A/B boot with secure U-Boot env for SWUpdate

[marin.doetterer]

Greetings,

While searching for an embedded update framework I came across SWUpdate.

The goal is to implement an OTA update for an A/B boot system.

It is important for me to meet the requirements for secure-boot, so that the attack vector for overwriteable u-boot env is bypassed.


As described in (https://elinux.org/images/e/e0/Josserand-schulz-secure-boot.pdf - page 18/45), " by default, the environment can be trusted only if it's in the U-Boot binary (ENV_IS_NOWHERE)".


I found in your mailing list the conversation about this topic (2021) where CONFIG_ENV_WRITEABLE_LIST and CONFIG_ENV_EMBEDDED are noted (https://groups.google.com/g/swupdate/c/jnX4x95yEs8).


Using "CONFIG_ENV_WRITEABLE_LIST" minimizes the attack vector, but it still poses a threat.

Unfortunately, I can't find any further information about CONFIG_ENV_EMBEDDED. Is this already supported by U-Boot?


Would you have any suggestions on how else to solve the problem with the U-Boot env variables?


Thanks a lot for your help!

Best regards,

Marin

[Stefano Babic]

Hi Marin,

On 09.10.23 22:20, 'marin.doetterer' via swupdate wrote:
> Greetings,
>
> While searching for an embedded update framework I came across SWUpdate.
>
> The goal is to implement an OTA update for an A/B boot system.
>
> It is important for me to meet the requirements for secure-boot, so that
> the attack vector for overwriteable u-boot env is bypassed.
>
>
> As described in
> (https://elinux.org/images/e/e0/Josserand-schulz-secure-boot.pdf
> <https://elinux.org/images/e/e0/Josserand-schulz-secure-boot.pdf> - page
> 18/45), " by default, the environment can be trusted only if it's in the
> U-Boot binary (ENV_IS_NOWHERE)".

ENV_IS_NOWHERE simply means there is no environment at all. The scripts
/ variables are linked (as array of strings) into U-Boot, and no
environment is stored.

>
>
> I found in your mailing list the conversation about this topic (2021)
> where CONFIG_ENV_WRITEABLE_LIST and CONFIG_ENV_EMBEDDED are noted
> (https://groups.google.com/g/swupdate/c/jnX4x95yEs8).

> <https://groups.google.com/g/swupdate/c/jnX4x95yEs8).>

Right - as signed environment was up now rejected by U-Boot, because
this means that "saveenv" does not work anymore, or U-Boot should be
able to sign itself.

>
>
> Using "CONFIG_ENV_WRITEABLE_LIST" minimizes the attack vector, but it
> still poses a threat.

And which are the drawbacks ?

With CONFIG_ENV_WRITEABLE_LIST, you can add a single variable (used for
toggling the software) and you can fix the type (int) to avoid that an
attacker can attach a script. The only thing that can happen is that an
attacker is able to toggle to the other partition, but this can be made
unusable after a successful update.

Think about that kernel should be signed as well to have secure boot
working.

Which are still the issues you see with CONFIG_ENV_WRITEABLE_LIST ?

>
> Unfortunately, I can't find any further information about
> CONFIG_ENV_EMBEDDED.

What is this supposed to do ?

> Is this already supported by U-Boot?

What should be supported and what should be done ?

ENV_IS_NOWHERE is too simplistic: it just drop the environment.

There are also other use cases where toggling is done without U-Boot
environment (see CONFIG_SSBLSWITCH), using other structures or switching
boot partition (for MBR partition table), but there is no additional
protection compared to CONFIG_ENV_WRITEABLE_LIST.

>
>
> Would you have any suggestions on how else to solve the problem with the
> U-Boot env variables?

The problem was solved with CONFIG_ENV_WRITEABLE_LIST. If there is still
an issue, please expose it.

Best regards,
Stefano Babic

>
>
> Thanks a lot for your help!
>
> Best regards,
>
> Marin
>

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