Locking down U-Boot env with ENV_WRITEABLE_LIST

Sean Anderson sean.anderson at seco.com
Tue Apr 6 23:54:18 CEST 2021

On 4/6/21 4:10 PM, Marek Vasut wrote:
 > On 4/6/21 9:52 PM, Tim Harvey wrote:
 >> On Mon, Apr 5, 2021 at 10:36 AM Marek Vasut <marex at denx.de> wrote:
 >>> On 4/5/21 6:27 PM, Tim Harvey wrote:
 >>>> On Sat, Apr 3, 2021 at 12:09 PM Marek Vasut <marex at denx.de> wrote:
 >>>>> On 4/3/21 6:43 PM, Tim Harvey wrote:
 >>>>> Hi,
 >>>>> [...]
 >>>>>>>>> And those config options I had enabled in u-boot defconfig:
 >>>>>>>>> CONFIG_ENV_IS_IN_MMC=y
 >>>>>>>>> CONFIG_ENV_APPEND=y
 >>>>>>>> Do you really define both ENV_IS_NOWHERE and ENV_IS_IN_MMC? From what
 >>>>>>>> I see if you define ENV_IS_NOWHERE none of the others will be used.
 >>>>>>> Yes, having two ENV drivers is mandatory. One provides the base env (the
 >>>>>>> nowhere) and the other is used to import the filtered extras from
 >>>>>>> external env.
 >>>>>> Enabling ENV_IS_NOWHERE does not work as you describe in my testing.
 >>>>>> I'm testing this with imx8mm_venice_defconfig on U-Boot 2021.01 and as
 >>>>>> soon as I define ENV_IS_NOWHERE then env_load is never called because
 >>>>>> when env_relocate is called env is not valid yet so env_set_default is
 >>>>>> called and env_load is 'never' called, thus mmc env is never loaded.
 >>>>>> This is all from
 >>>>>> https://elixir.bootlin.com/u-boot/v2021.01/source/env/common.c#L259
 >>>>> Maybe this [PATCH V3] env: Fix invalid env handling in env_init() patch
 >>>>> is missing ?
 >>>>> [...]
 >>>>>>     /* Link Definitions */
 >>>>>>     #define CONFIG_LOADADDR                        0x40480000
 >>>>>>     #define CONFIG_SYS_LOAD_ADDR           CONFIG_LOADADDR''
 >>>>>> With this configuration a blank env in flash results in the entire
 >>>>>> default env showing in an 'env print' (ie all the stuff from
 >>>>>> include/env_default.h 'default_environment') but as soon as I put an
 >>>>>> env in flash (ie saveenv) and reset now the only env is what was set
 >>>>>> via running code (ethaddr's fdtcontroladdr stdin/err/out). The only
 >>>>>> different I expected is that I expected the default env from
 >>>>>> include/env_default.h to be there on an initial boot with no valid mmc
 >>>>>> env.
 >>>>> Maybe the aforementioned patch is missing ?
 >>>> Marek,
 >>>> Yes, that patch fixes the issue I'm seeing of mmc not initializing and
 >>>> now the default env shows up as expected when MMC env is empty or
 >>>> populated.
 >>>> Thanks - hopefully that patch gets merged soon... I did respond with a
 >>>> Tested-By.
 >>> Oh, good. You might want to notify Tom about that, so it would get picked.
 >> Marek,
 >> One additional thing I did get stuck on is that if your writable
 >> var(s) appears in the default environment the default will take
 >> precedence over the FLASH env. This does make sense, but requires that
 >> you create a default FLASH env (ie mkenvimage) and put it in the
 >> appropriate place. I figured I would mention this for anyone else that
 >> ends up reading this thread for help.
 > Can you maybe write some better env documentation and submit a patch, now that you got it all figured out ?
 >> One last thing that I have not yet figured out how to work-around is
 >> how to best disable the shell completely so that if there is any
 >> failure in your bootcmd you don't simply drop into a completely
 >> insecure U-Boot shell. What is your recommendation there?
 > set bootdelay=-2 disables the prompt access.

This is insufficient. If the end of the boot command is reached, U-Boot
will fall back to the shell. One must ensure that the bootcmd never
exists. This can be done either by placing the whole command in a loop,
recursively calling another command, or by resetting the board if no
boot commands work. See e.g. [1] for a good writeup.


[1] https://labs.f-secure.com/assets/BlogFiles/2020-05-u-booting-securely-wp-final.pdf

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