[U-Boot] Verified boot of images without signatures

[U-Boot] Verified boot of images without signatures

[Patrick Doyle]

I am looking at enabling verified boot in the v2019.04-rc4 tag of
u-boot.  I was pleased when I learned how to embed the public
authentication key in my u-boot device tree, sign my kernel using my
private authentication key, and see u-boot validate the signature on
boot.

But then I was very surprised to learn that I could still boot an
unsigned image.  So I started looking at the code and I found
`fit_image_verify_with_data() in "common/image_fit.c", which does:

    if (IMAGE_ENABLE_VERIFY &&
        fit_image_verify_required_sigs(fit, image_noffset, data, size,
                       gd_fdt_blob(), &verify_all)) {
        err_msg = "Unable to verify required signature";
        goto error;
    }

    /* Process all hash subnodes of the component image node */
    fdt_for_each_subnode(noffset, fit, image_noffset) {
        const char *name = fit_get_name(fit, noffset, NULL);

        /*
         * Check subnode name, must be equal to "hash".
         * Multiple hash nodes require unique unit node
         * names, e.g. hash-1, hash-2, etc.
         */
        if (!strncmp(name, FIT_HASH_NODENAME,
                 strlen(FIT_HASH_NODENAME))) {
            if (fit_image_check_hash(fit, noffset, data, size,
                         &err_msg))
                goto error;
            puts("+ ");
        } else if (IMAGE_ENABLE_VERIFY && verify_all &&
                !strncmp(name, FIT_SIG_NODENAME,
                    strlen(FIT_SIG_NODENAME))) {
            ret = fit_image_check_sig(fit, noffset, data,
                            size, -1, &err_msg);

            /*
             * Show an indication on failure, but do not return
             * an error. Only keys marked 'required' can cause
             * an image validation failure. See the call to
             * fit_image_verify_required_sigs() above.
             */
            if (ret)
                puts("- ");
            else
                puts("+ ");
        }
    }

I see that if I create a "required" property in my signature block,
then u-boot will require that the signature match.  But if I don't
have that, then it will happily boot an unsigned image (or even one
that doesn't have any signature blocks).

Am I missing something here?

Has this been improved/addressed since v2019.04-rc4?

If the answers are "No" and "No", then I will go in and address it
myself.  I welcome any tips folks might care to give me in advance of
me just submitting a patch to address this.

--wpd

[U-Boot] Verified boot of images without signatures

[Alex Kiernan]

On Wed, Jun 12, 2019 at 7:00 PM Patrick Doyle <wpdster@gmail.com> wrote:
>
> I am looking at enabling verified boot in the v2019.04-rc4 tag of
> u-boot.  I was pleased when I learned how to embed the public
> authentication key in my u-boot device tree, sign my kernel using my
> private authentication key, and see u-boot validate the signature on
> boot.
>
> But then I was very surprised to learn that I could still boot an
> unsigned image.  So I started looking at the code and I found
> `fit_image_verify_with_data() in "common/image_fit.c", which does:
>
>     if (IMAGE_ENABLE_VERIFY &&
>         fit_image_verify_required_sigs(fit, image_noffset, data, size,
>                        gd_fdt_blob(), &verify_all)) {
>         err_msg = "Unable to verify required signature";
>         goto error;
>     }
>
>     /* Process all hash subnodes of the component image node */
>     fdt_for_each_subnode(noffset, fit, image_noffset) {
>         const char *name = fit_get_name(fit, noffset, NULL);
>
>         /*
>          * Check subnode name, must be equal to "hash".
>          * Multiple hash nodes require unique unit node
>          * names, e.g. hash-1, hash-2, etc.
>          */
>         if (!strncmp(name, FIT_HASH_NODENAME,
>                  strlen(FIT_HASH_NODENAME))) {
>             if (fit_image_check_hash(fit, noffset, data, size,
>                          &err_msg))
>                 goto error;
>             puts("+ ");
>         } else if (IMAGE_ENABLE_VERIFY && verify_all &&
>                 !strncmp(name, FIT_SIG_NODENAME,
>                     strlen(FIT_SIG_NODENAME))) {
>             ret = fit_image_check_sig(fit, noffset, data,
>                             size, -1, &err_msg);
>
>             /*
>              * Show an indication on failure, but do not return
>              * an error. Only keys marked 'required' can cause
>              * an image validation failure. See the call to
>              * fit_image_verify_required_sigs() above.
>              */
>             if (ret)
>                 puts("- ");
>             else
>                 puts("+ ");
>         }
>     }
>
> I see that if I create a "required" property in my signature block,
> then u-boot will require that the signature match.  But if I don't
> have that, then it will happily boot an unsigned image (or even one
> that doesn't have any signature blocks).
>
> Am I missing something here?
>

Probably... I went round a very similar loop too. You need the
required property in the U-Boot DTB, not in the image you're booting.
And if you're trying to do this for SPL loading U-Boot you need
CONFIG_SPL_LOAD_FIT_FULL. Oh and make sure you've disabled legacy
image support.

-- 
Alex Kiernan

[U-Boot] Verified boot of images without signatures

[Patrick Doyle]

On Wed, Jun 12, 2019 at 2:10 PM Alex Kiernan <alex.kiernan@gmail.com> wrote:
> On Wed, Jun 12, 2019 at 7:00 PM Patrick Doyle <wpdster@gmail.com> wrote:
> > Am I missing something here?
> >
>
> Probably... I went round a very similar loop too. You need the
> required property in the U-Boot DTB, not in the image you're booting.
> And if you're trying to do this for SPL loading U-Boot you need
> CONFIG_SPL_LOAD_FIT_FULL. Oh and make sure you've disabled legacy
> image support.
Hi Alex,
You nailed it.  I didn't understand that the "required" property
belonged to the u-boot dtb, not the fitImage.  Now that I understand
that, I see where that is described in signature.txt.  I'm great at
understanding documentation once I know what the documentation says
:-)


Thanks for the help.

--wpd

[U-Boot] Verified boot of images without signatures

[Simon Glass]

Hi Patrick,

On Wed, 12 Jun 2019 at 14:28, Patrick Doyle <wpdster@gmail.com> wrote:
>
> On Wed, Jun 12, 2019 at 2:10 PM Alex Kiernan <alex.kiernan@gmail.com> wrote:
> > On Wed, Jun 12, 2019 at 7:00 PM Patrick Doyle <wpdster@gmail.com> wrote:
> > > Am I missing something here?
> > >
> >
> > Probably... I went round a very similar loop too. You need the
> > required property in the U-Boot DTB, not in the image you're booting.
> > And if you're trying to do this for SPL loading U-Boot you need
> > CONFIG_SPL_LOAD_FIT_FULL. Oh and make sure you've disabled legacy
> > image support.
> Hi Alex,
> You nailed it.  I didn't understand that the "required" property
> belonged to the u-boot dtb, not the fitImage.  Now that I understand
> that, I see where that is described in signature.txt.  I'm great at
> understanding documentation once I know what the documentation says

A doc patch is welcome.

The 'required' property is in the 'trusted' DT since otherwise an
image could just omit it.

Regards,
Simon