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README on how boot images are created for secure TI devices
CONFIG_TI_SECURE_DEVICE:
Secure TI devices require a boot image that is authenticated by ROM
code to function. Without this, even JTAG remains locked and the
device is essentially useless. In order to create a valid boot image for
a secure device from TI, the initial public software image must be signed
and combined with various headers, certificates, and other binary images.
Information on the details on the complete boot image format can be obtained
from Texas Instruments. The tools used to generate boot images for secure
devices are part of a secure development package (SECDEV) that can be
downloaded from:
http://www.ti.com/mysecuresoftware (login required)
The secure development package is access controlled due to NDA and export
control restrictions. Access must be requested and granted by TI before the
package is viewable and downloadable. Contact TI, either online or by way
of a local TI representative, to request access.
Booting of U-Boot SPL
=====================
When CONFIG_TI_SECURE_DEVICE is set, the U-Boot SPL build process
requires the presence and use of these tools in order to create a
viable boot image. The build process will look for the environment
variable TI_SECURE_DEV_PKG, which should be the path of the installed
SECDEV package. If the TI_SECURE_DEV_PKG variable is not defined or
if it is defined but doesn't point to a valid SECDEV package, a
warning is issued during the build to indicate that a final secure
bootable image was not created.
Within the SECDEV package exists an image creation script:
${TI_SECURE_DEV_PKG}/scripts/create-boot-image.sh
This is called as part of the SPL/u-boot build process. As the secure
boot image formats and requirements differ between secure SOC from TI,
the purpose of this script is to abstract these details as much as
possible.
The script is basically the only required interface to the TI SECDEV
package for creating a bootable SPL image for secure TI devices.
Invoking the script for AM43xx Secure Devices
=============================================
create-boot-image.sh \
<IMAGE_FLAG> <INPUT_FILE> <OUTPUT_FILE> <SPL_LOAD_ADDR>
<IMAGE_FLAG> is a value that specifies the type of the image to
generate OR the action the image generation tool will take. Valid
values are:
SPI_X-LOADER - Generates an image for SPI flash (byte
swapped)
XIP_X-LOADER - Generates a single stage u-boot for
NOR/QSPI XiP
ISSW - Generates an image for all other boot modes
<INPUT_FILE> is the full path and filename of the public world boot
loaderbinary file (depending on the boot media, this is usually
either u-boot-spl.bin or u-boot.bin).
<OUTPUT_FILE> is the full path and filename of the final secure
image. The output binary images should be used in place of the standard
non-secure binary images (see the platform-specific user's guides and
releases notes for how the non-secure images are typically used)
u-boot-spl_HS_SPI_X-LOADER - byte swapped boot image for SPI flash
u-boot_HS_XIP_X-LOADER - boot image for NOR or QSPI flash
u-boot-spl_HS_ISSW - boot image for all other boot media
<SPL_LOAD_ADDR> is the address at which SOC ROM should load the
<INPUT_FILE>
Invoking the script for DRA7xx/AM57xx Secure Devices
====================================================
create-boot-image.sh <IMAGE_TYPE> <INPUT_FILE> <OUTPUT_FILE>
<IMAGE_TYPE> is a value that specifies the type of the image to
generate OR the action the image generation tool will take. Valid
values are:
X-LOADER - Generates an image for NOR or QSPI boot modes
MLO - Generates an image for SD/MMC/eMMC boot modes
ULO - Generates an image for USB/UART peripheral boot modes
Note: ULO is not yet used by the u-boot build process
<INPUT_FILE> is the full path and filename of the public world boot
loader binary file (for this platform, this is always u-boot-spl.bin).
<OUTPUT_FILE> is the full path and filename of the final secure image.
The output binary images should be used in place of the standard
non-secure binary images (see the platform-specific user's guides
and releases notes for how the non-secure images are typically used)
u-boot-spl_HS_MLO - boot image for SD/MMC/eMMC. This image is
copied to a file named MLO, which is the name that
the device ROM bootloader requires for loading from
the FAT partition of an SD card (same as on
non-secure devices)
u-boot-spl_HS_X-LOADER - boot image for all other flash memories
including QSPI and NOR flash
Booting of Primary U-Boot (u-boot.img)
======================================
The SPL image is responsible for loading the next stage boot loader,
which is the main u-boot image. For secure TI devices, the SPL will
be authenticated, as described above, as part of the particular
device's ROM boot process. In order to continue the secure boot
process, the authenticated SPL must authenticate the main u-boot
image that it loads.
The configurations for secure TI platforms are written to make the boot
process use the FIT image format for the u-boot.img (CONFIG_SPL_FRAMEWORK
and CONFIG_SPL_LOAD_FIT). With these configurations the binary
components that the SPL loads include a specific DTB image and u-boot
image. These DTB image may be one of many available to the boot
process. In order to secure these components so that they can be
authenticated by the SPL as they are loaded from the FIT image, the
build procedure for secure TI devices will secure these images before
they are integrated into the FIT image. When those images are extracted
from the FIT image at boot time, they are post-processed to verify that
they are still secure. The outlined security-related SPL post-processing
is enabled through the CONFIG_SPL_FIT_IMAGE_POST_PROCESS option which
must be enabled for the secure boot scheme to work. In order to allow
verifying proper operation of the secure boot chain in case of successful
authentication messages like "Authentication passed: CERT_U-BOOT-NOD" are
output by the SPL to the console for each blob that got extracted from the
FIT image. Note that the last part of this log message is the (truncated)
name of the signing certificate embedded into the blob that got processed.
The exact details of the how the images are secured is handled by the
SECDEV package. Within the SECDEV package exists a script to process
an input binary image:
${TI_SECURE_DEV_PKG}/scripts/secure-binary-image.sh
This is called as part of the u-boot build process. As the secure
image formats and requirements can differ between the various secure
SOCs from TI, this script in the SECDEV package abstracts these
details. This script is essentially the only required interface to the
TI SECDEV package for creating a u-boot.img image for secure TI
devices.
The SPL/u-boot code contains calls to dedicated secure ROM functions
to perform the validation on the secured images. The details of the
interface to those functions is shown in the code. The summary
is that they are accessed by invoking an ARM secure monitor call to
the device's secure ROM (fixed read-only-memory that is secure and
only accessible when the ARM core is operating in the secure mode).
Invoking the secure-binary-image script for Secure Devices
==========================================================
secure-binary-image.sh <INPUT_FILE> <OUTPUT_FILE>
<INPUT_FILE> is the full path and filename of the input binary image
<OUTPUT_FILE> is the full path and filename of the output secure image.
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