Internet DRAFT - draft-ietf-suit-architecture
draft-ietf-suit-architecture
SUIT B. Moran
Internet-Draft H. Tschofenig
Intended status: Informational Arm Limited
Expires: July 31, 2021 D. Brown
Linaro
M. Meriac
Consultant
January 27, 2021
A Firmware Update Architecture for Internet of Things
draft-ietf-suit-architecture-16
Abstract
Vulnerabilities in Internet of Things (IoT) devices have raised the
need for a reliable and secure firmware update mechanism suitable for
devices with resource constraints. Incorporating such an update
mechanism is a fundamental requirement for fixing vulnerabilities but
it also enables other important capabilities such as updating
configuration settings as well as adding new functionality.
In addition to the definition of terminology and an architecture this
document motivates the standardization of a manifest format as a
transport-agnostic means for describing and protecting firmware
updates.
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 https://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 July 31, 2021.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 5
2.1. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Stakeholders . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Functions . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Invoking the Firmware . . . . . . . . . . . . . . . . . . . . 13
4.1. The Bootloader . . . . . . . . . . . . . . . . . . . . . 14
5. Types of IoT Devices . . . . . . . . . . . . . . . . . . . . 15
5.1. Single MCU . . . . . . . . . . . . . . . . . . . . . . . 16
5.2. Single CPU with Secure - Normal Mode Partitioning . . . . 16
5.3. Symmetric Multiple CPUs . . . . . . . . . . . . . . . . . 16
5.4. Dual CPU, shared memory . . . . . . . . . . . . . . . . . 16
5.5. Dual CPU, other bus . . . . . . . . . . . . . . . . . . . 17
6. Manifests . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7. Securing Firmware Updates . . . . . . . . . . . . . . . . . . 19
8. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
10. Security Considerations . . . . . . . . . . . . . . . . . . . 25
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
12. Informative References . . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
Firmware updates can help to fix security vulnerabilities, and
performing updates is an important building block in securing IoT
devices. Due to rising concerns about insecure IoT devices the
Internet Architecture Board (IAB) organized a 'Workshop on Internet
of Things (IoT) Software Update (IOTSU)' [RFC8240] to take a look at
the bigger picture. The workshop revealed a number of challenges for
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developers and led to the formation of the IETF Software Updates for
Internet of Things (SUIT) working group.
Developing secure Internet of Things (IoT) devices is not an easy
task and supporting a firmware update solution requires skillful
engineers. Once devices are deployed, firmware updates play a
critical part in their lifecycle management, particularly when
devices have a long lifetime, or are deployed in remote or
inaccessible areas where manual intervention is cost prohibitive or
otherwise difficult. Firmware updates
for IoT devices are expected to work automatically, i.e. without user
involvement. Conversely, non-IoT devices are expected to account for
user preferences and consent when scheduling updates. Automatic
updates that do not require human intervention are key to a scalable
solution for fixing software vulnerabilities.
Firmware updates are done not only to fix bugs, but also to add new
functionality and to reconfigure the device to work in new
environments or to behave differently in an already deployed context.
The manifest specification has to allow that
- The firmware image is authenticated and integrity protected.
Attempts to flash a maliciously modified firmware image or an
image from an unknown, untrusted source must be prevented. In
examples this document uses asymmetric cryptography because it is
the preferred approach by many IoT deployments. The use of
symmetric credentials is also supported and can be used by very
constrained IoT devices.
- The firmware image can be confidentiality protected so that
attempts by an adversary to recover the plaintext binary can be
mitigated or at least made more difficult. Obtaining the firmware
is often one of the first steps to mount an attack since it gives
the adversary valuable insights into the software libraries used,
configuration settings and generic functionality. Even though
reverse engineering the binary can be a tedious process modern
reverse engineering frameworks have made this task a lot easier.
Authentication and integrity protection of firmware images must be
used in a deployment but the confidential protection of firmware is
optional.
