Re: Can't find a solution to a failed secure boot kernel loading

[Dimitri John Ledkov]

the MOK key as generated by Ubuntu/Debian tooling, creates a signing
certificate that self-limits itself to only support Kernel Module
signing.
Signatures made by such certificate, are not trusted by shim for the
purpose of code signing of bootloaders (i.e. grub) or kernels (i.e.
linux).
I also responded this on stackoverflow.

The automatically generated MOK key is only usable to sign kernel
modules, i.e. self-built DKMS modules.

-- 
okurrr,

Dimitri

On Tue, 10 May 2022 at 11:33, Łukasz Piątkowski <piontec@gmail.com> wrote:
>
> Hi everyone - I'm new here!
>
> Sorry for going with my problem directly to the grub-devel maling list, but 
> I'm pretty sure my problem is GRUB related. Still, I've spent some hours 
> trying to find a solution on the Internet and I failed :( So, here it comes - 
> if anyone has time to explain my problem to a layman, it would be awesome. 
> Even better, if you can maybe answer here on stackoverflow, where it can be 
> easier to find, I believe 
> (https://unix.stackexchange.com/questions/701612/cant-load-self-signed-kernel-with-secure-boot-on-bad-shim-signature).
>
> I'm running ubuntu with Secure Boot on. Everything works fine when I use a 
> kernel that comes packaged from cannonical. Still, I have issues running a 
> self-signed kernel (this is actually an externally built kernel, that I have 
> verified and want to use for my own machine). I'm pretty sure my signature 
> with MOK key is OK (verification below), but still when I try to boot the 
> kernel from grub, after selecting the correct entry, I get an error that 
> reads "Loading ... error: bad shim signature." I'm wrapping my head around it 
> and can't find a solution. Why, even though both kernels are signed with MOK 
> keys, one of them works and the other doesn't?
>
> Here's info about kernel signatures:
>
> root@T495:~# sbsign --key /var/lib/shim-signed/mok/MOK.priv --cert 
> /var/lib/shim-signed/mok/MOK.pem /boot/vmlinuz
> Image was already signed; adding additional signature
>
> root@T495:~# sbverify --list /boot/vmlinuz
> signature 1
> image signature issuers:
>  - /C=PL/ST=Poznan/L=Poznan/O=none/CN=Secure Boot 
> Signing/emailAddress=example@example.com
> image signature certificates:
>  - subject: /C=PL/ST=yes/L=yes/O=none/CN=Secure Boot 
> Signing/emailAddress=example@example.com
>    issuer:  /C=PL/ST=yes/L=yes/O=none/CN=Secure Boot 
> Signing/emailAddress=example@example.com
> signature 2
> image signature issuers:
>  - /CN=ubuntu Secure Boot Module Signature key
> image signature certificates:
>  - subject: /CN=ubuntu Secure Boot Module Signature key
>    issuer:  /CN=ubuntu Secure Boot Module Signature key
>
>
> And here about MOK keys:
>
> root@T495:~# openssl x509 -in /var/lib/shim-signed/mok/MOK.pem -fingerprint 
> -noout
> SHA1 Fingerprint=81:A2:93:CB:06:6F:52:BA:D9:E2:39:68:9D:FA:E2:2B:0C:95:3C:F7
> root@T495:~# mokutil --list-enrolled | grep "81:a2:93"
> SHA1 Fingerprint: 81:a2:93:cb:06:6f:52:ba:d9:e2:39:68:9d:fa:e2:2b:0c:95:3c:f7
>
> If there are any docs that help understand that, I'm happy to be redirected 
> there :)
>
> piontec
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