[U-Boot] Signed FIT image boots without public key

[Teddy Reed]

I also use the verified-boot features for several boards. The behavior
you're describing is my understanding too.

But I think the expectation: "do not enforce signature checks if there
are no public keys" is OK. The expectation "enforce a signature check
if there's a signature" doesn't seem complete to me.

On Fri, May 25, 2018 at 7:46 AM, Nelson, Mark <Mark.Nelson at commscope.com> wrote:
> Let me preface this by saying I'm using the xilinx-u-boot repo based on the 2017.4 tag.
>
> I've been studying how to sign FIT images, using mkimage. I successfully signed an image and verified the signature as follows:
>
> ${UBTOOLS4}/mkimage -T multi -A arm64 -O linux -f images/linux/fitimage.its -K images/linux/public.dtb -k ${SEC_KEYSTORE} -r images/linux/image_secure.ub
>
> ${UBTOOLS4}/fit_check_sign -f images/linux/image_secure.ub  -k images/linux/public.dtb
>
> I'm signing the configuration in the fitimage.its as follows:
>                 conf at 1 {
>                         description = "Boot Linux kernel with FDT blob + ramdisk";
>                         kernel = "kernel at 0";
>                         fdt = "fdt at 0";
>                         ramdisk = "ramdisk at 0";
>                         signature at 1 {
>                                 algo = "sha256,rsa2048";
>                                 key-name-hint = "linux_rsa_priv";
>                                 sign-images = "kernel", "fdt", "ramdisk";
>                         };
>                 };
>
>
>
> However, when I tested booting the FIT image without providing the public key in Uboots control fdt, it booted successfully. It obviously should not boot a signed image without the key information. When I looked into this, I found that there are two routines called to verify required signatures when booting: fit_image_verify_required_sigs() and fit_config_verify_required_sigs(). The latter is for verifying the signatures in configuration nodes. Each of these routines begins with this logic:
>        /* Work out what we need to verify */
>        *no_sigsp = 1;
>        sig_node = fdt_subnode_offset(sig_blob, 0, FIT_SIG_NODENAME);
>        if (sig_node < 0) {
>              debug("%s: No signature node found: %s\n", __func__,
>                    fdt_strerror(sig_node));
>              return 0;
>        }
>
> The problem is that if the fdt signature node(with the public key info) is not provided, this simply returns 0 for success, and the boot process continues successfully after it checks the hashes of the images. To fix the problem, I simply changed fit_config_verify_required_sigs() to return sig_node instead. If the signature is not there, the returned value is FDT_ERR_NOTFOUND, and the boot processing ends with:
> Bad Data Hash
> ERROR: can't get kernel image!
>
> Notice that I didn't change the logic in fit_image_verify_required_sigs(), because I'm not including signatures on the images, just hashes. In that case, I want it to ignore the fact that it didn't find a signature node.
>
> But surely, this solution is not the correct way. I know that IMAGE_ENABLE_VERIFY is used to gate access to fit_image_verify_required_sigs(), but there is nothing gating access to fit_config_verify_required_sigs() that I can see. I don't see any way to tell u-boot which signature method it should be using to verify: images or configs. It would seem that an additional preprocessor directive is needed. Am I missing something?
>

The suggestion of a build-time option to enforce a signature check
regardless of the availability of public keys is interesting. A
complementary build option to choose image or config could work too.

This could enforce a failure if (A) there are no public keys available
and also (B) there are no signatures available for verifying. I am
interested in others' thoughts!

>
> [cid:image002.png at 01D357D5.908FF380]<http://www.commscope.com/>
> Mark Nelson
> Principal Engineer
> CommScope
> 250 Apollo Drive, Suite 100
> Chelmsford, MA  01824
> Mobile: 352-444-0150
>
>
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-- 
Teddy Reed V