While the standardization work has been informed by and optimized for
firmware update use cases of Class 1 devices (according to the device
class definitions in RFC 7228 [RFC7228]), there is nothing in the
architecture that restricts its use to only these constrained IoT
devices. Moreover, this architecture is not limited to managing
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firmware and software updates, but can also be applied to managing
the delivery of arbitrary data, such as configuration information and
keys. Unlike higher end devices, like laptops and desktop PCs, many
IoT devices do not have user interfaces; and support for unattended
updates is, therefore, essential for the design of a practical
solution. Constrained IoT devices often use a software engineering
model where a developer is responsible for creating and compiling all
software running on the device into a single, monolithic firmware
image. On higher end devices application software is, on the other
hand, often downloaded separately and even obtained from developers
different to the developers of the lower level software. The details
for how to obtain those application layer software binaries then
depends heavily on the platform, programming language used and the
sandbox in which the software is executed.
While the IETF standardization work has been focused on the manifest
format, a fully interoperable solution needs more than a standardized
manifest. For example, protocols for transferring firmware images
and manifests to the device need to be available as well as the
status tracker functionality. Devices also require a mechanism to
discover the status tracker(s) and/or firmware servers, for example
using pre-configured hostnames or DNS-SD [RFC6763]. These building
blocks have been developed by various organizations under the
umbrella of an IoT device management solution. The LwM2M protocol
[LwM2M] is one IoT device management protocol.
There are, however, several areas that (partially) fall outside the
scope of the IETF and other standards organizations but need to be
considered by firmware authors, as well as device and network
operators. Here are some of them, as highlighted during the IOTSU
workshop:
- Installing firmware updates in a robust fashion so that the update
does not break the device functionality of the environment this
device operates in. This requires proper testing and offering
recovery strategies when a firmware update is unsuccessful.
- Making firmware updates available in a timely fashion considering
the complexity of the decision making process for updating
devices, potential re-certification requirements, the length of a
supply chain an update needs to go through before it reaches the
end customer, and the need for user consent to install updates.
- Ensuring an energy efficient design of a battery-powered IoT
device because a firmware update, particularly radio communication
and writing the firmware image to flash, is an energy-intensive
task for a device.
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- Creating incentives for device operators to use a firmware update
mechanism and to demand the integration of it from IoT device
vendors.
- Ensuring that firmware updates addressing critical flaws can be
obtained even after a product is discontinued or a vendor goes out
of business.
This document starts with a terminology followed by the description
of the architecture. We then explain the bootloader and how it
integrates with the firmware update mechanism. Subsequently, we
offer a categorization of IoT devices in terms of their hardware
capabilities relevant for firmware updates. Next, we talk about the
manifest structure and how to use it to secure firmware updates. We
conclude with a more detailed example.
2. Conventions and Terminology
2.1. Terms
This document uses the following terms:
- Firmware Image: The firmware image, or simply the "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. Often it is
also a compressed archive that contains code, configuration data,
and even the entire file system. The image may consist of a
differential update for performance reasons.
The terms, firmware image, firmware, and image, are used in this
document and are interchangeable. We use the term application
firmware image to differentiate it from a firmware image that
contains the bootloader. An application firmware image, as the
name indicates, contains the application program often including
all the necessary code to run it (such as protocol stacks, and
embedded operating system).
- Manifest: The manifest contains meta-data about the firmware
image. The manifest is protected against modification and
provides information about the author.
- 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
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often used interchangeably with MCU, but MCU tends to imply more
limited peripheral functions.
- 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.
- Software: Similar to firmware, but typically dynamically loaded by
an Operating System. Used interchangeably with firmware in this
document.
- System on Chip (SoC): An SoC is an integrated circuit that
contains all components of a computer, such as CPU, memory, input/
output ports, secondary storage, a bus to connect the components,
and other hardware blocks of logic.
- Trust Anchor: A trust anchor, as defined in [RFC6024], 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.
- Trust Anchor Store: A trust anchor store, as defined in [RFC6024],
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.
- Trusted Applications (TAs): An application component that runs in
a TEE.
- Trusted Execution Environments (TEEs): An execution environment
that runs alongside of, but is isolated from, an REE. For more
information about TEEs see [I-D.ietf-teep-architecture].
2.2. Stakeholders
The following stakeholders are used in this document:
- Author: The author is the entity that creates the firmware image.
There may be multiple authors involved in producing firmware
running on an IoT device. Section 5 talks about those IoT device
deployment cases.
