Bootloader

The "bootloader" is the code that loads the Mynewt OS image into memory and conducts some checks before allowing the OS to be run. It manages images for the embedded system and upgrades of those images using protocols over various interfaces (e.g. serial, BLE, etc.). Typically, systems with bootloaders have at least two program images coexisting on the same microcontroller, and hence must include branch code that performs a check to see if an attempt to update software is already underway and manage the progress of the process.

The bootloader in the Apache Mynewt project verifies the cryptographic signature of the firmware image before running it. It maintains a detailed status log for each stage of the boot process. For verification of the authenticity of the OS image, it:

The "secure bootloader" should be placed in protected memory on a given microcontroller.

The Mynewt bootloader comprises two packages:

  • The bootutil library (boot/bootutil)
  • The boot application (apps/boot)

The Mynewt code is thus structured so that the generic bootutil library performs most of the functions of a boot loader. The final step of actually jumping to the main image is kept out of the bootutil library. This last step should instead be implemented in an architecture-specific project. Boot loader functionality is separated in this manner for the following two reasons:

  1. By keeping architecture-dependent code separate, the bootutil library can be reused among several boot loaders.
  2. By excluding the last boot step from the library, the bootloader can be unit tested since a library can be unit tested but an applicant can't.

Limitations

The boot loader currently only supports images with the following characteristics:

  • Built to run from flash.
  • Build to run from a fixed location (i.e., position-independent).

Image Format

The following definitions describe the image header format.

#define IMAGE_MAGIC                 0x96f3b83c
#define IMAGE_MAGIC_NONE            0xffffffff

struct image_version {
    uint8_t iv_major;
    uint8_t iv_minor;
    uint16_t iv_revision;
    uint32_t iv_build_num;
};

/** Image header.  All fields are in little endian byte order. */
struct image_header {
    uint32_t ih_magic;
    uint16_t ih_tlv_size; /* Trailing TLVs */
    uint8_t  ih_key_id;
    uint8_t  _pad1;
    uint16_t ih_hdr_size;
    uint16_t _pad2;
    uint32_t ih_img_size; /* Does not include header. */
    uint32_t ih_flags;
    struct image_version ih_ver;
    uint32_t _pad3;
};

The ih_hdr_size field indicates the length of the header, and therefore the offset of the image itself. This field provides for backwards compatibility in case of changes to the format of the image header.

The following are the image header flags available.

#define IMAGE_F_PIC                    0x00000001
#define IMAGE_F_SHA256                 0x00000002 /* Image contains hash TLV */
#define IMAGE_F_PKCS15_RSA2048_SHA256  0x00000004 /* PKCS15 w/RSA and SHA */
#define IMAGE_F_ECDSA224_SHA256        0x00000008 /* ECDSA256 over SHA256 */
#define IMAGE_F_NON_BOOTABLE           0x00000010
#define IMAGE_HEADER_SIZE              32

Optional type-length-value records (TLVs) containing image metadata are placed after the end of the image. For example, security data gets added as a footer at the end of the image.

/** Image trailer TLV format. All fields in little endian. */
struct image_tlv {
    uint8_t  it_type;   /* IMAGE_TLV_[...]. */
    uint8_t  _pad;
    uint16_t it_len     /* Data length (not including TLV header). */
};

/*
 * Image trailer TLV types.
 */
#define IMAGE_TLV_SHA256            1   /* SHA256 of image hdr and body */
#define IMAGE_TLV_RSA2048           2   /* RSA2048 of hash output */
#define IMAGE_TLV_ECDSA224          3   /* ECDSA of hash output */

Flash Map

A Mynewt device's flash is partitioned according to its flash map. At a high level, the flash map maps numeric IDs to flash areas. A flash area is a region of disk with the following properties:

  1. An area can be fully erased without affecting any other areas.
  2. A write to one area does not restrict writes to other areas.

The boot loader uses the following flash areas:

#define FLASH_AREA_BOOTLOADER                    0
#define FLASH_AREA_IMAGE_0                       1
#define FLASH_AREA_IMAGE_1                       2
#define FLASH_AREA_IMAGE_SCRATCH                 3

Image Slots

A portion of the flash memory is partitioned into two image slots: a primary slot and a secondary slot. The boot loader will only run an image from the primary slot, so images must be built such that they can run from that fixed location in flash. If the boot loader needs to run the image resident in the secondary slot, it must swap the two images in flash prior to booting.

In addition to the two image slots, the boot loader requires a scratch area to allow for reliable image swapping.

