u-boot: signature check for u-boot scripts

[Sean Anderson]

On 1/11/23 01:13, Heiko Schocher wrote:
> Hello Sean,
> 
> Thanks for your answer!
> 
> On 10.01.23 17:27, Sean Anderson wrote:
>> On 1/10/23 08:18, Heiko Schocher wrote:
>>> Hello Simon,
> [...]
>>> While writting this email ... in [3] the line
>>>
>>>  require = "conf"
>>>
>>> poped into my eyes .... and in fit_image_verify_required_sigs() there is check:
>>>
>>>                 if (!required || strcmp(required, "image"))
>>>                         continue;
>>>
>>> and yes! changing in [3]
>>>
>>> -required = "conf";
>>> +required = "image";
>>>
>>> makes sourcing the signed script working (error in case of no
>>> signature or wrong signature)! ... but booting the signed fitimage
>>> now breaks ... so it seems, I cannot use configuration signing with
>>> images signing ?
>>>
>>> I tried to add two key nodes in signature node of u-boot dtb ... one with
>>> require = "conf" and one with require = "image" ... but no luck...
>>>
>>> Also adding a configurations section to scripts its file did not helped
>>> (which will not prevent the problem sourcing a not signed script)
>> 
>> As you discovered, you must either have required = "image", in which case
>> 
>>     source :
>> 
>> will be secure. Otherwise, you must use
>> 
>>     source \#
>> 
>> Any other way is not secure.
> 
> My "hack" checks a configuration signature in fitimage with script in it...
> so also "secure" ...
> 
> BTW: why we need a env variable to enable checking in cmd/source.c?
>      I would say, if verify fit images is enabled we always should check
>      signature ... but this is another question...

I think it's to allow disabling things for debugging. If the variable
does not exist, it defaults to verifying.

> So I tried your suggestion:
> 
> => tftp 100000 script.bin.signed;setenv verify 1;source \#100000
> Speed: 1000, full duplex
> Using ethernet at 24000 device
> TFTP from server 192.168.3.1; our IP address is 192.168.3.40
> Filename 'script.bin.signed'.
> Load address: 0x100000
> Loading: #
>          233.4 KiB/s
> done
> Bytes transferred = 1679 (68f hex)
> ## Executing script at 00000000
> Wrong image format for "source" command
> =>
> 
> same for
> 
> => source \#100000:script-1
> ## Executing script at 00000000
> Wrong image format for "source" command
> =>
> 
> Which is the error message from the switch in image_source_script()
> from cmd/source.c ...

Right.

What kind of image is your script? Do you have CONFIG_FIT (and *only*
CONFIG_FIT) enabled?

> (check if fitimage "is okay"):
> => source 100000
> ## Executing script at 00100000
> sha256+ sha256,rsa2048:dev+ Hallo from script
> => source 100000:script-1
> ## Executing script at 00100000
> sha256+ sha256,rsa2048:dev+ Hallo from script
> => source 100000:script-2
> ## Executing script at 00100000
> Can't find 'script-2' FIT subimage
> =>
> 
> and changing hash in fitimages signature leads to:
> => mw 1001c0 0 1
> => source 100000:script-1
> ## Executing script at 00100000
> sha256+ sha256,rsa2048:dev- Hallo from script
> =>
> 
> As I described ... problem "hash is detected, but script is executed",
> as public key in u-boots dtb has required = "conf"; (as it is used also
> for fitimage boot, where we use conf signing)
>
> May you have an example (u-boot.dtb, its and complete working command
> for a signed fitimage script)?

$ cat << EOF >> dm-verity.its
/dts-v1/;

/ {
    description = "dm-verity boot parameters";
    #address-cells = <1>;

    images {
        dm-verity {
            data = /incbin/("dm-verity.scr");
            type = "script";
            arch = "arm64";
            compression = "none";
            hash-1 {
                algo = "sha256";
            };
        };
    };

    configurations {
        default = "conf";
        conf {
            description = "Load dm-verity boot parameters";
            script = "dm-verity";
            signature {
                algo = "sha256,rsa2048";
                key-name-hint = "u-boot";
                sign-images = "script";
            };
        };
    };
};
EOF
$ uboot-mkimage -EB 0x40 -f dm-verity.its dm-verity.itb
$ uboot-mkimage -EB 0x40 -r -F -k /path/to/keys dm-verity.itb

> The main problem is (I think) that we check for fitimages which are
> used for booting kernels, a "signed configuration" and in fitimage for scripts
> only "image" signatures ... and a combination of both is not possible
> (except I also sign the image nodes in kernel fitimage too ... which
> than leads in checking configuration signature and image signature on
> boot... but may a way to go (and disabling hash check) ?

Yes, which is why you need to have # in the source command to force only
using configurations. IMO we should also check image-only FITs, but
Simon disagrees.

--Sean