[U-Boot] U-Boot: Verified Boot: signed configuration and mix and match attack

[Simon Glass]

Hi Johann,

On 2 August 2018 at 07:20, Johann Neuhauser
<jneuhauser at dh-electronics.de> wrote:
> Hello Simon,
>
>> > Dear U-Boot devs,
>> >
>> > I've setup verified boot on a imx6 board and want to protect my device
>> against the "mix and match" attacks mentioned in
>> "doc/uImage.FIT/signature.txt".
>> > That's why I have only implemented signed configurations and no signed
>> images as in doc/uImage.FIT/signed-configs.its.
>> > My public key in my embedded fdt has the property required = "conf";
>> >
>> > Booting a signed config with "bootm ${loadaddr}#conf at 1" and an
>> embedded public key required for configurations does work as expected and
>> do fail to boot if I modify the config, image, hash, signature and so on.
>> >
>> > If I boot any fit image(signed and unsigned) for example with "bootm
>> ${loadaddr}:kernel at 1 - fdt at 1" to select the subimages directly, I could boot
>> every image combination without signature verification although a signature
>> is enforced for a configuration.
>> >
>> > Is this the expected behavior?
>> >
>> > I thought if I had set the public key in in the embedded fdt as required for
>> configurations, bootm does only boot signed configurations and no
>> subimages directly...
>>
>> I don't think there is any restriction on that at the moment. You are explicitly
>> asking to boot particular images rather than a config. So I suppose it would be
>> odd if U-Boot tried to enforce a config. Are you thinking it should try to find a
>> config that has those images in it?
>
> No, I expected that I cannot boot sub images directly if there is a required public key for a configuration.
> After a dive into the bootm source I think this is not easily possible to enforce this behavior.

It should be easy enough to change fit_image_load() to return an error
if fit_uname_configp doesn't have a config.

>
>> But why not just specify the config to bootm?
>
> At first I wanted to use a simple boot script wrapped in a fit image (unsigned) and
> have only the needed commands enabled in U-Boot.
> Now I switched to a signed U-Boot script as boot script and can be sure that this one gets not tampered.
> The only bad thing is here that the source command does only have support for fit sub images and
> I have to sign the config and the image of my system image if I had a required certificate for images and configs.

I don't quite get this, sorry. What is a FIT sub-image? Do you mean a
FIT image? You can actually sign individual images if you want to, but
as the docs mention, this is not fully secure since it is vulnerable
to mix-and-match attacks.

But if do want to group images together, so that only certain ones can
be used together, then a FIT config is the mechanism for that. You
must sign each config simple because otherwise there is nothing
protecting against the attack.

>
> Probably this behavior should be mentioned in the doc.

Please send a patch.

>
> Many thanks for the clarification.

You are welcome. But I suspect I have misunderstood this in some way,
so let me know :-)

Regards,
Simon