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- Device Operator: The device operator is responsible for the day-
to-day operation of a fleet of IoT devices. Customers of IoT
devices, as the owners of IoT devices - such as enterprise
customers or end users - interact with their IoT devices
indirectly through the device operator via web or smart phone
apps.
- Network Operator: The network operator is responsible for the
operation of a network to which IoT devices connect.
- Trust Provisioning Authority (TPA): The TPA distributes trust
anchors and authorization policies to devices and various
stakeholders. The TPA may also delegate rights to stakeholders.
Typically, the Original Equipment Manufacturer (OEM) or Original
Design Manufacturer (ODM) will act as a TPA, however complex
supply chains may require a different design. In some cases, the
TPA may decide to remain in full control over the firmware update
process of their products.
- User: The end-user of a device. The user may interact with
devices via web or smart phone apps, as well as through direct
user interfaces.
2.3. Functions
- (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 successfully perform an update.
- Status Tracker: The status tracker has a client and a server
component and performs three tasks: 1) It communicates the
availability of a new firmware version. This information will
flow from the server to the client.
2) It conveys information about software and hardware
characteristics of the device. The information flow is from the
client to the server.
3) It can remotely trigger the firmware update process. The
information flow is from the server to the client.
For example, a device operator may want to read the installed
firmware version number running on the device and information
about available flash memory. Once an update has been triggered,
the device operator may want to obtain information about the state
of the firmware update. If errors occurred, the device operator
may want to troubleshoot problems by first obtaining diagnostic
information (typically using a device management protocol).
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We make no assumptions about where the server-side component is
deployed. The deployment of status trackers is flexible: they may
be found at cloud-based servers or on-premise servers, or they may
be embedded in edge computing devices. A status tracker server
component may even be deployed on an IoT device. For example, if
the IoT device contains multiple MCUs, then the main MCU may act
as a status tracker towards the other MCUs. Such deployment is
useful when updates have to be synchronized across MCUs.
The status tracker may be operated by any suitable stakeholder;
typically the Author, Device Operator, or Network Operator.
- 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 client (locally).
- 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 and these two
entities are often physically separated on different devices for
scalability reasons.
- Bootloader: A bootloader is a piece of software that is executed
once a microcontroller has been reset. It is responsible for
deciding what code to execute.
3. Architecture
More devices today than ever before are 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
RS-232. Sending updates over the Internet requires the device to
fetch the new firmware image as well as the manifest.
Hence, the following components are necessary on a device for a
firmware update solution:
- the Internet protocol stack for firmware downloads. Because
firmware images are often multiple kilobytes, sometimes exceeding
one hundred kilobytes, for low-end IoT devices and even several
megabytes for IoT devices running full-fledged operating systems
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like Linux, the protocol mechanism for retrieving these images
needs to offer features like congestion control, flow control,
fragmentation and reassembly, and mechanisms to resume interrupted
or corrupted transfers.
- the capability to write the received firmware image to persistent
storage (most likely flash memory).
- a manifest parser with code to verify a digital signature or a
message authentication code.
- the ability to unpack, to decompress and/or to decrypt the
received firmware image.
- a status tracker.
The features listed above are most likely offered by code in the
application firmware image running on the device rather than by the
bootloader itself. Note that cryptographic algorithms will likely
run in a trusted execution environment, on a separate MCU, in a
hardware security module, or in a secure element rather than in the
same context with the application code.
Figure 1 shows the architecture where a firmware image is created by
an author, and made available to a firmware server. For security
reasons, the author will not have the permissions to upload firmware
images to the firmware server and to initiate an update directly.
Instead, authors will make firmware images available to the device
operators. Note that there may be a longer supply chain involved to
pass software updates from the author all the way to the party that
can then finally make a decision to deploy it with IoT devices.
As a first step in the firmware update process, the status tracker
server needs to inform the status tracker client that a new firmware
update is available. This can be accomplished via polling (client-
initiated), push notifications (server-initiated), or more complex
mechanisms (such as a hybrid approach):
- Client-initiated updates take the form of a status tracker client
proactively checking (polling) for updates.
