid manifest and firmware must
not be tricked into installing such firmware since a vulnerability in
the old firmware image may allow an attacker to gain control of the
device.
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3.5. High reliability
A power failure at any time must not cause a failure of the device.
A failure to validate any part of an update must not cause a failure
of the device. One way to achieve this functionality is to provide a
minimum of two storage locations for firmware and one bootable
location for firmware. An alternative approach is to use a 2nd stage
bootloader with build-in full featured firmware update functionality
such that it is possible to return to the update process after power
down.
Note: This is an implementation requirement rather than a requirement
on the manifest format.
3.6. Operate with a small bootloader
Throughout this document we assume that the bootloader itself is
distinct from the role of the fw consumer and therefore does not
manage the firmware update process. This may give the impression
that the bootloader itself is a completely separate component, which
is mainly responsible for selecting a firmware image to boot.
The overlap between the firmware update process and the bootloader
functionality comes in two forms, namely
- First, a bootloader must verify the firmware image it boots as
part of the secure boot process. Doing so requires meta-data to
be stored alongside the firmware image so that the bootloader can
cryptographically verify the firmware image before booting it to
ensure it has not been tampered with or replaced. This meta-data
used by the bootloader may well be the same manifest obtained with
the firmware image during the update process (with the severable
fields stripped off).
- Second, an IoT device needs a recovery strategy in case the
firmware update / boot process fails. The recovery strategy may
include storing two or more firmware images on the device or
offering the ability to have a second stage bootloader perform the
firmware update process again using firmware updates over serial,
USB or even wireless connectivity like a limited version of
Bluetooth Smart. In the latter case the fw consumer functionality
is contained in the second stage bootloader and requires the
necessary functionality for executing the firmware update process,
including manifest parsing.
In general, it is assumed that the bootloader itself, or a minimal
part of it, will not be updated since a failed update of the
bootloader poses a risk in reliability.
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All information necessary for a device to make a decision about the
installation of a firmware update must fit into the available RAM of
a constrained IoT device. This prevents flash write exhaustion.
This is typically not a difficult requirement to accomplish because
there are not other task/processing running while the bootloader is
active (unlike it may be the case when running the application
firmware).
Note: This is an implementation requirement.
3.7. Small Parsers
Since parsers are known sources of bugs they must be minimal.
Additionally, it must be easy to parse only those fields that are
required to validate at least one signature or MAC with minimal
exposure.
3.8. Minimal impact on existing firmware formats
The design of the firmware update mechanism must not require changes
to existing firmware formats.
3.9. Robust permissions
When a device obtains a monolithic firmware image from a single
author without any additional approval steps then the authorization
flow is relatively simple. There are, however, other cases where
more complex policy decisions need to be made before updating a
device.
In this architecture the authorization policy is separated from the
underlying communication architecture. This is accomplished by
separating the entities from their permissions. For example, an
author may not have the authority to install a firmware image on a
device in critical infrastructure without the authorization of a
device operator. In this case, the device may be programmed to
reject firmware updates unless they are signed both by the firmware
author and by the device operator.
Alternatively, a device may trust precisely one entity, which does
all permission management and coordination. This entity allows the
device to offload complex permissions calculations for the device.
3.10. Operating modes
There are three broad classifications of update operating modes.
- Client-initiated Update
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- Server-initiated Update
- Hybrid Update
Client-initiated updates take the form of a firmware consumer on a
device proactively checking (polling) for new firmware images.
Server-initiated updates are important to consider because timing of
updates may need to be tightly controlled in some high- reliability
environments. In this case the status tracker determines what
devices qualify for a firmware update. Once those devices have been
selected the firmware server distributes updates to the firmware
consumers.
Note: This assumes that the status tracker is able to reach the
device, which may require devices to keep reachability information at
the status tracker up-to-date. This may also require keeping state
at NATs and stateful packet filtering firewalls alive.
Hybrid updates are those that require an interaction between the
firmware consumer and the status tracker. The status tracker pushes
notifications of availability of an update to the firmware consumer,
and it then downloads the image from a firmware server as soon as
possible.
An alternative view to the operating modes is to consider the steps a
device has to go through in the course of an update:
- Notification
- Pre-authorisation
- Dependency resolution
- Download
- Installation
The notification step consists of the status tracker informing the
firmware consumer that an update is available. This can be
accomplished via polling (client-initiated), push notifications
(server-initiated), or more complex mechanisms.