Boot States

Logically, you can think of a pair of flags associated with each image slot: pending and confirmed. On startup, the boot loader determines the state of the device by inspecting each pair of flags. These flags have the following meanings:

  • pending: image gets tested on next reboot; absent subsequent confirm command, revert to original image on second reboot.
  • confirmed: always use image unless excluded by a test image.

In English, when the user wants to run the secondary image, they set the pending flag for the second slot and reboot the device. On startup, the boot loader will swap the two images in flash, clear the secondary slot's pending flag, and run the newly-copied image in slot 0. This is a temporary state; if the device reboots again, the boot loader swaps the images back to their original slots and boots into the original image. If the user doesn't want to revert to the original state, they can make the current state permanent by setting the confirmed flag in slot 0.

Switching to an alternate image is a two-step process (set + confirm) to prevent a device from becoming "bricked" by bad firmware. If the device crashes immediately upon booting the second image, the boot loader reverts to the working image, rather than repeatedly rebooting into the bad image.

The following set of tables illustrate the three possible states that the device can be in:

               | slot-0 | slot-1 |
---------------+--------+--------|
       pending |        |        |
     confirmed |   X    |        |
---------------+--------+--------'
Image 0 confirmed;               |
No change on reboot              |
---------------------------------'

               | slot-0 | slot-1 |
---------------+--------+--------|
       pending |        |   X    |
     confirmed |   X    |        |
---------------+--------+--------'
Image 0 confirmed;               |
Test image 1 on next reboot      |
---------------------------------'

               | slot-0 | slot-1 |
---------------+--------+--------|
       pending |        |        |
     confirmed |        |   X    |
---------------+--------+--------'
Testing image 0;                 |
Revert to image 1 on next reboot |
---------------------------------'

Boot Vector

At startup, the boot loader determines which of the above three boot states a device is in by inspecting the boot vector. The boot vector consists of two records (called "image trailers"), one written at the end of each image slot. An image trailer has the following structure:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~                       MAGIC (16 octets)                       ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~                                                               ~
~             Swap status (128 * min-write-size * 3)            ~
~                                                               ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Copy done   |     0xff padding (up to min-write-sz - 1)     ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Image OK    |     0xff padding (up to min-write-sz - 1)     ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

These records are at the end of each image slot. The offset immediately following such a record represents the start of the next flash area.

Note: min-write-size is a property of the flash hardware. If the hardware allows individual bytes to be written at arbitrary addresses, then min-write-size is 1. If the hardware only allows writes at even addresses, then min-write-size is 2, and so on.

The fields are defined as follows:

  1. MAGIC: The following 16 bytes, written in host-byte-order:

    const uint32_t boot_img_magic[4] = {
        0xf395c277,
        0x7fefd260,
        0x0f505235,
        0x8079b62c,
    };
    
  2. Swap status: A series of single-byte records. Each record corresponds to a flash sector in an image slot. A swap status byte indicate the location of the corresponding sector data. During an image swap, image data is moved one sector at a time. The swap status is necessary for resuming a swap operation if the device rebooted before a swap operation completed.

  3. Copy done: A single byte indicating whether the image in this slot is complete (0x01=done, 0xff=not done).

  4. Image OK: A single byte indicating whether the image in this slot has been confirmed as good by the user (0x01=confirmed; 0xff=not confirmed).

The boot vector records are structured around the limitations imposed by flash hardware. As a consequence, they do not have a very intuitive design, and it is difficult to get a sense of the state of the device just by looking at the boot vector. It is better to map all the possible vector states to the swap types (None, Test, Revert) via a set of tables. These tables are reproduced below. In these tables, the "pending" and "confirmed" flags are shown for illustrative purposes; they are not actually present in the boot vector.

State I
                 | slot-0 | slot-1 |
-----------------+--------+--------|
           magic | Unset  | Unset  |
        image-ok | Any    | N/A    |
-----------------+--------+--------'
         pending |        |        |
      confirmed  |   X    |        |
-----------------+--------+--------'
 swap: none                        |
-----------------------------------'


State II
                 | slot-0 | slot-1 |
-----------------+--------+--------|
           magic | Any    | Good   |
        image-ok | Any    | N/A    |
-----------------+--------+--------'
         pending |        |   X    |
      confirmed  |   X    |        |
-----------------+--------+--------'
 swap: test                        |
-----------------------------------'