- With Server-initiated updates the server-side component of the
status tracker learns about a new firmware version and determines
which devices qualify for a firmware update. Once the relevant
devices have been selected, the status tracker informs these
devices and the firmware consumers obtain those images and
manifests. Server-initiated updates are important because they
allow a quick response time. Note that in this mode the client-
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side status tracker needs to be reachable by the server-side
component. This may require devices to keep reachability
information on the server-side up-to-date and state at NATs and
stateful packet filtering firewalls alive.
- Using a hybrid approach the server-side of the status tracker
pushes notifications of availability of an update to the client
side and requests the firmware consumer to pull the manifest and
the firmware image from the firmware server.
Once the device operator triggers an update via the status tracker,
it will keep track of the update process on the device. This allows
the device operator to know what devices have received an update and
which of them are still pending an update.
Firmware images can be conveyed to devices in a variety of ways,
including USB, UART, WiFi, BLE, low-power WAN technologies, mesh
networks and many more. At the application layer a variety of
protocols are also available: MQTT, CoAP, and HTTP are the most
popular application layer protocols used by IoT devices. This
architecture does not make assumptions about how the firmware images
are distributed to the devices and therefore aims to support all
these technologies.
In some cases it may be desirable to distribute firmware images using
a multicast or broadcast protocol. This architecture does not make
recommendations for any such protocol. However, given that broadcast
may be desirable for some networks, updates must cause the least
disruption possible both in metadata and firmware transmission. For
an update to be broadcast friendly, it cannot rely on link layer,
network layer, or transport layer security. A solution has to rely
on security protection applied to the manifest and firmware image
instead. In addition, the same manifest 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.
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+----------+
| |
| Author |
| |
+----------+
Firmware + Manifest |
+----------------------------------+ | Firmware +
| | | Manifest
| ---+------- |
| ---- | --|-
| //+----------+ | \\
-+-- // | | | \
----/ | ---- |/ | Firmware |<-+ | \
// | \\ | | Server | | | \
/ | \ / | | + + \
/ | \ / +----------+ \ / |
/ +--------+--------+ \ / | |
/ | v | \ / v |
| | +------------+ | | | +----------------+ |
| | | Firmware | | | Device | |
| | | Consumer | | | | | Management | |
| | +------------+ | | | | | |
| | +------------+ | | | | +--------+ | |
| | | Status |<-+--------------------+-> | | | |
| | | Tracker | | | | | | Status | | |
| | | Client | | | | | | Tracker| | |
| | +------------+ | | | | | Server | | |
| | Device | | | | +--------+ | |
| +-----------------+ | \ | | /
\ / \ +----------------+ /
\ Network / \ /
\ Operator / \ Device Operator /
\\ // \ \ //
---- ---- ---- ----
----- -----------
Figure 1: Architecture.
Firmware images and manifests may be conveyed as a bundle or
detached. The manifest format must support both approaches.
For distribution as a bundle, the firmware image is embedded into the
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 firmware update happens via a USB sticks or short range
radio technologies (such as Bluetooth Smart).
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Alternatively, the manifest is distributed detached from the firmware
image. Using this approach, the firmware consumer is presented with
the manifest first and then needs to obtain one or more firmware
images as dictated in the manifest.
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.
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/1.1-based firmware server is
client-initiated. Pushing a manifest and firmware image to the
Package resource of the LwM2M Firmware Update object [LwM2M] is
server-initiated update.
If the firmware consumer has downloaded a new firmware image and is
ready to install it, 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. Sometimes the final decision
may require confirmation of the user of the device for safety
reasons.
Installation is the act of processing the payload into a format that
the IoT device can recognize and the bootloader is responsible for
then booting from the newly installed firmware image. This process
is different when a bootloader is not involved. For example, when an
application is updated in a full-featured operating system, the
updater may halt and restart the application in isolation. Devices
must not fail when a disruption, such as a power failure or network
interruption, occurs during the update process.
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4. Invoking the Firmware
Section 3 describes the steps for getting the firmware image and the
manifest from the author to the firmware consumer on the IoT device.
Once the firmware consumer has retrieved and successfully processed
the manifest and the firmware image it needs to invoke the new
firmware image. This is managed in many different ways, depending on
the type of device, but it typically involves halting the current
version of the firmware, handing control over to a firmware with a
higher privilege/trust level (the firmware verifier), verifying the
new firmware's authenticity & integrity, and then invoking it.