The pre-authorisation step involves verifying whether the entity
signing the manifest is indeed authorized to perform an update. The
firmware consumer must also determine whether it should fetch and
process a firmware image, which is referenced in a manifest.
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A dependency resolution phase is needed when more than one component
can be updated or when a differential update is used. The necessary
dependencies must be available prior to installation.
The download step is the process of acquiring a local copy of the
firmware image. When the download is client-initiated, this means
that the firmware consumer chooses when a download occurs and
initiates the download process. When a download is server-initiated,
this means that the status tracker tells the device when to download
or that it initiates the transfer directly to the firmware consumer.
For example, a download from an HTTP-based firmware server is client-
initiated. Pushing a manifest and firmware image to the transfer to
the Package resource of the LwM2M Firmware Update object [LwM2M] is
server-initiated.
If the firmware consumer has downloaded a new firmware image and is
ready to install it, it 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 (such as
delaying the update process to a later time when end users are less
impacted by the update process).
Installation is the act of processing the payload into a format that
the IoT device can recognise and the bootloader is responsible for
then booting from the newly installed firmware image.
Each of these steps may require different permissions.
3.11. Suitability to software and personalization data
The work on a standardized manifest format initially focused on the
most constrained IoT devices and those devices contain code put
together by a single author (although that author may obtain code
from other developers, some of it only in binary form).
Later it turns out that other use cases may benefit from a
standardized manifest format also for conveying software and even
personalization data alongside software. Trusted Execution
Environments (TEEs), for example, greatly benefit from a protocol for
managing the lifecycle of trusted applications (TAs) running inside a
TEE. TEEs may obtain TAs from different authors and those TAs may
require personalization data, such as payment information, to be
securely be conveyed to the TEE.
To support this wider range of use cases the manifest format should
therefore be extensible to convey other forms of payloads as well.
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4. Claims
Claims in the manifest offer a way to convey instructions to a device
that impact the firmware update process. To have any value the
manifest containing those claims must be authenticated and integrity
protected. The credential used to must be directly or indirectly
related to the trust anchor installed at the device by the Trust
Provisioning Authority.
The baseline claims for all manifests are described in
[I-D.ietf-suit-information-model]. For example, there are:
- Do not install firmware with earlier metadata than the current
metadata.
- Only install firmware with a matching vendor, model, hardware
revision, software version, etc.
- Only install firmware that is before its best-before timestamp.
- Only allow a firmware installation if dependencies have been met.
- Choose the mechanism to install the firmware, based on the type of
firmware it is.
5. Communication Architecture
Figure 1 shows the communication architecture where a firmware image
is created by an author, and uploaded to a firmware server. The
firmware image/manifest is distributed to the device either in a push
or pull manner using the firmware consumer residing on the device.
The device operator keeps track of the process using the status
tracker. This allows the device operator to know and control what
devices have received an update and which of them are still pending
an update.
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Firmware + +----------+ Firmware + +-----------+
Manifest | |-+ Manifest | |-+
+--------->| Firmware | |<---------------| | |
| | Server | | | Author | |
| | | | | | |
| +----------+ | +-----------+ |
| +----------+ +-----------+
|
|
|
-+-- ------
---- | ---- ---- ----
// | \\ // \\
/ | \ / \
/ | \ / \
/ | \ / \
/ | \ / \
| v | | |
| +------------+ |
| | Firmware | | | |
| | Consumer | | Device | +--------+ |
| +------------+ | Management| | | |
| | |<------------------------->| Status | |
| | Device | | | | Tracker| |
| +------------+ | || | | |
| | || +--------+ |
| | | |
| | \ /
\ / \ /
\ / \ Device /
\ Network / \ Operator /
\ Operator / \\ //
\\ // ---- ----
---- ---- ------
-----
Figure 1: Architecture.
End-to-end security mechanisms are used to protect the firmware image
and the manifest although Figure 2 does not show the manifest itself
since it may be distributed independently.
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+-----------+
+--------+ | | +--------+
| | Firmware Image | Firmware | Firmware Image | |
| Device |<-----------------| Server |<------------------| Author |
| | | | | |
+--------+ +-----------+ +--------+
^ *
* *
************************************************************
End-to-End Security
Figure 2: End-to-End Security.
