This document describes the OpenBmc software implementing the secure flash update mechanism.
The primary details are here.
This supports two methods of providing the image to stage. You can send the file over IPMI packets, which is a very slow process. A 32-MiB image can take ~3 hours to send via this method. This can be done in <1 minutes via the PCI bridge, or just a few minutes via LPC depending on the size of the mapped area.
This is implemented as a phosphor blob handler.
The image must be signed via the production or development keys, the former being required for production builds. The image itself and the image signature are separately sent to the BMC for verification. The verification package source is beyond the scope of this design.
Basically the IPMI OEM handler receives the image in one fashion or another and then triggers the verify_image service. Then, the user polls until the result is reported. This is because the image verification process can exceed 10 seconds.
The image flashing mechanism itself is the initramfs stage during reboot. It will check for files named "image-*" and flash them appropriately for each name to section. The IPMI command creates a file /run/initramfs/bmc-image and writes the contents there. It was found that writing it in /tmp could cause OOM errors moving it on low memory systems, whereas renaming a file within the same folder seems to only update the directory inode's contents.
The staging file path can be controlled via software configuration. The image is assumed to be the tarball contents and is written into /tmp/{tarball_name}.gz
TODO: Flesh out the UBI approach.