In an execute-in-place microcontroller, this is often done by
rebooting into a bootloader (simultaneously halting the application &
handing over to the higher privilege level) then executing a secure
boot process (verifying and invoking the new image).
In a rich OS, this may be done by halting one or more processes, then
invoking new applications. In some OSs, this implicitly involves the
kernel verifying the code signatures on the new applications.
The invocation process is security sensitive. An attacker will
typically try to retrieve a firmware image from the device for
reverse engineering or will try to get the firmware verifier to
execute an attacker-modified firmware image. The firmware verifier
will therefore have to perform security checks on the firmware image
before it can be invoked. These security checks by the firmware
verifier happen in addition to the security checks that took place
when the firmware image and the manifest were downloaded by the
firmware consumer.
The overlap between the firmware consumer and the firmware verifier
functionality comes in two forms, namely
- A firmware verifier 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 firmware
verifier 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 firmware verifier may well be the same
manifest obtained with the firmware image during the update
process.
- An IoT device needs a recovery strategy in case the firmware
update / invocation process fails. The recovery strategy may
include storing two or more application firmware images on the
device or offering the ability to invoke a recovery image to
perform the firmware update process again using firmware updates
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over serial, USB or even wireless connectivity like Bluetooth
Smart. In the latter case the firmware consumer functionality is
contained in the recovery image and requires the necessary
functionality for executing the firmware update process, including
manifest parsing.
While this document assumes that the firmware verifier itself is
distinct from the role of the firmware consumer and therefore does
not manage the firmware update process, this is not a requirement and
these roles may be combined in practice.
Using a bootloader as the firmware verifier requires some special
considerations, particularly when the bootloader implements the
robustness requirements identified by the IOTSU workshop [RFC8240].
4.1. The Bootloader
In most cases the MCU must restart in order to hand over control to
the bootloader. Once the MCU has initiated a restart, the bootloader
determines whether a newly available firmware image should be
executed. If the bootloader concludes that the newly available
firmware image is invalid, a recovery strategy is necessary. There
are only two approaches for recovering from an invalid firmware:
either the bootloader must be able to select a different, valid
firmware, or it must be able to obtain a new, valid firmware. Both
of these approaches have implications for the architecture of the
update system.
Assuming the first approach, there are (at least) three firmware
images available on the device:
- First, the bootloader is also firmware. If a bootloader is
updatable then its firmware image is treated like any other
application firmware image.
- Second, the firmware image that has to be replaced is still
available on the device as a backup in case the freshly downloaded
firmware image does not boot or operate correctly.
- Third, there is the newly downloaded firmware image.
Therefore, the firmware consumer must know where to store the new
firmware. In some cases, this may be implicit, for example replacing
the least-recently-used firmware image. In other cases, the storage
location of the new firmware must be explicit, for example when a
device has one or more application firmware images and a recovery
image with limited functionality, sufficient only to perform an
update.
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Since many low end IoT devices do not use position-independent code,
either the bootloader needs to copy the newly downloaded application
firmware image into the location of the old application firmware
image and vice versa or multiple versions of the firmware need to be
prepared for different locations.
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 reliability risk.
For a bootloader to offer a secure boot functionality it needs to
implement the following functionality:
- The bootloader needs to fetch the manifest from nonvolatile
storage and parse its contents for subsequent cryptographic
verification.
- Cryptographic libraries with hash functions, digital signatures
(for asymmetric crypto), message authentication codes (for
symmetric crypto) need to be accessible.
- The device needs to have a trust anchor store to verify the
digital signature. (Alternatively, access to a key store for use
with the message authentication code.)
- There must be an ability to expose boot process-related data to
the application firmware (such as to the status tracker). This
allows sharing information about the current firmware version, and
the status of the firmware update process and whether errors have
occurred.
- Produce boot measurements as part of an attestation solution. See
[I-D.ietf-rats-architecture] for more information. (optional)
- The bootloader must be able to decrypt firmware images, in case
confidentiality protection was applied. This requires a solution
for key management. (optional)
5. Types of IoT Devices
There are billions of MCUs used in devices today produced by a large
number of silicon manufacturers. While MCUs can vary significantly
in their characteristics, there are a number of similiaries allowing
us to categorize in groups.
The firmware update architecture, and the manifest format in
particular, needs to offer enough flexibility to cover these common
deployment cases.