Whether the firmware image and the manifest is pushed to the device
or fetched by the device is a deployment specific decision.
The following assumptions are made to allow the firmware consumer to
verify the received firmware image and manifest before updating
software:
- To accept an update, a device needs to verify the signature
covering the manifest. There may be one or multiple manifests
that need to be validated, potentially signed by different
parties. The device needs to be in possession of the trust
anchors to verify those signatures. Installing trust anchors to
devices via the Trust Provisioning Authority happens in an out-of-
band fashion prior to the firmware update process.
- Not all entities creating and signing manifests have the same
permissions. A device needs to determine whether the requested
action is indeed covered by the permission of the party that
signed the manifest. Informing the device about the permissions
of the different parties also happens in an out-of-band fashion
and is also a duty of the Trust Provisioning Authority.
- For confidentiality protection of firmware images the author needs
to be in possession of the certificate/public key or a pre-shared
key of a device. The use of confidentiality protection of
firmware images is deployment specific.
There are different types of delivery modes, which are illustrated
based on examples below.
There is an option for embedding a firmware image into a manifest.
This is a useful approach for deployments where devices are not
connected to the Internet and cannot contact a dedicated firmware
server for the firmware download. It is also applicable when the
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firmware update happens via a USB stick or via Bluetooth Smart.
Figure 3 shows this delivery mode graphically.
/------------\ /------------\
/Manifest with \ /Manifest with \
|attached | |attached |
\firmware image/ \firmware image/
\------------/ +-----------+ \------------/
+--------+ | | +--------+
| |<.................| Firmware |<................| |
| Device | | Server | | Author |
| | | | | |
+--------+ +-----------+ +--------+
Figure 3: Manifest with attached firmware.
Figure 4 shows an option for remotely updating a device where the
device fetches the firmware image from some file server. The
manifest itself is delivered independently and provides information
about the firmware image(s) to download.
/--------\ /--------\
/ \ / \
| Manifest | | Manifest |
\ / \ /
\--------/ \--------/
+-----------+
+--------+ | | +--------+
| |<.................| Status |................>| |
| Device | | Tracker | -- | Author |
| |<- | | --- | |
+--------+ -- +-----------+ --- +--------+
-- ---
--- ---
-- +-----------+ --
-- | | --
/------------\ -- | Firmware |<- /------------\
/ \ -- | Server | / \
| Firmware | | | | Firmware |
\ / +-----------+ \ /
\------------/ \------------/
Figure 4: Independent retrieval of the firmware image.
This architecture does not mandate a specific delivery mode but a
solution must support both types.
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6. Manifest
In order for a device to apply an update, it has to make several
decisions about the update:
- Does it trust the author of the update?
- Has the firmware been corrupted?
- Does the firmware update apply to this device?
- Is the update older than the active firmware?
- When should the device apply the update?
- How should the device apply the update?
- What kind of firmware binary is it?
- Where should the update be obtained?
- Where should the firmware be stored?
The manifest encodes the information that devices need in order to
make these decisions. It is a data structure that contains the
following information:
- information about the device(s) the firmware image is intended to
be applied to,
- information about when the firmware update has to be applied,
- information about when the manifest was created,
- dependencies on other manifests,
- pointers to the firmware image and information about the format,
- information about where to store the firmware image,
- cryptographic information, such as digital signatures or message
authentication codes (MACs).
The manifest information model is described in
[I-D.ietf-suit-information-model].
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7. Device Firmware Update Examples
Although these documents attempt to define a firmware update
architecture that is applicable to both existing systems, as well as
yet-to-be-conceived systems; it is still helpful to consider existing
architectures.
7.1. Single CPU SoC
The simplest, and currently most common, architecture consists of a
single MCU along with its own peripherals. These SoCs generally
contain some amount of flash memory for code and fixed data, as well
as RAM for working storage. These systems either have a single
firmware image, or an immutable bootloader that runs a single image.
A notable characteristic of these SoCs is that the primary code is
generally execute in place (XIP). Combined with the non-relocatable
nature of the code, firmware updates need to be done in place.