State III
                 | slot-0 | slot-1 |
-----------------+--------+--------|
           magic | Good   | Unset  |
        image-ok | 0xff   | N/A    |
-----------------+--------+--------'
         pending |        |        |
      confirmed  |        |   X    |
-----------------+--------+--------'
 swap: revert (test image running) |
-----------------------------------'


State IV
                 | slot-0 | slot-1 |
-----------------+--------+--------|
           magic | Good   | Unset  |
        image-ok | 0x01   | N/A    |
-----------------+--------+--------'
         pending |        |        |
      confirmed  |   X    |        |
-----------------+--------+--------'
 swap: none (confirmed test image) |
-----------------------------------'

High-level Operation

With the terms defined, we can now explore the boot loader's operation. First, a high-level overview of the boot process is presented. Then, the following sections describe each step of the process in more detail.

Procedure:

A. Inspect swap status region; is an interrupted swap is being resumed?
    Yes: Complete the partial swap operation; skip to step C.
    No: Proceed to step B.

B. Inspect boot vector; is a swap requested?
    Yes.
        1. Is the requested image valid (integrity and security check)?
            Yes.
                a. Perform swap operation.
                b. Persist completion of swap procedure to boot vector.
                c. Proceed to step C.
            No.
                a. Erase invalid image.
                b. Persist failure of swap procedure to boot vector.
                c. Proceed to step C.
    No: Proceed to step C.

C. Boot into image in slot 0.

Image Swapping

The boot loader swaps the contents of the two image slots for two reasons:

  • User has issued an "image test" operation; the image in slot-1 should be run once (state II).
  • Test image rebooted without being confirmed; the boot loader should revert to the original image currently in slot-1 (state III).

If the boot vector indicates that the image in the secondary slot should be run, the boot loader needs to copy it to the primary slot. The image currently in the primary slot also needs to be retained in flash so that it can be used later. Furthermore, both images need to be recoverable if the boot loader resets in the middle of the swap operation. The two images are swapped according to the following procedure:

1. Determine how many flash sectors each image slot consists of.  This
   number must be the same for both slots.
2. Iterate the list of sector indices in descending order (i.e., starting
   with the greatest index); current element = "index".
    b. Erase scratch area.
    c. Copy slot0[index] to scratch area.
    d. Write updated swap status (i).

    e. Erase slot1[index]
    f. Copy slot0[index] to slot1[index]
        - If these are the last sectors (i.e., first swap being perfomed),
          copy the full sector *except* the image trailer.
        - Else, copy entire sector contents.
    g. Write updated swap status (ii).

    h. Erase slot0[index].
    i. Copy scratch area slot0[index].
    j. Write updated swap status (iii).

3. Persist completion of swap procedure to slot 0 image trailer.

The additional caveats in step 2f are necessary so that the slot 1 image trailer can be written by the user at a later time. With the image trailer unwritten, the user can test the image in slot 1 (i.e., transition to state II).

The particulars of step 3 vary depending on whether an image is being tested or reverted:

* test:
    o Write slot0.copy_done = 1
    (should now be in state III)

* revert:
    o Write slot0.magic = BOOT_MAGIC
    o Write slot0.copy_done = 1
    o Write slot0.image_ok = 1
    (should now be in state IV)

Swap Status

The swap status region allows the boot loader to recover in case it restarts in the middle of an image swap operation. The swap status region consists of a series of single-byte records. These records are written independently, and therefore must be padded according to the minimum write size imposed by the flash hardware. In the below figure, a min-write-size of 1 is assumed for simplicity. The structure of the swap status region is illustrated below. In this figure, a min-write-size of 1 is assumed for simplicity.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|sec127,state 0 |sec127,state 1 |sec127,state 2 |sec126,state 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|sec126,state 1 |sec126,state 2 |sec125,state 0 |sec125,state 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|sec125,state 2 |                                               |
+-+-+-+-+-+-+-+-+                                               +
~                                                               ~
~               [Records for indices 124 through 1              ~
~                                                               ~
~               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~               |sec000,state 0 |sec000,state 1 |sec000,state 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

And now, in English...

Each image slot is partitioned into a sequence of flash sectors. If we were to enumerate the sectors in a single slot, starting at 0, we would have a list of sector indices. Since there are two image slots, each sector index would correspond to a pair of sectors. For example, sector index 0 corresponds to the first sector in slot 0 and the first sector in slot 1. Furthermore, we impose a limit of 128 indices. If an image slot consists of more than 128 sectors, the flash layout is not compatible with this boot loader. Finally, reverse the list of indices such that the list starts with index 127 and ends with 0. The swap status region is a representation of this reversed list.