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5.1. Single MCU
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. A notable characteristic of these SoCs
is that the primary code is generally execute in place (XIP). Due to
the non-relocatable nature of the code, the firmware image needs to
be placed in a specific location in flash since the code cannot be
executed from an arbitrary location in flash. Hence, when the
firmware image is updated it is necessary to swap the old and the new
image.
5.2. 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.
5.3. Symmetric Multiple CPUs
In more complex SoCs with symmetric multi-processing support,
advanced operating systems, such as Linux, are often used. These
SoCs frequently use an external storage medium, such as raw NAND
flash or eMMC. Due to the higher quantity of resources, these
devices are often capable of storing multiple copies of their
firmware images and selecting the most appropriate one to boot. Many
SoCs also support bootloaders that are capable of updating the
firmware image, however this is typically a last resort because it
requires the device to be held in the bootloader while the new
firmware is downloaded and installed, which results in down-time for
the device. Firmware updates in this class of device are typically
not done in-place.
5.4. Dual CPU, shared memory
This configuration has two or more heterogeneous CPUs in a single SoC
that share memory (flash and RAM). Generally, there will be a
mechanism to prevent one CPU from unintentionally accessing memory
currently allocated to the other. Upgrades in this case will
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typically be done by one of the CPUs, and is similar to the single
CPU with secure mode.
5.5. Dual CPU, other bus
This configuration has two or more heterogeneous 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 the
primary CPU, and will direct the other CPU to do the upgrade. This
configuration is commonly used to offload specific work to other
CPUs. Firmware 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.
6. Manifests
In order for a firmware consumer to apply an update, it has to make
several decisions using manifest-provided information and data
available on the device itself. For more detailed information and a
longer list of information elements in the manifest consult the
information model specification [I-D.ietf-suit-information-model],
which offers justifications for each element, and the manifest
specification [I-D.ietf-suit-manifest] for details about how this
information is included in the manifest.
Table 1 provides examples of decisions to be made.
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+----------------------------+--------------------------------------+
| Decision | Information Elements |
+----------------------------+--------------------------------------+
| Should I trust the author | Trust anchors and authorization |
| of the firmware? | policies on the device |
| | |
| Has the firmware been | Digital signature and MAC covering |
| corrupted? | the firmware image |
| | |
| Does the firmware update | Conditions with Vendor ID, Class ID |
| apply to this device? | and Device ID |
| | |
| Is the update older than | Sequence number in the manifest (1) |
| the active firmware? | |
| | |
| When should the device | Wait directive |
| apply the update? | |
| | |
| How should the device | Manifest commands |
| apply the update? | |
| | |
| What kind of firmware | Unpack algorithms to interpret a |
| binary is it? | format. |
| | |
| Where should the update be | Dependencies on other manifests and |
| obtained? | firmware image URI in Manifest |
| | |
| Where should the firmware | Storage Location and Component |
| be stored? | Identifier |
+----------------------------+--------------------------------------+
Table 1: Firmware Update Decisions.
(1): 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.
Keeping the code size and complexity of a manifest parsers small is
important for constrained IoT devices. Since the manifest parsing
code may also be used by the bootloader it can be part of the trusted
computing base.
A manifest may be used to protect not only firmware images but also
configuration data such as network credentials or personalization
data related to firmware or software. Personalization data
demonstrates the need for confidentiality to be maintained between
two or more stakeholders that both deliver images to the same device.
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Personalization data is used with Trusted Execution Environments
(TEEs), which 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 conveyed to the
TEE. The TA's author does not want to expose the TA's code to any
other stakeholder or third party. The user does not want to expose
the payment information to any other stakeholder or third party.
7. Securing Firmware Updates
Using firmware updates to fix vulnerabilities in devices is important
but securing this update mechanism is equally important since
security problems are exacerbated by the update mechanism: update is
essentially authorized remote code execution, so any security
problems in the update process expose that remote code execution
system. Failure to secure the firmware update process will help
attackers to take control over devices.
End-to-end security mechanisms are used to protect the firmware image
and the manifest. The following assumptions are made to allow the
firmware consumer to verify the received firmware image and manifest
before updating software:
- Authentication ensures that the device can cryptographically
identify the author(s) creating firmware images and manifests.