7.2. Single CPU with Secure - Normal Mode Partitioning
Another configuration consists of a similar architecture to the
previous, with a single CPU. However, this CPU supports a security
partitioning scheme that allows memory (in addition to other things)
to be divided into secure and normal mode. There will generally be
two images, one for secure mode, and one for normal mode. In this
configuration, firmware upgrades will generally be done by the CPU in
secure mode, which is able to write to both areas of the flash
device. In addition, there are requirements to be able to update
either image independently, as well as to update them together
atomically, as specified in the associated manifests.
7.3. Dual CPU, shared memory
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.
Upgrades in this case will typically be done by one of the CPUs, and
is similar to the single CPU with secure mode.
7.4. Dual CPU, other bus
This configuration has two or more CPUs, each having their own
memory. There will be a communication channel between them, but it
will be used as a peripheral, not via shared memory. In this case,
each CPU will have to be responsible for its own firmware upgrade.
It is likely that one of the CPUs will be considered a master, and
will direct the other CPU to do the upgrade. This configuration is
commonly used to offload specific work to other CPUs. Firmware
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dependencies are similar to the other solutions above, sometimes
allowing only one image to be upgraded, other times requiring several
to be upgraded atomically. Because the updates are happening on
multiple CPUs, upgrading the two images atomically is challenging.
8. Bootloader
More devices today than ever before are being connected to the
Internet, which drives the need for firmware updates to be provided
over the Internet rather than through traditional interfaces, such as
USB or RS232. Updating a device over the Internet requires the
device to fetch not only the firmware image but also the manifest.
Hence, the following building blocks are necessary for a firmware
update solution:
- the Internet protocol stack for (possibly large) firmware
downloads,
- the capability to write the received firmware image to persistent
storage (most likely flash memory) prior to performing the update,
- the ability to unpack, decompress or otherwise process the
received firmware image,
- the features to verify an image and a manifest, including digital
signature verification or checking a message authentication code,
- a manifest parsing library, and
- integration of the device into a device management server to
perform automatic firmware updates and to track their progress.
All these features are most likely offered by the application, i.e.
firmware consumer, running on the device (except for basic security
algorithms that may run either on a trusted execution environment or
on a separate hardware security MCU/module) rather than by the
bootloader itself.
Once manifests have been processed and firmware images successfully
downloaded and verified the device needs to hand control over to the
bootloader. In most cases this requires the MCU to restart. Once
the MCU has initiated a restart, the bootloader takes over control
and determines whether the newly downloaded firmware image should be
executed.
The boot process is security sensitive because the firmware images
may, for example, be stored in off-chip flash memory giving attackers
easy access to the image for reverse engineering and potentially also
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for modifying the binary. The bootloader will therefore have to
perform security checks on the firmware image before it can be
booted. These security checks by the bootloader happen in addition
to the security checks that happened when the firmware image and the
manifest were downloaded.
The manifest may have been stored alongside the firmware image to
allow re-verification of the firmware image during every boot
attempt. Alternatively, secure boot-specific meta-data may have been
created by the application after a successful firmware download and
verification process. Whether to re-use the standardized manifest
format that was used during the initial firmware retrieval process or
whether it is better to use a different format for the secure boot-
specific meta-data depends on the system design. The manifest format
does, however, have the capability to serve also as a building block
for secure boot with its severable elements that allow shrinking the
size of the manifest by stripping elements that are no longer needed.
If the application image contains the firmware consumer
functionality, as described above, then it is necessary that a
working image is left on the device to ensure that the bootloader can
roll back to a working firmware image to re-do the firmware download
since the bootloader itself does not have enough functionality to
fetch a firmware image plus manifest from a firmware server over the
Internet. A multi-stage bootloader may soften this requirement at
the expense of a more sophisticated boot process.
For a bootloader to offer a secure boot mechanism it needs to provide
the following features:
- ability to access security algorithms, such as SHA-256 to compute
a fingerprint over the firmware image and a digital signature
algorithm.
- access keying material directly or indirectly to utilize the
digital signature. The device needs to have a trust anchor store.
- ability to expose boot process-related data to the application
firmware (such as to the device management software). This allows
a device management server to determine whether the firmware
update has been successful and, if not, what errors occurred.
- to (optionally) offer attestation information (such as
measurements).
While the software architecture of the bootloader and its security
mechanisms are implementation-specific, the manifest can be used to
control the firmware download from the Internet in addition to
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augmenting secure boot process. These building blocks are highly
relevant for the design of the manifest.