During a swap operation, each sector index transitions through four separate states:

0. slot 0: image 0,   slot 1: image 1,   scratch: N/A
1. slot 0: image 0,   slot 1: N/A,       scratch: image 1 (1->s, erase 1)
2. slot 0: N/A,       slot 1: image 0,   scratch: image 1 (0->1, erase 0)
3. slot 0: image 1,   slot 1: image 0,   scratch: N/A     (s->0)

Each time a sector index transitions to a new state, the boot loader writes a record to the swap status region. Logically, the boot loader only needs one record per sector index to keep track of the current swap state. However, due to limitations imposed by flash hardware, a record cannot be overwritten when an index's state changes. To solve this problem, the boot loader uses three records per sector index rather than just one.

Each sector-state pair is represented as a set of three records. The record values map to the above four states as follows

        | rec0 | rec1 | rec2
--------+------+------+------
state 0 | 0xff | 0xff | 0xff
state 1 | 0x01 | 0xff | 0xff
state 2 | 0x01 | 0x02 | 0xff
state 3 | 0x01 | 0x02 | 0x03

The swap status region can accommodate 128 sector indices. Hence, the size of the region, in bytes, is 128 * min-write-size * 3. The number 128 is chosen somewhat arbitrarily and will likely be made configurable. The only requirement for the index count is that is is great enough to account for a maximum-sized image (i.e., at least as great as the total sector count in an image slot). If a device's image slots use less than 128 sectors, the first record that gets written will be somewhere in the middle of the region. For example, if a slot uses 64 sectors, the first sector index that gets swapped is 63, which corresponds to the exact halfway point within the region.

Reset Recovery

If the boot loader resets in the middle of a swap operation, the two images may be discontiguous in flash. Bootutil recovers from this condition by using the boot vector to determine how the image parts are distributed in flash.

The first step is determine where the relevant swap status region is located. Because this region is embedded within the image slots, its location in flash changes during a swap operation. The below set of tables map boot vector contents to swap status location. In these tables, the "source" field indicates where the swap status region is located.

          | slot-0     | scratch    |
----------+------------+------------|
    magic | Good       | Any        |
copy-done | 0x01       | N/A        |
----------+------------+------------'
source: none                        |
------------------------------------'

          | slot-0     | scratch    |
----------+------------+------------|
    magic | Good       | Any        |
copy-done | 0xff       | N/A        |
----------+------------+------------'
source: slot 0                      |
------------------------------------'

          | slot-0     | scratch    |
----------+------------+------------|
    magic | Any        | Good       |
copy-done | Any        | N/A        |
----------+------------+------------'
source: scratch                     |
------------------------------------'

          | slot-0     | scratch    |
----------+------------+------------|
    magic | Unset      | Any        |
copy-done | 0xff       | N/A        |
----------+------------+------------|
source: varies                      |
------------------------------------+------------------------------+
This represents one of two cases:                                  |
o No swaps ever (no status to read, so no harm in checking).       |
o Mid-revert; status in slot 0.                                    |
-------------------------------------------------------------------'

If the swap status region indicates that the images are not contiguous, bootutil completes the swap operation that was in progress when the system was reset. In other words, it applies the procedure defined in the previous section, moving image 1 into slot 0 and image 0 into slot 1. If the boot status file indicates that an image part is present in the scratch area, this part is copied into the correct location by starting at step e or step h in the area-swap procedure, depending on whether the part belongs to image 0 or image 1.

After the swap operation has been completed, the boot loader proceeds as though it had just been started.

Integrity Check

An image is checked for integrity immediately before it gets copied into the primary slot. If the boot loader doesn't perform an image swap, then it doesn't perform an integrity check.

During the integrity check, the boot loader verifies the following aspects of an image:

  • 32-bit magic number must be correct (0x96f3b83c).
  • Image must contain a SHA256 TLV.
  • Calculated SHA256 must matche SHA256 TLV contents.
  • Image may contain a signature TLV. If it does, its contents must be verifiable using a key embedded in the boot loader.

Image Signing and Verification

As indicated above, the final step of the integrity check is signature verification. The boot loader can have one or more public keys embedded in it at build time. During signature verification, the boot loader verifies that an image was signed with a private key that corresponds to one of its public keys. The image signature TLV indicates the index of the key that is has been signed with. The boot loader uses this index to identify the corresponding public key.

For information on embedding public keys in the boot loader, as well as producing signed images, see: boot/bootutil/signed_images.md