Authenticated identities may be used as input to the authorization
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 a duty of the Trust Provisioning Authority.
- Integrity protection ensures that no third party can modify the
manifest or the firmware image. 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.
- For confidentiality protection of the firmware image, it must be
done in such a way that the intended firmware consumer(s), other
authorized parties, and no one else can decrypt it. The
information that is encrypted individually for each device/
recipient must be done in a way that is usable with Content
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Distribution Networks, bulk storage, and broadcast protocols. 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 optional.
A manifest specification must support different cryptographic
algorithms and algorithm extensibility. Moreover, since RSA- and
ECC-based signature schemes may become vulnerable to quantum-
accelerated key extraction in the future, unchangeable bootloader
code in ROM is recommended to use post-quantum secure signature
schemes such as hash-based signatures [RFC8778]. A bootloader author
must carefully consider the service lifetime of their product and the
time horizon for quantum-accelerated key extraction. The worst-case
estimate, at time of writing, for the time horizon to key extraction
with quantum acceleration is approximately 2030, based on current
research [quantum-factorization].
When a device obtains a monolithic firmware image from a single
author without any additional approval steps, 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.
8. Example
Figure 2 illustrates an example message flow for distributing a
firmware image to a device. The firmware and manifest are stored on
the same firmware server and distributed in a detached manner.
+--------+ +-----------------+ +-----------------------------+
| | | Firmware Server | | IoT Device |
| Author | | Status Tracker | | +------------+ +----------+ |
+--------+ | Server | | | Firmware | |Bootloader| |
| +-----------------+ | | Consumer | | | |
| | | +------------+ +----------+ |
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| | | | | |
| | | +-----------------------+ |
| Create Firmware | | | Status Tracker Client | |
|--------------+ | | +-----------------------+ |
| | | `''''''''''''''''''''''''''''
|<-------------+ | | | |
| | | | |
| Upload Firmware | | | |
|------------------>| | | |
| | | | |
| Create Manifest | | | |
|---------------+ | | | |
| | | | | |
|<--------------+ | | | |
| | | | |
| Sign Manifest | | | |
|-------------+ | | | |
| | | | | |
|<------------+ | | | |
| | | | |
| Upload Manifest | | | |
|------------------>| Notification of | | |
| | new firmware image | | |
| |----------------------------->| |
| | | | |
| | |Initiate| |
| | | Update | |
| | |<-------| |
| | | | |
| | Query Manifest | | |
| |<--------------------| . |
| | | . |
| | Send Manifest | . |
| |-------------------->| . |
| | | Validate |
| | | Manifest |
| | |--------+ |
| | | | |
| | |<-------+ |
| | | . |
| | Request Firmware | . |
| |<--------------------| . |
| | | . |
| | Send Firmware | . |
| |-------------------->| . |
| | | Verify . |
| | | Firmware |
| | |--------+ |
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| | | | |
| | |<-------+ |
| | | . |
| | | Store . |
| | | Firmware |
| | |--------+ |
| | | | |
| | |<-------+ |
| | | . |
| | | . |
| | | . |
| | | | |
| | | Update | |
| | |Complete| |
| | |------->| |
| | | |
| | Firmware Update Completed | |
| |<-----------------------------| |
| | | |
| | Reboot | |
| |----------------------------->| |
| | | | |
| | | | |
| | |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| | | |
| | +---+----------------+--+
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| | | . |
| | | | |
| | . | |
| | Device running new firmware | |
| |<-----------------------------| |
| | . | |
| | | |
Figure 2: First Example Flow for a Firmware Update.
Figure 3 shows an exchange that starts with the status tracker
querying the device for its current firmware version. Later, a new
firmware version becomes available and since this device is running
an older version the status tracker server interacts with the device
to initiate an update.
The manifest and the firmware are stored on different servers in this
example. When the device processes the manifest it learns where to
download the new firmware version. 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. Finally, the device reports the
successful boot of the new firmware version.