9. Example
Figure 5 illustrates an example message flow for distributing a
firmware image to a device starting with an author uploading the new
firmware to firmware server and creating a manifest. The firmware
and manifest are stored on the same 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.
+--------+ +-----------------+ +------------+ +----------+
| Author | | Firmware Server | |FW Consumer | |Bootloader|
+--------+ +-----------------+ +------------+ +----------+
| | | +
| Create Firmware | | |
|--------------+ | | |
| | | | |
|<-------------+ | | |
| | | |
| Upload Firmware | | |
|------------------>| | |
| | | |
| Create Manifest | | |
|---------------+ | | |
| | | | |
|<--------------+ | | |
| | | |
| Sign Manifest | | |
|-------------+ | | |
| | | | |
|<------------+ | | |
| | | |
| Upload Manifest | | |
|------------------>| | |
| | | |
| | Query Manifest | |
| |<--------------------| |
| | | |
| | Send Manifest | |
| |-------------------->| |
| | | Validate |
| | | Manifest |
| | |---------+ |
| | | | |
| | |<--------+ |
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| | | |
| | Request Firmware | |
| |<--------------------| |
| | | |
| | Send Firmware | |
| |-------------------->| |
| | | Verify |
| | | Firmware |
| | |--------------+ |
| | | | |
| | |<-------------+ |
| | | |
| | | Store |
| | | Firmware |
| | |-------------+ |
| | | | |
| | |<------------+ |
| | | |
| | | |
| | | Trigger Reboot |
| | |--------------->|
| | | |
| | | |
| | +---+----------------+--+
| | S| | | |
| | E| | Verify | |
| | C| | Firmware | |
| | U| | +--------------| |
| | R| | | | |
| | E| | +------------->| |
| | | | | |
| | B| | Activate new | |
| | O| | Firmware | |
| | O| | +--------------| |
| | T| | | | |
| | | | +------------->| |
| | P| | | |
| | R| | Boot new | |
| | O| | Firmware | |
| | C| | +--------------| |
| | E| | | | |
| | S| | +------------->| |
| | S| | | |
| | +---+----------------+--+
| | | |
Figure 5: First Example Flow for a Firmware Upate.
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Figure 6 shows an example follow 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. Also omitted is the secure boot process following the
successful firmware update process.
The exchange starts with the device interacting with the status
tracker; the details of such exchange will vary with the different
device management systems being used. In any case, the status
tracker learns about the firmware version of the devices it manages.
In our example, the device under management is using firmware version
A.B.C. At a later point in time the author uploads a new firmware
along with the manifest to the firmware server and the status
tracker, respectively. While there is no need to store the manifest
and the firmware on different servers this example shows a common
pattern used in the industry. The status tracker may then
automatically, based on human intervention or based on a more complex
policy decide to inform the device about the newly available firmware
image. In our example, it does so by pushing the manifest to the FW
consumer. The firmware consumer downloads the firmware image with
the newer version X.Y.Z after successful validation of the manifest.
Subsequently, a reboot is initiated and the secure boot process
starts.
+---------+ +-----------------+ |-----------------------------.
| Status | | Firmware Server | | +------------+ +----------+ |
| Tracker | | | | |FW Consumer | |Bootloader| |
+---------+ +-----------------+ | +------------+ +----------+ |
| | | | IoT Device | |
| | `''''''''''''''''''''''''''''
| | | |
| Query Firmware Version | |
|------------------------------------->| |
| Firmware Version A.B.C | |
|<-------------------------------------| |
| | | |
| <<some time later>> | |
| | | |
_,...._ _,...._ | |
,' `. ,' `. | |
| New | | New | | |
\ Manifest / \ Firmware / | |
`.._ _,,' `.._ _,,' | |
`'' `'' | |
| Push manifest | |
|----------------+-------------------->| |
| | | |
| ' | '
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| | | Validate |
| | | Manifest |
| | |---------+ |
| | | | |
| | |<--------+ |
| | Request firmware | |
| | X.Y.Z | |
| |<--------------------| |
| | | |
| | Firmware X.Y.Z | |
| |-------------------->| |
| | | |
| | | Verify |
| | | Firmware |
| | |--------------+ |
| | | | |
| | |<-------------+ |
| | | |
| | | Store |
| | | Firmware |
| | |-------------+ |
| | | | |
| | |<------------+ |
| | | |
| | | |
| | | Trigger Reboot |
| | |--------------->|
| | | |
| | | |
| | | __..-------..._'
| | ,-' `-.
| | | Secure Boot |
| | `-. _/
| | |`--..._____,,.,-'
| | | |
Figure 6: Second Example Flow for a Firmware Upate.
