MCUBOOT

[YveWit]

For our project we switched over to using DIRECT_XIP mone primarily because of the faster boot time but it automatically enables downgrade protection. For our project we need the ability to downgrade. So is there anyway to have DIRECT_XIP while still allowing for the ability to downgrade.

[AHaug]

Hi,

Currently this is not supported as all of the variations of CONFIG_BOOT_BUILD_DIRECT_XIP_VARIANT, i.e CONFIG_MCUBOOT_BOOTLOADER_MODE_DIRECT_XIP and CONFIG_MCUBOOT_BOOTLOADER_MODE_DIRECT_XIP_WITH_REVERT selects MCUBOOT_BOOTLOADER_NO_DOWNGRADE due it's not possible to downgrade a running application.

Could you expand a bit upon the motivation for the need of not having downgrade protection while using DIRECT_XIP? Is it an option to investigate the use of non-direct_xip with downgrade protection disabled?

Kind regards,
Andreas

[YveWit]

We require the use of DIRECT_XIP for the boot speed. Because it does not copy over the secondary slot to the primary slot. As far as I know there is no other option that allows one to run directly from the slot that was updated. It is a requirement for our product to have the ability to update to any version. I just want to know if there is anyway to use DIRECT_XIP but disabling  the version checking 

[YveWit]

Why Can I not enable  DIRECT_XIP and just set  CONFIG_MCUBOOT_BOOTLOADER_NO_DOWNGRADE to no. Why would that now work exactly?

[AHaug]

Hi,

What do you see when you set it to n and remove all dependencies?

Kind regards,
Andreas

[YveWit]

When I set it to n it say that it is still set to y in build. I am not sure it is possible to actually disable it with DIRECT_XIP:

[AHaug]

This would mean that you don't remove all dependencies. The configuration is selected by something else, which you can see in the KConfig documentation: https://docs.nordicsemi.com/bundle/ncs-latest/page/kconfig/index.html#CONFIG_MCUBOOT_BOOTLOADER_NO_DOWNGRADE 

You can modify the KConfigs in the SDK to make a custom fork of the SDK (disclaimer do not do this without knowing what you're doing) as an experiment to see if you're able to disable it.

Kind regards,
Andreas

[YveWit]

Hi Sorry for the late reply, but is it possible to modify the version number of the current image safely to allow downgrading? Creating a fork of the SDK is not really a viable option. If for example we were in the middle of modifying the version and we lost power would that essentially brick the unit as we would have no image with a valid version number a.k.a no bootable image?

[Einar Thorsrud]

Hi,

I would not recomend this approach, but theoretically you could modfiy the version and security counter in some cases in a semi-safe way by writing to the image trailer without erasing. That would only let you flip bits from 1 to 0.

But why do you want to do this? If. you really need to downgrade while using downgrade protection, a possibility could be to build another variant of the old firmware with a higher version number. So that it is a higher version seen from the bootloader, but in fact is in other ways identical with the older version.