+---------+ +-----------------+ +-----------------------------+
| Status | | Firmware Server | | +------------+ +----------+ |
| Tracker | | Status Tracker | | | Firmware | |Bootloader| |
| Server | | Server | | | Consumer | | | |
+---------+ +-----------------+ | | +Status | +----------+ |
| | | | Tracker | | |
| | | | Client | | |
| | | +------------+ | |
| | | | IoT Device | |
| | `''''''''''''''''''''''''''''
| | | |
| Query Firmware Version | |
|------------------------------------->| |
| Firmware Version A.B.C | |
|<-------------------------------------| |
| | | |
| <<some time later>> | |
| | | |
_,...._ _,...._ | |
,' `. ,' `. | |
| New | | New | | |
\ Manifest / \ Firmware / | |
`.._ _,,' `.._ _,,' | |
`'' `'' | |
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| Push manifest | |
|----------------+-------------------->| |
| | | |
| ' | '
| | | Validate |
| | | Manifest |
| | |---------+ |
| | | | |
| | |<--------+ |
| | Request firmware | |
| | X.Y.Z | |
| |<--------------------| |
| | | |
| | Firmware X.Y.Z | |
| |-------------------->| |
| | | |
| | | Verify |
| | | Firmware |
| | |--------------+ |
| | | | |
| | |<-------------+ |
| | | |
| | | Store |
| | | Firmware |
| | |-------------+ |
| | | | |
| | |<------------+ |
| | | |
| | | |
| | | Trigger Reboot |
| | |--------------->|
| | | |
| | | |
| | | __..-------..._'
| | ,-' `-.
| | | Secure Boot |
| | `-. _/
| | |`--..._____,,.,-'
| | | |
| Device running firmware X.Y.Z | |
|<-------------------------------------| |
| | | |
| | | |
Figure 3: Second Example Flow for a Firmware Update.
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9. IANA Considerations
This document does not require any actions by IANA.
10. Security Considerations
This document describes terminology, requirements and an architecture
for firmware updates of IoT devices. The content of the document is
thereby focused on improving security of IoT devices via firmware
update mechanisms and informs the standardization of a manifest
format.
An in-depth examination of the security considerations of the
architecture is presented in [I-D.ietf-suit-information-model].
11. 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
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- Markus Gueller
- Henk Birkholz
- Jintao Zhu
- Takeshi Takahashi
- Jacob Beningo
- Kathleen Moriarty
- Bob Briscoe
- Roman Danyliw
- Brian Carpenter
- Theresa Enghardt
- Rich Salz
- Mohit Sethi
- Eric Vyncke
- Alvaro Retana
- Barry Leiba
- Benjamin Kaduk
- Martin Duke
- Robert Wilton
We would also like to thank the WG chairs, Russ Housley, David
Waltermire, and Dave Thaler, for their support and their reviews.
12. Informative References
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture",
draft-ietf-rats-architecture-08 (work in progress),
December 2020.
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[I-D.ietf-suit-information-model]
Moran, B., Tschofenig, H., and H. Birkholz, "An
Information Model for Firmware Updates in IoT Devices",
draft-ietf-suit-information-model-08 (work in progress),
October 2020.
[I-D.ietf-suit-manifest]
Moran, B., Tschofenig, H., Birkholz, H., and K. Zandberg,
"A Concise Binary Object Representation (CBOR)-based
Serialization Format for the Software Updates for Internet
of Things (SUIT) Manifest", draft-ietf-suit-manifest-11
(work in progress), December 2020.
[I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", draft-ietf-teep-architecture-13 (work in
progress), November 2020.
[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>.
[quantum-factorization]
Jiang, S., Britt, K., McCaskey, A., Humble, T., and S.
Kais, "Quantum Annealing for Prime Factorization",
December 2018,
<https://www.nature.com/articles/s41598-018-36058-z>.
[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>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[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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[RFC8778] Housley, R., "Use of the HSS/LMS Hash-Based Signature
Algorithm with CBOR Object Signing and Encryption (COSE)",
RFC 8778, DOI 10.17487/RFC8778, April 2020,
<https://www.rfc-editor.org/info/rfc8778>.
Authors' Addresses
Brendan Moran
Arm Limited
EMail: Brendan.Moran@arm.com
Hannes Tschofenig
Arm Limited
EMail: hannes.tschofenig@arm.com
David Brown
Linaro
EMail: david.brown@linaro.org
Milosch Meriac
Consultant
EMail: milosch@meriac.com
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