10. IANA Considerations
This document does not require any actions by IANA.
11. Security Considerations
Firmware updates fix security vulnerabilities and are considered to
be an important building block in securing IoT devices. Due to the
importance of firmware updates for IoT devices the Internet
Architecture Board (IAB) organized a 'Workshop on Internet of Things
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(IoT) Software Update (IOTSU)', which took place at Trinity College
Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at
the big picture. A report about this workshop can be found at
[RFC8240]. A standardized firmware manifest format providing end-to-
end security from the author to the device will be specified in a
separate document.
There are, however, many other considerations raised during the
workshop. Many of them are outside the scope of standardization
organizations since they fall into the realm of product engineering,
regulatory frameworks, and business models. The following
considerations are outside the scope of this document, namely
- installing firmware updates in a robust fashion so that the update
does not break the device functionality of the environment this
device operates in.
- installing firmware updates in a timely fashion considering the
complexity of the decision making process of updating devices,
potential re-certification requirements, and the need for user
consent to install updates.
- 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.
- key management required for verifying the digital signature
protecting the manifest.
- incentives for manufacturers to offer a firmware update mechanism
as part of their IoT products.
12. Mailing List Information
The discussion list for this document is located at the e-mail
address suit@ietf.org [1]. Information on the group and information
on how to subscribe to the list is at
https://www1.ietf.org/mailman/listinfo/suit
Archives of the list can be found at: https://www.ietf.org/mail-
archive/web/suit/current/index.html
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13. Acknowledgements
We would like to thank the following persons for their feedback:
- Geraint Luff
- Amyas Phillips
- Dan Ros
- Thomas Eichinger
- Michael Richardson
- Emmanuel Baccelli
- Ned Smith
- Jim Schaad
- Carsten Bormann
- Cullen Jennings
- Olaf Bergmann
- Suhas Nandakumar
- Phillip Hallam-Baker
- Marti Bolivar
- Andrzej Puzdrowski
- Markus Gueller
- Henk Birkholz
- Jintao Zhu
- Takeshi Takahashi
- Jacob Beningo
We would also like to thank the WG chairs, Russ Housley, David
Waltermire, Dave Thaler for their support and their reviews.
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14. References
14.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC7925] Tschofenig, H., Ed. and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925, DOI
10.17487/RFC7925, July 2016, <https://www.rfc-
editor.org/info/rfc7925>.
14.2. Informative References
[I-D.ietf-suit-information-model]
Moran, B., Tschofenig, H., and H. Birkholz, "Firmware
Updates for Internet of Things Devices - An Information
Model for Manifests", draft-ietf-suit-information-model-03
(work in progress), July 2019.
[I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Wheeler, D., Atyeo, A., and D.
Liu, "Trusted Execution Environment Provisioning (TEEP)
Architecture", draft-ietf-teep-architecture-03 (work in
progress), July 2019.
[LwM2M] OMA, ., "Lightweight Machine to Machine Technical
Specification, Version 1.0.2", February 2018,
<http://www.openmobilealliance.org/release/LightweightM2M/
V1_0_2-20180209-A/
OMA-TS-LightweightM2M-V1_0_2-20180209-A.pdf>.
[RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649, DOI
10.17487/RFC5649, September 2009, <https://www.rfc-
editor.org/info/rfc5649>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <https://www.rfc-editor.org/info/rfc6024>.
[RFC8240] Tschofenig, H. and S. Farrell, "Report from the Internet
of Things Software Update (IoTSU) Workshop 2016", RFC
8240, DOI 10.17487/RFC8240, September 2017,
<https://www.rfc-editor.org/info/rfc8240>.
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14.3. URIs
[1] mailto:suit@ietf.org
Authors' Addresses
Brendan Moran
Arm Limited
EMail: Brendan.Moran@arm.com
Milosch Meriac
Consultant
EMail: milosch@meriac.com
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@arm.com
David Brown
Linaro
EMail: david.brown@linaro.org
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